0001293971-21-000013 | 10-K | XBRL Viewer

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Cover - USD ($)	12 Months Ended
Cover - USD ($)	Dec. 31, 2020	Feb. 18, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-35966
Entity Registrant Name	bluebird bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	13-3680878
Entity Address, Address Line One	60 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	339
Local Phone Number	499-9300
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	BLUE
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4,040,592,242
Entity Common Stock, Shares Outstanding		67,141,044
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Proxy Statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001293971

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 317,705	$ 327,214
Marketable securities	833,546	779,246
Prepaid expenses	37,472	32,888
Receivables and other current assets	26,814	12,826
Total current assets	1,215,537	1,152,174
Marketable securities	122,891	131,506
Property, plant and equipment, net	162,831	151,176
Intangible assets, net	10,041	14,326
Goodwill	13,128	13,128
Operating lease right-of-use assets	184,019	185,885
Restricted cash and other non-current assets	72,805	79,229
Total assets	1,781,252	1,727,424
Current liabilities:
Accounts payable	21,602	42,995
Accrued expenses and other current liabilities	145,406	141,556
Operating lease liability, current portion	25,024	20,175
Deferred revenue, current portion	2,320	8,474
Collaboration research advancement, current portion	9,236	10,380
Total current liabilities	203,588	223,580
Deferred revenue, net of current portion	25,762	9,791
Collaboration research advancement, net of current portion	21,581	27,834
Operating lease liability, net of current portion	167,997	170,812
Other non-current liabilities	7,268	10,414
Total liabilities	426,196	442,431
Commitments and contingencies Note 9
Stockholders' equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at December 31, 2020 and December 31, 2019	0	0
Common stock, $0.01 par value, 125,000 shares authorized; 66,432 and 55,368 shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	665	554
Additional paid-in capital	4,260,443	3,568,184
Accumulated other comprehensive loss	(5,505)	(1,893)
Accumulated deficit	(2,900,547)	(2,281,852)
Total stockholders' equity	1,355,056	1,284,993
Total liabilities and stockholders' equity	$ 1,781,252	$ 1,727,424

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value per share (in dollars per share)	$ 10.00	$ 10.00
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 10.00	$ 10.00
Common stock, shares authorized (in shares)	125,000,000	125,000,000
Common stock, shares issued (in shares)	66,432,000	55,368,000
Common stock, shares outstanding (in shares)	66,432,000	55,368,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Collaborative arrangement revenue	$ 115,594	$ 5,740	$ 7,820
Total revenues	250,734	44,674	54,579
Operating expenses:
Research and development	587,956	582,413	448,589
Selling, general and administrative	286,896	271,362	174,129
Cost of royalty and other revenue	5,396	2,978	885
Change in fair value of contingent consideration	(6,468)	2,747	2,999
Total operating expenses	873,780	859,500	626,602
Loss from operations	(623,046)	(814,826)	(572,023)
Interest income, net	11,539	34,761	14,624
Other (expense) income, net	(6,502)	(10,088)	1,961
Loss before income taxes	(618,009)	(790,153)	(555,438)
Income tax (expense) benefit	(686)	545	(187)
Net loss	$ (618,695)	$ (789,608)	$ (555,625)
Net loss per share - basic and diluted (in dollars per share)	$ (9.95)	$ (14.31)	$ (10.68)
Weighted-average number of common shares used in computing net loss per share - basic and diluted (in shares)	62,178	55,191	52,032
Other comprehensive (loss) income:
Other comprehensive income	$ (3,612)	$ 1,734	$ 578
Comprehensive loss	(622,307)	(787,874)	(555,047)
Service revenue
Revenue:
Revenue	114,064	30,729	44,533
Royalty and other revenue
Revenue:
Revenue	$ 21,076	$ 8,205	$ 2,226

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Other Comprehensive Income (Loss), Tax	$ 0.0	$ (1.2)	$ (0.4)

- Definition

Amount of tax expense (benefit) allocated to other comprehensive income (loss).

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+ Details

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018
Statement of Financial Position [Abstract]
Costs from public offering	$ 33,645	$ 34,588

Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
Consolidated Statements of Cash Flows $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash flows from operating activities:
Net loss	$ (618,695)	$ (789,608)	$ (555,625)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(6,468)	2,747	2,999
Depreciation and amortization	19,356	17,434	17,158
Stock-based compensation expense	156,631	160,629	110,836
Unrealized loss (gain) on equity securities	7,217	9,297	(2,154)
Other non-cash items	458	(11,000)	(5,880)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(10,089)	(13,913)	(24,288)
Operating lease right-of-use assets	21,281	22,496	0
Accounts payable	(20,100)	23,600	3,614
Accrued expenses and other liabilities	(4,982)	46,291	37,832
Operating lease liabilities	(17,380)	(9,944)	0
Deferred revenue	9,817	(16,674)	(41,872)
Collaboration research advancement	(7,397)	(5,739)	43,954
Net cash used in operating activities	(470,351)	(564,384)	(413,426)
Cash flows from investing activities:
Purchase of property, plant and equipment	(28,986)	(71,028)	(55,737)
Purchases of marketable securities	(1,003,525)	(756,570)	(1,517,982)
Sales of marketable securities	29,878	0	0
Proceeds from maturities of marketable securities	918,288	1,340,629	894,284
Purchase of intangible assets	0	(5,224)	0
Net cash provided by (used in) investing activities	(84,345)	507,807	(679,435)
Cash flows from financing activities:
Reimbursement of assets under financing lease obligation	0	0	3,098
Payments on financing lease obligation	0	0	(1,017)
Proceeds from exercise of stock options and ESPP contributions	5,179	21,187	31,759
Net cash provided by financing activities	546,715	21,187	737,692
Decrease in cash, cash equivalents and restricted cash	(7,981)	(35,390)	(355,169)
Cash, cash equivalents and restricted cash at beginning of year	381,709	417,099	772,268
Cash, cash equivalents and restricted cash at end of year	373,728	381,709	417,099
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	317,705	327,214	402,579
Restricted cash included in receivables and other current assets	1,500	0	364
Restricted cash included in restricted cash and other non-current assets	54,523	54,495	14,156
Supplemental cash flow disclosures:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	2,854	5,286	7,449
Right-of-use assets obtained in exchange for operating lease liabilities	19,414	23,939	0
Cash paid during the period for interest	0	0	15,494
Cash paid during the period for income taxes	361	637	358
Public Offering
Cash flows from financing activities:
Proceeds from issuance of common stock	541,536	0	649,368
Regeneron Collaborative Arrangement
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0	$ 0	$ 54,484

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance (in shares) at Dec. 31, 2017			49,406,000
Beginning balance at Dec. 31, 2017	$ 1,623,432	$ (29,375)	$ 494	$ 2,540,951	$ (4,205)	$ (913,808)	$ (29,375)
Vesting of restricted stock units (in shares)			152,000
Vesting of restricted stock units	0		$ 2	(2)
Issuance of common stock upon public offering, net of issuance costs (in shares)			4,169,000
Issuance of common stock upon public offering, net of issuance costs of $34,588	649,368		$ 42	649,326
Issuance of common stock to Regeneron (in shares)			420,000
Issuance of common stock to Regeneron	54,484		$ 4	54,480
Exercise of stock options (in shares)			575,000
Exercise of stock options	29,768		$ 5	29,763
Purchase of common stock under ESPP (in shares)			16,000
Purchase of common stock under ESPP	1,604			1,604
Stock-based compensation	110,836			110,836
Other comprehensive income	578				578
Net loss	(555,625)					(555,625)
Ending balance (in shares) at Dec. 31, 2018			54,738,000
Ending balance at Dec. 31, 2018	$ 1,885,070	$ 6,564	$ 547	3,386,958	(3,627)	(1,498,808)	$ 6,564
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Vesting of restricted stock units (in shares)			251,000
Vesting of restricted stock units	$ 0		$ 3	(3)
Exercise of stock options (in shares)			354,000
Exercise of stock options	17,838		$ 4	17,834
Purchase of common stock under ESPP (in shares)			25,000
Purchase of common stock under ESPP	2,766			2,766
Stock-based compensation	160,629			160,629
Other comprehensive income	1,734				1,734
Net loss	$ (789,608)					(789,608)
Ending balance (in shares) at Dec. 31, 2019	55,368,000		55,368,000
Ending balance at Dec. 31, 2019	$ 1,284,993		$ 554	3,568,184	(1,893)	(2,281,852)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Vesting of restricted stock units (in shares)			434,000
Vesting of restricted stock units	$ 0		$ 4	(4)
Issuance of common stock upon public offering, net of issuance costs (in shares)			10,455,000
Issuance of common stock upon public offering, net of issuance costs of $34,588	$ 541,536		$ 105	541,431
Exercise of stock options (in shares)	95,000		95,000
Exercise of stock options	$ 1,847		$ 1	1,846
Purchase of common stock under ESPP (in shares)			80,000
Purchase of common stock under ESPP	3,775		$ 1	3,774
Stock-based compensation	145,212			145,212
Other comprehensive income	(3,612)				(3,612)
Net loss	$ (618,695)					(618,695)
Ending balance (in shares) at Dec. 31, 2020	66,432,000		66,432,000
Ending balance at Dec. 31, 2020	$ 1,355,056		$ 665	$ 4,260,443	$ (5,505)	$ (2,900,547)

Description of the business	12 Months Ended
Description of the business	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the business	Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company is a biotechnology company committed to researching, developing and commercializing potentially transformative gene therapies for severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide selling, general and administrative support for these operations, including commercial-readiness activities. The Company’s programs in severe genetic diseases include betibeglogene autotemcel (beti-cel; formerly LentiGlobin for β-thalassemia gene therapy) as a treatment for transfusion-dependent β-thalassemia, or TDT; its LentiGlobin® product candidate as a treatment for sickle cell disease, or SCD; and elivaldogene autotemcel (eli-cel; formerly Lenti-D gene therapy) as a treatment for cerebral adrenoleukodystrophy, or CALD. The Company’s programs in oncology are focused on developing novel T cell-based immunotherapies, including chimeric antigen receptor (CAR) and T cell receptor (TCR) T cell therapies. Idecabtagene vicleucel, or ide-cel, and bb21217 are product candidates in oncology under the Company’s collaboration arrangement with Bristol-Myers Squibb ("BMS"). ide-cel and bb21217 are CAR T cell product candidates for the treatment of multiple myeloma. Please refer to Note 11, Collaborative arrangements, for further discussion of the Company’s collaboration with BMS. In June 2019, the Company received conditional marketing authorization from the European Commission for beti-cel as a treatment of patients 12 years and older with TDT who do not have a β0/β0 genotype, for whom hematopoietic stem cell (HSC) transplantation is appropriate but a human leukocyte-matched related HSC donor is not available. beti-cel is being marketed as ZYNTEGLO™ in the European Union. Since receiving conditional marketing authorization for ZYNTEGLO, the Company has continued to advance its commercial readiness activities. Through December 31, 2020, the Company had not generated any revenue from product sales. In February 2021 the Company temporarily suspended marketing of ZYNTEGLO in light of safety events in the HGB-206 study of LentiGlobin gene therapy for SCD which is manufactured using the same vector as ZYNTEGLO. Additionally, the European Medicines Agency, or EMA, has paused the renewal procedure for ZYNTEGLO's conditional marketing authorization while the EMA's pharmacovigilance risk assessment committee reviews the risk-benefit assessment for ZYNTEGLO and determines whether any additional pharmacovigilance measures are necessary. In January 2021, the Company announced its intent to separate its severe genetic disease and oncology programs into two separate, independent publicly traded companies, bluebird bio, Inc. and a new company, referred to as Oncology NewCo in these consolidated financial statements. bluebird bio, Inc. intends to retain focus on its severe genetic disease programs and Oncology NewCo is expected to focus on the Company's oncology programs. The transaction is expected to be completed in late 2021 and is anticipated to be tax-free, subject to receipt of a favorable Internal Revenue Service (“IRS”) ruling. In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern, the Company evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company has incurred losses since inception and to date has financed its operations primarily through the sale of equity securities and, to a lesser extent, through collaboration agreements and grants from governmental agencies and charitable foundations. As of December 31, 2020, the Company had an accumulated deficit of $2.90 billion. During the year ended December 31, 2020, the Company incurred a loss of $618.7 million and used $470.4 million of cash in operations. The Company expects to continue to generate operating losses and negative operating cash flows for the next few years and will need additional funding to support its planned operating activities through profitability. The transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. As of December 31, 2020, the Company had cash, cash equivalents and marketable securities of $1.27 billion. The Company expects its cash, cash equivalents and marketable securities will be sufficient to fund current planned operations for at least the next twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing additional collaborations with other companies. Management's expectations with respect to its ability to fund current planned operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management's estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any of these strategic or financing opportunities will be executed or executed on favorable terms, and some could be dilutive to existing stockholders. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product and product candidates.

Summary of significant accounting policies and basis of presentation

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Summary of significant accounting policies and basis of presentation

Basis of presentation

The accompanying consolidated financial statements have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the ASC and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States GAAP as found in the ASC and ASUs of the FASB.

Certain items in the prior year’s consolidated financial statements have been reclassified to conform to the current presentation. No subtotals in the prior year consolidated financial statements were impacted by these reclassifications.

Amounts reported are computed based on thousands, except percentages, per share amounts or as otherwise noted. As a result, certain totals may not sum due to rounding.

Principles of consolidation

The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

The Company continually assesses whether it is the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in consolidation or deconsolidation of one or more collaborators or partners. In determining whether it is the primary beneficiary of an entity in which the Company has a variable interest, management applies a qualitative approach that determines whether the Company has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity.

Use of estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements.

Estimates and judgments are used in the following areas, among others: future undiscounted cash flows and subsequent fair value estimates used to assess potential and measure any impairment of long-lived assets, including goodwill and intangible assets, and the measurement of right-of-use assets and lease liabilities, contingent consideration, stock-based compensation expense, accrued expenses, income taxes, and the assessment of the Company's ability to fund its operations for at least the next twelve months from the date of issuance of these financial statements. In addition, estimates and judgments are used in the Company’s accounting for its revenue-generating arrangements, in particular as it relates to determining the standalone selling price of performance obligations, evaluating whether an option to acquire additional goods and services represents a material right, estimating the total transaction price, including estimating variable consideration and the probability of achieving future potential development and regulatory milestones, assessing the period of performance over which revenue may be recognized, and accounting for modifications to revenue-generating arrangements.

Foreign currency translation

The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss) in stockholders’ equity. Foreign currency transaction gains and losses are included in other income (expense), net in the results of operations.

Segment information

The Company operates in a single segment, focusing on researching, developing and commercializing potentially transformative gene therapies for severe genetic diseases and cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources at a global, consolidated level. Therefore, results of the Company's operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States.

Cash and cash equivalents

The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents comprise marketable securities with maturities of less than 90 days when purchased. Cash equivalents are reported at fair value.

Marketable securities

The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current assets. The Company’s marketable securities are maintained by investment managers and consist of U.S. government agency securities and treasuries, equity securities, corporate bonds, and commercial paper. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium arising at purchase is amortized to the earliest call date and any discount arising at purchase is accreted to maturity. Amortization and accretion of premiums and discounts are recorded in interest income, net. Equity securities with readily determinable fair values are also carried at fair value with unrealized gains and losses included in other (expense) income, net. Realized gains and losses on both debt and equity securities are determined using the specific identification method and are included in other (expense) income, net.

The Company classifies equity securities with readily determinable fair values, which would be available for use in its current operations, as current assets even though the Company may not dispose of such marketable securities within the next 12 months. Equity securities are included in the balance of marketable securities on the Company's consolidated balance sheet.

Effective January 1, 2020, the Company adopted ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements (“ASU 2016-13” or “ASC 326”), using the effective date method. As the Company had never recorded any other-than-temporary-impairment adjustments to its available-for-sale debt securities prior to the effective date, no transition provisions are applicable to the Company.

The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASC 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the consolidated statements of operations and comprehensive loss as credit loss expense within other (expense) income, net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities.

Accrued interest receivable related to the Company's available-for-sale debt securities is presented within receivables and other current assets on the Company's consolidated balance sheets. The Company has elected the practical expedient available to exclude accrued interest receivable from both the fair value and the amortized cost basis of available-for-sale debt securities for the purposes of identifying and measuring any impairment. The Company writes off accrued interest receivable once it has determined that the asset is not realizable. Any write offs of accrued interest receivable are recorded by reversing interest income, recognizing credit loss expense, or a combination of both. To date, the Company has not written off any accrued interest receivables associated with its marketable securities.

Concentrations of credit risk and off-balance sheet risk

Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and available-for-sale securities. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s marketable securities primarily consist of U.S. Treasury securities, U.S. government agency securities, certificates of deposit, corporate bonds, and commercial paper, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure.

Fair value of financial instruments

The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements:

Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2—Fair values are determined utilizing quoted prices for identical or similar assets or liabilities in active markets or other market observable inputs such as interest rates, yield curves and foreign currency spot rates.

Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Items measured at fair value on a recurring basis include marketable securities (see Note 3, Marketable securities, and Note 4, Fair value measurements) and contingent consideration (see Note 4, Fair value measurements). The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term nature.

Business combinations

Business combinations are accounted for using the acquisition method of accounting. Using this method, the tangible and intangible assets acquired and the liabilities assumed are recorded as of the acquisition date at their respective fair values. The Company evaluates a business as an integrated set of activities and assets that is capable of being managed for the purpose of providing a return in the form of dividends, lower costs or other economic benefits and consists of inputs and processes that provide or have the ability to provide outputs. In an acquisition of a business, the excess of the fair value of the consideration transferred over the fair value of the net assets acquired is recorded as goodwill. In an acquisition of net assets that does not constitute a business, no goodwill is recognized.

The consolidated financial statements include the results of operations of an acquired business after the completion of the acquisition. See Note 4, Fair value measurements, for additional information.

Goodwill

Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. During the fourth quarter of 2019, the Company early adopted ASC 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"), which removes the second step of the goodwill impairment test. Under this ASU, the Company performs a one-step quantitative test and records the amount of goodwill impairment, if any, as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The Company has not recognized any impairment charges related to goodwill to date.

Intangible assets, net

Intangible assets, net consist of acquired core technology and in-licensed rights with finite lives, net of accumulated amortization. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. The Company has not recognized any impairment charges related to intangible assets to date.

Contingent consideration

Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and records within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones may be achieved, and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, Fair value measurements, for additional information.

Property, plant and equipment

Property, plant and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:


Asset						Estimated useful life
Building						40 years
Computer equipment and software						3 years
Furniture and fixtures						2-5 years
Laboratory equipment						2-5 years
Leasehold improvements						Shorter of the useful life or remaining lease term

Prior to the adoption of ASU 2016-02, Leases (Topic 842) (“ASU 2016-02” or “ASC 842”), on January 1, 2019 (discussed further below), the Company recorded certain costs incurred and reported by a landlord as a building asset and corresponding financing lease obligation on the consolidated balance sheets. See Note 8, Leases, for additional information.

Impairment of long-lived assets

The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets.

Leases

Effective January 1, 2019, the Company adopted ASC 842 using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, amounts for the year ended December 31, 2018 are presented in accordance with the previous guidance in ASC 840, Leases (“ASC 840”).

At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and current and non-current lease liabilities, as applicable. The Company does not have material financing leases.

Operating lease liabilities and their corresponding right-of-use assets are initially recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items

such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. To estimate its incremental borrowing rate, a credit rating applicable to the Company is estimated using a synthetic credit rating analysis since the Company does not currently have a rating agency-based credit rating. Prospectively, the Company will adjust the right-of-use assets for straight-line rent expense or any incentives received and remeasure the lease liability at the net present value using the same incremental borrowing rate that was in effect as of the lease commencement or transition date.

The Company has elected not to recognize leases with an original term of one year or less on the balance sheet. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless there is reasonable certainty that the Company will renew.

Assumptions made by the Company at the commencement date are re-evaluated upon occurrence of certain events, including a lease modification. A lease modification results in a separate contract when the modification grants the lessee an additional right of use not included in the original lease and when lease payments increase commensurate with the standalone price for the additional right of use. When a lease modification results in a separate contract, it is accounted for in the same manner as a new lease.

ASC 842 transition practical expedients and application of transition provisions to leases at the transition date

The Company elected the following practical expedients, which must be elected as a package and applied consistently to all of its leases at the transition date (including those for which the entity is a lessee or a lessor): (i) the Company did not reassess whether any expired or existing contracts are or contain leases; (ii) the Company did not reassess the lease classification for any expired or existing leases (that is, all existing leases that were classified as operating leases in accordance with ASC 840 are classified as operating leases, and all existing leases that were classified as capital leases in accordance with ASC 840 are classified as finance leases); and (iii) the Company did not reassess initial direct costs for any existing leases.

For leases that existed prior to the date of initial application of ASC 842 (which were previously classified as operating leases), a lessee may elect to use either the total lease term measured at lease inception under ASC 840 or the remaining lease term as of the date of initial application of ASC 842 in determining the period for which to measure its incremental borrowing rate. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates.

Application of ASC 842 policy elections to leases post adoption

The Company has made certain policy elections to apply to its leases executed post adoption, or subsequent to January 1, 2019, as further described below.

In accordance with ASC 842, components of a lease should be split into three categories: lease components, non-lease components, and non-components. The fixed and in-substance fixed contract consideration (including any consideration related to non-components) must be allocated based on the respective relative fair values to the lease components and non-lease components.

Entities may elect not to separate lease and non-lease components. Rather, entities would account for each lease component and related non-lease component together as a single lease component. The Company has elected to account for lease and non-lease components together as a single lease component for all underlying assets and allocate all of the contract consideration to the lease component only.

ASC 842 allows for the use of judgment in determining whether the assumed lease term is for a major part of the remaining economic life of the underlying asset and whether the present value of lease payments represents substantially all of the fair value of the underlying asset. The Company applies the bright line thresholds referenced in ASC 842-10-55-2 to assist in evaluating leases for appropriate classification. The aforementioned bright lines are applied consistently to the Company’s entire portfolio of leases.

Revenue recognition

Under ASC Topic 606, Revenue from Contracts with Customers (“Topic 606”), an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer.

Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The identification of material rights requires judgments related to the determination of the value of the underlying good or service relative to the option exercise price. The exercise of a material right is accounted for as a contract modification for accounting purposes.

The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct.

The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations.

If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment.

If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received.

For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company

recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied.

In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements.

The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method.

The Company recognizes revenue within the following financial statement captions:

Service revenue

To date, the Company’s service revenue has primarily been generated from the elements of its collaboration arrangement with BMS that are accounted for pursuant to Topic 606, using the five-step model described above. As discussed further below, the Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements (“ASC 808”) or Topic 606. For the elements of a collaboration arrangement which are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606, the Company records the related revenue as service revenue on the consolidated statement of operations and comprehensive loss. Refer below for additional discussion around the Company’s policy for recognizing collaborative arrangement revenue and the determination of whether elements of a collaboration arrangement are within the scope of ASC 808 or Topic 606.

Collaborative arrangement revenue

To date, the Company’s collaborative arrangement revenue has been generated from its collaboration arrangements with BMS and Regeneron Pharmaceuticals, Inc. (“Regeneron”), as further described in Note 11, Collaborative arrangements.

The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, which includes determining whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606 (refer above for further discussion of the Company's policy for recognizing service revenue). For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. Amounts that are owed to collaboration partners are recognized as an offset to collaborative arrangement revenues as such amounts are incurred by the collaboration partner. Where amounts owed to a collaboration partner exceed the Company’s collaborative arrangement revenues in each quarterly period, such amounts are classified as research and development expense.

Prior to the adoption of ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”) on January 1, 2020, the Company presented all revenue recognized under its collaborative arrangements as collaboration revenue on its consolidated statements of operations and comprehensive loss. However, as the Company recognizes revenue under its collaborative arrangements both within and outside the scope of Topic 606, the Company has revised its presentation of revenue on its consolidated statements of operations and comprehensive loss as follows: service revenue includes revenue from collaborative partners recognized within the scope of Topic 606 and collaborative arrangement revenue includes revenue from collaborative partners recognized outside the scope of Topic 606. The disaggregation of revenue recognized under Topic 606 and outside of Topic 606 had previously otherwise been disclosed in the notes to consolidated financial statements.

Royalty and other revenue

The Company enters into out-licensing agreements that are within the scope of Topic 606. The Company does not have any material license arrangements that contain more than one performance obligation. The terms of such out-license agreements include the license of functional intellectual property, given the functionality of the intellectual property is not expected to change substantially as a result of the licensor’s ongoing activities, and typically include payment of one or more of the following: non-refundable up-front license fees; development and regulatory milestone payments and milestone payments based on the level of sales; and royalties on net sales of licensed products. Nonrefundable up-front license fees are recognized as revenue at a point in time when the licensed intellectual property is made available for the customer’s use and benefit, which is generally at the inception of the arrangement. Development and regulatory milestone fees, which are a type of variable consideration, are recognized as revenue to the extent that it is probable that a significant reversal will not occur. The Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied.

For a complete discussion of accounting for collaboration and other revenue-generating arrangements, see Note 11, Collaborative arrangements, and Note 12, Royalty and other revenue.

Research and development expenses

Research and development costs are charged to expense as costs are incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, clinical study and related clinical manufacturing costs, license and milestone fees, contract services, manufacturing costs for pre-launch inventory that did not qualify for capitalization, and other related costs. Up-front fees and milestones paid to third parties in connection with technologies which have not reached technological feasibility and do not have an alternative future use are expensed as research and development expense as incurred. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense. The Company accrues costs for clinical trial activities based upon estimates of the services received and related expenses incurred that have yet to be invoiced by the contract research organizations, clinical study sites, laboratories, consultants, or other clinical trial vendors that perform the activities. Where amounts owed to a collaboration partner exceed the Company’s collaborative arrangement revenues in each quarterly period, such amounts are classified as research and development expense.

Cost of royalty and other revenue

Cost of royalty and other revenue represents expense associated with amounts owed to third parties as a result of revenue recognized under the Company’s out-license arrangements.

Stock-based compensation

The Company’s share-based compensation programs grant awards that have included stock options, restricted stock units, restricted stock awards, and shares issued under its employee stock purchase plan. Grants are awarded to employees and non-employees, including the Company's board of directors.

The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock-based payments, including grants of stock options and restricted stock units and modifications to existing stock options, to be recognized in the consolidated statements of operations and comprehensive loss based on their fair values.

The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable.

The Company estimates the fair value of its option awards using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Effective January 1, 2020, the Company eliminated the use of a representative peer group and uses only its own historical volatility data in its estimate of expected volatility given that there is now a sufficient amount of historical information regarding the volatility of its own stock price. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the

arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future.

The Company accounts for forfeitures as they occur. Stock-based compensation expense recognized in the financial statements is based on awards for which performance or service conditions are expected to be satisfied.

Prior to the adoption of ASU No. 2018-07 on July 1, 2018, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), the measurement date for non-employee awards was generally the date the services are completed, resulting in financial reporting period adjustments to stock-based compensation during the vesting terms for changes in the fair value of the awards. After adoption of ASU 2018-07 on July 1, 2018, the measurement date for non-employee awards is the date of grant without changes in the fair value of the award.

Interest income, net

Interest income, net consists primarily of interest income earned on investments, net of amortization of premium and accretion of discount. Interest income was approximately $11.5 million, $34.8 million, and $30.1 million for the years ended December 31, 2020, 2019, and 2018, respectively. Interest expense was $0.0 million, $0.0 million, and $15.5 million for the years ended December 31, 2020, 2019, and 2018, respectively. Please refer to Note 8, Leases, for further discussion of interest expense incurred on the 60 Binney Street lease.

Other (expense) income, net

Other (expense) income, net consists primarily of gains and losses on equity securities held by the Company, gains and losses on disposal of assets, and gains and losses on foreign currency.

Net loss per share

Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation.

The Company follows the two-class method when computing net loss per share in periods when issued shares that meet the definition of participating securities are outstanding. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to received dividends as if all income for the period had been distributed. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders when participating securities are outstanding, losses are not allocated to the participating securities.

Income taxes

Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits

of the tax position as well as consideration of the available facts and circumstances. The Company accrues for potential interest and penalties related to unrecognized tax benefits in income tax expense.

Comprehensive loss

Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on debt securities, foreign currency translation adjustments and other items.

Recent accounting pronouncements

ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements, ASU No. 2019-5 Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief, ASU No. 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses

In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements. The new standard, as amended, requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The targeted transition relief standard allows filers an option to irrevocably elect the fair value option of ASC 825-10, Financial Instruments-Overall, applied on an instrument-by-instrument basis for eligible instruments. The Company adopted this standard on January 1, 2020 on a prospective basis and the adoption did not have a material impact on its financial position and results of operations.

ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. The new standard removes certain disclosures, modifies certain disclosures, and adds additional disclosures related to fair value measurement. The Company adopted this standard on January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption.

ASU No. 2018-15, Intangibles-Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract

In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The Company adopted this standard on a prospective basis as of January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption.

ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606

In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, (“ASU 2018-18”). The amendments in this update clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers (“Topic 606” or “ASC 606”) when the counter party is a customer in the context of a unit of account. ASU 2018-18 also precludes companies from presenting transactions with collaborative partners that are outside the scope of Topic 606 together with revenue within the scope of Topic 606. The Company adopted this standard on a retrospective basis on January 1, 2020. As a result, revenue for prior periods are presented in accordance with the new standard.

Prior to the adoption of ASU 2018-18, the Company presented all revenue recognized under its collaborative arrangements as collaboration revenue on its consolidated statements of operations and comprehensive loss. However, as the Company recognizes revenue under its collaborative arrangements both within and outside the scope of Topic 606, the Company has revised its presentation of revenue on its consolidated statements of operations and comprehensive loss as follows: service revenue includes revenue from collaborative partners recognized within the scope of Topic 606 and collaborative arrangement revenue includes revenue from collaborative partners recognized outside the scope of Topic 606. The disaggregation of revenue

recognized under Topic 606 and outside of Topic 606 had previously otherwise been disclosed in the notes to consolidated financial statements.

ASU No. 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments

In April 2019, the FASB issued ASU 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. This update provides clarifications for three topics related to financial instruments accounting, some of which apply to the Company. The Company adopted this standard on January 1, 2020 on a prospective basis, and it did not have a material impact on its financial position and results of operations upon adoption.

Not yet adopted

ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (“ASU 2019-12”), which is intended to simplify the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. The new standard will be effective beginning January 1, 2021. The adoption of ASU 2019-12 is not expected to have a material impact on the Company's financial position or results of operations upon adoption.

ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity

In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU 2020-06”). ASU 2020-06 simplifies the complexity associated with applying U.S. GAAP for certain financial instruments with characteristics of liabilities and equity. More specifically, the amendments focus on the guidance for convertible instruments and derivative scope exception for contracts in an entity's own equity. The Company will early adopt the new standard, effective January 1, 2021. The adoption of ASU 2020-06 is not expected to have an impact on the Company's financial position or results of operations upon adoption.

ASU No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs

In October 2020, the FASB issued ASU 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs (“ASU 2020-08”) to provide further clarification and update the previously issued guidance in ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20: Premium Amortization on Purchased Callable Debt Securities) (“ASU 2017-08”). ASU 2017-08 shortened the amortization period for certain callable debt securities purchased at a premium by requiring that the premium be amortized to the earliest call date. ASU 2020-08 requires that at each reporting period, to the extent that the amortized cost of an individual callable debt security exceeds the amount repayable by the issuer at the next call date, the excess premium shall be amortized to the next call date. The new standard will be effective beginning January 1, 2021. The adoption of ASU 2020-08 is not expected to have a material impact on the Company's financial position or results of operations upon adoption.

ASU No. 2020-10, Codification Improvements

In October 2020, the FASB issued ASU 2020-10, Codification Improvements ("ASU 2020-10"). The amendments in this ASU represent changes to clarify the ASC, correct unintended application of the guidance, or make minor improvements to the ASC that are not expected to have a significant effect on current accounting practice or create a significant administrative cost to most entities. This new standard will be effective beginning January 1, 2021. The adoption of ASU 2020-10 is not expected to have a material impact on the Company's financial position or results of operations upon adoption.

Marketable securities

12 Months Ended

Dec. 31, 2020

Investments, Debt and Equity Securities [Abstract]

Marketable securities

Marketable securitiesThe following table summarizes the marketable securities held at December 31, 2020 and 2019 (in thousands):


	Amortized cost / cost		Unrealized gains		Unrealized losses		Fair value
December 31, 2020
U.S. government agency securities and treasuries	$	675,043	$	302	$	(74)	$	675,271
Corporate bonds	197,171		432		(40)		197,563
Commercial paper	77,949		1		—		77,950
Equity securities	20,017		—		(14,364)		5,653
Total	$	970,180	$	735	$	(14,478)	$	956,437
December 31, 2019
U.S. government agency securities and treasuries	$	633,970	$	2,014	$	(48)	$	635,936
Certificates of deposit	960		—		—		960
Corporate bonds	185,827		824		(43)		186,608
Commercial paper	74,378		—		—		74,378
Equity securities	20,017		—		(7,147)		12,870
Total	$	915,152	$	2,838	$	(7,238)	$	910,752

No available-for-sale debt securities held as of December 31, 2020 or 2019 had remaining maturities greater than five years.

Fair value measurements

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair value measurements

The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2020 and 2019 (in thousands):


	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
December 31, 2020
Assets:
Cash and cash equivalents	$	317,705	$	317,705	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries	675,271		—		675,271		—
Corporate bonds	197,563		—		197,563		—
Commercial paper	77,950		—		77,950		—
Equity securities	5,653		5,653		—		—
Total assets	$	1,274,142	$	323,358	$	950,784	$	—
Liabilities:
Contingent consideration	$	1,509	$	—	$	—	$	1,509
Total liabilities	$	1,509	$	—	$	—	$	1,509
December 31, 2019
Assets:
Cash and cash equivalents	$	327,214	$	311,245	$	15,969	$	—
Marketable securities:
U.S. government agency securities and treasuries	635,936		—		635,936		—
Certificates of deposit	960		—		960		—
Corporate bonds	186,608		—		186,608		—
Commercial paper	74,378		—		74,378
Equity securities	12,870		12,870		—		—
Total assets	$	1,237,966	$	324,115	$	913,851	$	—
Liabilities:
Contingent consideration	$	7,977	$	—	$	—	$	7,977
Total liabilities	$	7,977	$	—	$	—	$	7,977

Cash and cash equivalents

The Company considers all highly liquid securities with original final maturities of 90 days or less from the date of purchase to be cash equivalents. As of December 31, 2020, cash and cash equivalents comprise funds in cash and money market accounts. As of December 31, 2019, cash and cash equivalents comprise funds in cash, money market accounts, and commercial paper.

Marketable securities

Marketable securities classified as Level 2 within the valuation hierarchy generally consist of certificates of deposit, U.S. government agency securities and treasuries, corporate bonds, and commercial paper. The Company estimates the fair values of these marketable securities by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for the same or similar securities, issuer credit spreads, benchmark yields, and other observable inputs. The Company validates the prices provided by its third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances.

The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. At December 31, 2020 and 2019, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale debt securities. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the year ended December 31, 2020 or 2019.

Accrued interest receivable on the Company's available-for-sale debt securities totaled $3.1 million and $3.6 million as of December 31, 2020 and 2019, respectively. No accrued interest receivable was written off during the twelve months ended December 31, 2020 or 2019.

The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2020 and 2019 (in thousands):


	Less than 12 months				12 months or greater				Total
Description	Fair value		Unrealized losses		Fair value		Unrealized losses		Fair value		Unrealized losses
December 31, 2020
U.S. government agency securities and treasuries	$	211,384	$	(74)	$	—	$	—	$	211,384	$	(74)
Corporate bonds	76,598		(40)		1,205		—		77,803		(40)
Total	$	287,982	$	(114)	$	1,205	$	—	$	289,187	$	(114)
December 31, 2019
U.S. government agency securities and treasuries	$	13,234	$	(3)	$	79,618	$	(45)	$	92,852	$	(48)
Corporate bonds	53,983		(43)		—		—		53,983		(43)
Total	$	67,217	$	(46)	$	79,618	$	(45)	$	146,835	$	(91)

The Company determined that there was no material change in the credit risk of the above investments during the twelve months ended December 31, 2020. As such, an allowance for credit losses was not recognized. As of December 31, 2020, the Company does not intend to sell such securities and it is not more likely than not that the Company will be required to sell the securities before recovery of their amortized cost bases.

The Company holds equity securities with an aggregate fair value of $5.7 million and $12.9 million at December 31, 2020 and 2019, respectively within current marketable securities on its consolidated balance sheet. The Company recorded unrealized losses of $7.2 million and $9.3 million and an unrealized gain of $2.2 million during the years ended December 31, 2020, 2019, and 2018 respectively, related to its equity securities, which are included in other (expense) income, net on the consolidated statements of operations and comprehensive loss. In January 2021, the Company sold a portion of its equity securities for proceeds of $31.3 million. The fair value of the remaining equity securities held as of the trade date is $7.3 million.

Contingent consideration

In connection with its prior acquisition of Precision Genome Engineering, Inc. (“Pregenen”), the Company may be required to pay future consideration that is contingent upon the achievement of specified development, regulatory approvals or sales-based milestone events. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the consolidated statements of operations and comprehensive loss. In the absence of new information related to the probability of milestone achievement, changes in fair value will reflect changing discount rates and the passage of time. Contingent consideration is included in accrued expenses and other current liabilities and other non-current liabilities on the consolidated balance sheets.

Please refer to Note 9, Commitments and contingencies, for further information.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property, plant and equipment, net

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Property, plant and equipment, net

Property, plant and equipment, net, consists of the following (in thousands):


	As of December 31,
	2020		2019
Land	$	1,210	$	1,210
Building	15,745		15,664
Computer equipment and software	6,950		6,947
Office equipment	7,665		7,599
Laboratory equipment	55,521		44,560
Leasehold improvements	34,286		33,788
Construction-in-progress	92,514		77,981
Total property, plant and equipment	213,891		187,749
Less accumulated depreciation and amortization	(51,060)		(36,573)
Property, plant and equipment, net	$	162,831	$	151,176

Depreciation and amortization expense related to property, plant and equipment was $15.1 million, $13.4 million, and $13.4 million for the years ended December 31, 2020, 2019, and 2018, respectively.

North Carolina manufacturing facility

In November 2017, the Company acquired a manufacturing facility in Durham, North Carolina for the future manufacture of lentiviral vector for the Company’s gene therapies. As of December 31, 2020, a portion of the facility has been placed into service and the remainder of the facility is still in process of construction and qualification, which is required for the facility to be ready for its intended use. Construction-in-progress as of December 31, 2020, and 2019, includes $91.1 million and $74.2 million, respectively, related to the North Carolina manufacturing facility.

Restricted cash

12 Months Ended

Dec. 31, 2020

Other Assets, Noncurrent [Abstract]

Restricted cash

As of both December 31, 2020 and 2019, the Company maintained letters of credit of $54.5 million, which are collateralized with bank accounts at financial institutions in accordance with the agreements. Total restricted cash as of December 31, 2020 and 2019 consisted of the following (in thousands):


	As of December 31,
	2020		2019
60 Binney Street lease	$	13,763	$	13,763
50 Binney Street sublease	40,072		40,072
Other	2,188		660
Total restricted cash	$	56,023	$	54,495

Refer to Note 8, Leases, for further information on the Company's letters of credit.

Accrued expenses and other current liabilities

12 Months Ended

Dec. 31, 2020

Payables and Accruals [Abstract]

Accrued expenses and other current liabilities

Accrued expenses and other current liabilitiesAccrued expenses and other current liabilities consist of the following (in thousands):


	As of December 31,
	2020		2019
Employee compensation	$	55,802	$	44,679
Manufacturing costs	22,571		23,126
Clinical and contract research organization costs	23,766		16,799
Collaboration research costs	20,004		27,142
Property, plant, and equipment	789		2,354
License and milestone fees	278		300
Professional fees	1,541		1,827
Other	20,655		25,329
Total accrued expenses and other current liabilities	$	145,406	$	141,556

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

The Company leases certain office and laboratory space. Additionally, the Company has embedded leases at contract manufacturing organizations.

60 Binney Street lease

In September, 2015, the Company entered into a lease agreement for office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”), which is now the Company’s corporate headquarters. Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. The Company currently maintains a $13.8 million collateralized letter of credit and, subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease to $9.2 million over time. The $13.8 million of cash collateralizing the letter of credit is classified as restricted cash and other non-current assets on the Company's consolidated balance sheets. Pursuant to a work letter entered into in connection with the 60 Binney Street Lease, the landlord contributed an aggregate of $42.4 million toward the cost of construction and tenant improvements for the Building.

The Company occupied the Building beginning on March 27, 2017 and the 60 Binney Street Lease will continue until March 31, 2027. The Company has the option to extend the 60 Binney Street Lease for two successive five-year terms. In applying the ASC 842 transition guidance, the Company classified this lease as an operating lease and recorded a right-of-use asset and lease liability on the effective date. The Company is recognizing rent expense on a straight-line basis throughout the remaining term of the lease.

50 Binney Street sublease

In April 2019, the Company entered into a sublease agreement for office space located at 50 Binney Street in Cambridge, Massachusetts (the “50 Binney Street Sublease”) to supplement the Company’s corporate headquarters located at 60 Binney Street in Cambridge, Massachusetts. Under the terms of the 50 Binney Street Sublease, the Company will lease 267,278 square feet of office space for $99.95 per square foot, or $26.7 million per year in base rent subject to certain operating expenses, taxes and annual rent increases of approximately 3%. The lease will commence when the space is available for use by the Company, which is anticipated to be in the first half of 2022, which reflects the sublessor's exercise of its option to postpone the commencement date of the sublease, and end on December 31, 2030, unless other conditions specified in the 50 Binney Street Sublease occur. Upon signing the 50 Binney Street Sublease, the Company executed a $40.1 million cash-collateralized letter of credit, which may be reduced in the future subject to the terms of the 50 Binney Street Sublease and certain reduction requirements specified therein. The $40.1 million of cash collateralizing the letter of credit is classified as restricted cash and other non-current assets on the Company’s consolidated balance sheets. Payments will commence at the earlier of (i) the date which is 90 days following the commencement date and (ii) the date the Company takes occupancy of all or any portion of the premises. In connection with the execution of the 50 Binney Street Sublease, the Company also entered into a Purchase Agreement for furniture and equipment (the “Furniture Purchase Agreement”) located on the premises upon lease commencement. Upon execution of the Furniture Purchase Agreement, the Company made an upfront payment of $7.5 million, all of which was recorded within restricted cash and other non-current assets on the Company’s consolidated balance sheets as

of December 31, 2020. The Company will assess the lease classification of the 50 Binney Street Sublease and commence recognition of the associated rent expense upon lease commencement.

Seattle, Washington leases

In July 2018, the Company entered into a lease agreement for office and laboratory space located in a portion of a building in Seattle, Washington. The lease was amended in October 2018 to increase the total rentable space to approximately 36,126 square feet at $54.00 per square foot in base rent per year, which is subject to scheduled annual rent increases of 2.5% plus certain operating expenses and taxes. The lease commenced on January 1, 2019 and the lease term will continue through January 31, 2027 ("the Initial Term"). The Company moved into the facility in June 2019. The Company determined the classification of this lease to be an operating lease and recorded a right-of-use asset and lease liability at lease commencement.

In September 2019, the Company entered into a second amendment to the lease (the “Second Amendment”). The Second Amendment added approximately 22,188 square feet to the existing space and extended the lease term of the entire premises by 16 months, or until April 2028. Fixed monthly rent for the expanded space will be incurred at a rate of $62.80 per square foot per year beginning in January 2021, subject to annual increases of 2.5%. The Second Amendment includes a five-year option to extend the term.

Upon the execution of the Second Amendment, which was deemed to be a lease modification, the Company re-evaluated the assumptions made at the original lease commencement date. The Company determined the Second Amendment consists of two separate contracts under ASC 842. One contract is related to a new right-of-use for the expanded 22,188 square feet of space, which is to be accounted for as a new lease, and the other is related to the modification of term for the original 36,126 square feet of space. The Company recorded an additional right-of-use asset and lease liability upon lease commencement of the expanded space. In September 2020, the Company entered into a sublease agreement for the 22,188 square feet added under the Second Amendment at a fixed monthly rent of $62.80 per square foot per year beginning in January 2021, subject to annual increases of 2.5%. The sublease term will continue through April 2028. The Company is recognizing rent expense on a straight-line basis through the remaining extended term of the respective leases. The head lease and the sublease will be accounted for as two separate contracts with the income from the sublease presented separately from the lease expense on the head lease.

Embedded leases

In June 2016, the Company entered into a manufacturing agreement for the future commercial production of the Company’s beti-cel and eli-cel drug products with a contract manufacturing organization. Under this 12-year agreement, the contract manufacturing organization will complete the design, construction, validation and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company paid $12.0 million upon the achievement of certain contractual milestones. Construction was completed in March 2018 and beginning in April 2018, the Company pays $5.1 million per year, subject to annual inflationary adjustments, in fixed suite fees, as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services. The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company concluded in prior periods that this agreement contained an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company recorded a right-of-use asset and lease liability for this operating lease on the effective date of ASC 842 and is recognizing rent expense on a straight-line basis throughout the remaining term of the embedded lease.

In November 2016, the Company entered into an agreement for clinical and commercial production of the Company’s ZYNTEGLO, LentiGlobin for SCD, and eli-cel drug products with a contract manufacturing organization at an existing facility. The Company concluded that this agreement contains an embedded operating lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement. The term of the agreement is five years with subsequent three-year renewals at the mutual option of each party. As a result, the Company recorded a right-of-use asset and lease liability for this operating lease on the effective date of ASC 842, and is recognizing rent expense on a straight-line basis throughout the estimated remaining term of the embedded lease. In March 2020, the Company amended its agreement with the contract manufacturing organization, resulting in a lease modification. Under the terms of the amended arrangement, the Company may be required to pay annual maintenance and production fees of up to €16.5 million, depending on its production needs, and may terminate this agreement with twelve months’ notice and a one-time termination fee. The amendment also provides for an option to reserve an additional clean room for a one-time option fee plus annual maintenance fees. As a result, the Company increased the right-of-use asset and lease liability related to this embedded operating lease during the first quarter of 2020.

In July 2020, the Company entered into an agreement reserving manufacturing capacity with a contract manufacturing organization. The Company concluded that this agreement contains an embedded lease as a controlled environment room at the

facility is designated for the Company's exclusive use during the term of the agreement, with the option to sublease the space if the Company provides notice that it will not utilize it for a specified duration of time. Under the terms of the agreement, the Company will be required to pay up to $5.4 million per year in maintenance fees in addition to the cost of any services provided and may terminate this agreement with eighteen months' notice. The term of the agreement is five years, with the option to extend, and is expected to commence in the first half of 2021. The Company intends to assess the lease classification of the embedded lease and commence recognition of the associated rent expense upon lease commencement.

Summary of all lease costs recognized under ASC 842

The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the years ended December 31, 2020 and 2019 (in thousands):


	For the year ended December 31,
	2020			2019
Lease cost (1)
Operating lease cost	$	35,049		$	35,346
Total lease cost	$	35,049		$	35,346

Other information
Operating cash flows used for operating leases	$	32,097		$	31,026
Weighted average remaining lease term	6.4 years			7.4 years
Weighted average discount rate	6.35		%	6.70		%

(1) Short-term lease costs and variable lease costs incurred by the Company for the twelve months ended December 31, 2020 and 2019 were immaterial.

Rent expense is calculated on a straight-line basis over the term of the lease. Rent expense recognized under all leases, including additional charges for utilities, parking, maintenance, and real estate taxes, was $47.7 million, $45.3 million, and $9.8 million for the years ended December 31, 2020, 2019 and 2018, respectively. Note that the Company adopted ASC 842 effective January 1, 2019 using the required modified retrospective approach and utilizing the effective date as its date of initial application. Therefore, amounts disclosed pertaining to the year ended December 31, 2018 are presented under previous accounting guidance and are therefore not comparable to the amounts recorded during the years ended December 31, 2020 and 2019 under ASC 842.

As of December 31, 2020, future minimum commitments under ASC 842 under the Company’s operating leases were as follows (in thousands):


	As of
Maturity of lease liabilities	December 31, 2020
2021	$	36,430
2022	36,899
2023	37,387
2024	37,398
2025	32,082
2026 and thereafter	56,095
Total lease payments	236,291
Less: imputed interest	(43,270)
Total operating lease liabilities	$	193,021

The above table excludes legally binding minimum lease payments for leases executed but not yet commenced as of December 31, 2020.

Commitments and contingencies

12 Months Ended

Dec. 31, 2020

Commitments and Contingencies Disclosure [Abstract]

Commitments and contingencies

Lease commitments

The Company leases certain office and laboratory space and has embedded leases at contract manufacturing organizations. Refer to Note 8, Leases, for further information on the terms of these lease agreements.

Contingent consideration related to business combinations

On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to an additional $120.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain clinical and commercial milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value.

Other funding commitments

The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones that may be met in subsequent periods or royalties on future sales of specified products, which includes the collaboration agreement entered into with Regeneron in August 2018. Please refer to Note 11, Collaborative arrangements, for further information on the collaboration agreement with Regeneron.

Additionally, the Company is party to various contracts with contract research organizations and contract manufacturers that generally provide for termination on notice, with the exact amounts in the event of termination to be based on the timing of the termination and the terms of the agreement.

Based on our development plans as of December 31, 2020, the Company may be obligated to make future development, regulatory and commercial milestone payments and royalty payments on future sales of specified products associated with the Company's collaboration and license agreements. Payments under these agreements generally become due and payable upon achievement of such milestones or sales. When the achievement of these milestones or sales have not occurred, such contingencies are not recorded in the Company’s financial statements. As further discussed in Note 11, Collaborative arrangements, BMS assumed responsibility for amounts due to licensors as a result of any future ex-U.S. sales of ide-cel and bb21217.

The Company has various manufacturing development and license agreements to support clinical and commercial product needs. The following table presents non-cancelable contractual obligations arising from these arrangements:


Years ended December 31,	Purchase commitment
2021	$	83,885
2022	12,611
2023	—
2024	—
2025	—
2026 and thereafter	—
Total purchase commitments	$	96,496

Litigation

From time to time, the Company is party to various claims and complaints arising in the ordinary course of business, including securities class action litigation. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. Management

does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, management cannot give any assurance regarding the ultimate outcome of any claims, and their resolution could be material to operating results for any particular period.

The Company also indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company's request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and by-laws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director or officer in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company's exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, it has not recognized any liabilities relating to these obligations.

Common stock and preferred stock

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Common stock and preferred stock

The Company is authorized to issue 125.0 million shares of common stock. Holders of common stock are entitled to one vote per share. Holders of common stock are entitled to receive dividends, if and when declared by the Company’s board of directors, and to share ratably in the Company’s assets legally available for distribution to the Company’s shareholders in the event of liquidation. Holders of common stock have no preemptive, subscription, redemption or conversion rights. As of December 31, 2020, and 2019, the Company had 66.4 million and 55.4 million shares of common stock issued and outstanding, respectively.

In January 2018, the Company sold 0.3 million shares of common stock pursuant to the partial exercise of an overallotment option granted to the underwriters in connection with the December 2017 underwritten public offering at a price of $185.00 per share for aggregate net proceeds of $48.7 million. In July 2018, the Company sold 3.9 million shares of common stock (excluding any shares sold pursuant to an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $162.50 per share for aggregate net proceeds of $600.6 million. In August 2018, the Company sold 0.4 million shares of common stock to Regeneron in connection with a collaboration arrangement at a price of $238.10 per share for aggregate net proceeds of $100.0 million, of which $45.5 million was attributed to a prepayment of joint research activities.

In May 2020, the Company sold 10.5 million shares of common stock (inclusive of shares sold pursuant to an option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $55.00 per share for aggregate net proceeds of $541.5 million.

The Company is authorized to issue 5.0 million shares of preferred stock in one or more series and to fix the powers, designations, preferences and relative participating option or other rights thereof, including dividend rights, conversion rights, voting rights, redemption terms, liquidation preferences and the number of shares constituting any series, without any further vote or action by the Company’s shareholders. As of December 31, 2020 and 2019, the Company had no shares of preferred stock issued or outstanding.

Reserved for future issuance

The Company has reserved for future issuance the following number of shares of common stock (in thousands):


	As of December 31,
	2020	2019
Options to purchase common stock	6,262	5,483
Restricted stock units	1,495	1,127
2013 Stock Option and Incentive Plan	2,545	2,007
2013 Employee Stock Purchase Plan	67	147
	10,369	8,764

Collaborative arrangements

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborative arrangements

To date, the Company’s service and collaborative arrangement revenue has been primarily generated from its collaboration arrangements with BMS, formerly Celgene Corporation ("Celgene") prior to its acquisition by BMS in November 2019, and Regeneron, each as further described below.

Bristol-Myers Squibb

BMS Original Collaboration Agreement

In March 2013, the Company entered into a Master Collaboration Agreement (the “BMS Collaboration Agreement”) with Celgene (now BMS following its acquisition of Celgene in November 2019) to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR T cells, to target and destroy cancer cells. Additionally, in March 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with BMS pursuant to which the Company obtained a sublicense to certain intellectual property from BMS, originating under BMS’s license from Baylor College of Medicine, for use in the collaboration.

Under the terms of the BMS Collaboration Agreement, the Company received an up-front, non-refundable, non-creditable payment of $75.0 million. The Company was responsible for conducting discovery, research and development activities through completion of phase 1 clinical studies, if any, during the initial term of the BMS Collaboration Agreement, or three years.

BMS Amended Collaboration Agreement

In June 2015, the Company and BMS amended and restated the BMS Collaboration Agreement (the “Amended BMS Collaboration Agreement”). Under the Amended BMS Collaboration Agreement, the parties narrowed the focus of the collaboration to exclusively work on anti-B-cell maturation antigen (“BCMA”) product candidates for a new three-year term. In connection with the Amended BMS Collaboration Agreement, the Company received an up-front, one-time, non-refundable, non-creditable payment of $25.0 million to fund research and development under the collaboration. Under the terms of the Amended BMS Collaboration Agreement, for up to two product candidates selected for development under the collaboration, the Company was responsible for conducting and funding all research and development activities performed up through completion of the initial phase 1 clinical study of such product candidates.

On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial phase 1 clinical study for such product candidate, the Company had granted BMS an option to obtain an exclusive worldwide license to develop and commercialize such product. Following BMS’s license of each product candidate, the Company is entitled to elect to co-develop and co-promote each product candidate in the U.S.

BMS Ide-cel License Agreement

In February 2016, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize ide-cel, the first product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“Ide-cel License Agreement”) entered into by the parties in February 2016 and paid to the Company the associated $10.0 million option fee. Pursuant to the Ide-cel License Agreement, BMS was responsible for development and related funding of ide-cel after the substantial completion of the phase 1 clinical trial. The Company was responsible for the manufacture of vector and associated payload throughout development and upon BMS’s request, throughout commercialization, the costs of which were reimbursable by BMS in accordance with the terms of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement, as further described below. BMS was responsible for the manufacture of drug product throughout development and commercialization. Under the Ide-cel License Agreement, the Company was eligible to receive U.S. milestones of up to $85.0 million for the first indication to be addressed by ide-cel and royalties for U.S. sales of ide-cel. Additionally, the Company was eligible to receive ex-U.S. milestones of up to $55.0 million and royalties for ex-U.S. sales of ide-cel.

BMS Ide-cel Co-Development, Co-Promote and Profit Share Agreement

In March 2018, the Company elected to co-develop and co-promote ide-cel within the U.S. pursuant to the execution of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Ide-cel CCPS”), which replaced the Ide-cel License Agreement. As a result of executing the Ide-cel CCPS, the responsibilities of the parties remain unchanged from

those under the Ide-cel License Agreement, however, the Company will share equally in all profits and losses relating to developing, commercializing and manufacturing ide-cel within the U.S. and has the right to participate in the development and promotion of ide-cel in the U.S. BMS is responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. As a result of electing to co-develop and co-promote ide-cel within the U.S., the milestones and royalties payable under the Ide-cel License Agreement were adjusted. Under the Ide-cel CCPS, the Company was eligible to receive a $10.0 million milestone related to the development of ide-cel in the U.S. and, for the first indication to be addressed by ide-cel, ex-U.S. regulatory and commercial milestones of up to $60.0 million. Under the Ide-cel CCPS, the $10.0 million development milestone was achieved in the second quarter of 2019 and subsequently paid by BMS.

In May 2020, the First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (as amended, the "Amended Ide-cel CCPS") was executed, which amended the Ide-cel CCPS. Under the Amended Ide-cel CCPS, the parties will continue to share equally in all profits and losses related to developing, commercializing and manufacturing ide-cel within the U.S. Under the Amended Ide-cel CCPS and the Amended bb21217 License Agreement, described further below, BMS was relieved of its obligations to pay the Company for future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 in exchange for an up-front, non-refundable, non-creditable payment of $200.0 million, which represents the aggregate of the probability-weighted, net present value of the future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217. In connection with these amendments, BMS assumed the contract manufacturing agreements related to ide-cel adherent lentiviral vector. Over time, BMS is assuming responsibility for manufacturing ide-cel suspension lentivrial vector outside of the U.S., with bluebird responsible for manufacturing ide-cel suspension lentiviral vector in the U.S. In addition, under the Amended Ide-cel CCPS and the Amended bb21217 License Agreement, described further below, the parties are released from future exclusivity related to BCMA-directed T cell therapies. There are no remaining milestones or royalties under the Amended Ide-cel CCPS.

BMS bb21217 License Agreement

In September 2017, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“bb21217 License Agreement”) entered into by the parties in September 2017 and paid the Company an option fee of $15.0 million. Pursuant to the bb21217 License Agreement, BMS is responsible for development and related funding of bb21217 after the substantial completion of the ongoing phase 1 clinical trial. In 2019, the parties amended the protocol for the ongoing phase 1 clinical trial to enroll additional patients for which the Company will be reimbursed based upon an agreed-upon amount per patient. Under the bb21217 License Agreement, the Company is eligible to receive U.S. milestones of up to $85.0 million for the first indication to be addressed by bb21217 and royalties for U.S. sales of bb21217. Additionally, the Company was eligible to receive ex-U.S. milestones of up to $55.0 million and royalties for ex-U.S. sales of bb21217.

In May 2020, the Second Amended and Restated License Agreement ("Amended bb21217 License Agreement") was executed, which replaced the bb21217 License Agreement. Under the Amended bb21217 License Agreement, over time, BMS is assuming responsibility for manufacturing suspension lentiviral vector outside of the U.S., with bluebird responsible for manufacturing suspension lentiviral vector in the U.S. Under the Amended bb21217 License Agreement, expenses incurred by the Company associated with these activities are fully reimbursable by BMS at cost plus a mark-up. Throughout both development and commercialization, BMS is responsible for the manufacture of drug product. There are no remaining milestones and royalties related to the ex-U.S. development or commercialization of bb21217 following execution of the Amended bb21217 License Agreement.

The Company currently expects it will exercise its option to co-develop and co-promote bb21217 within the U.S. The Company’s election to co-develop and co-promote bb21217 must be made by the substantial completion of the on-going phase 1 clinical trial of bb21217. If elected, the Company expects the responsibilities of the parties to remain largely unchanged, however, the Company expects it will share equally in all profits and losses relating to developing, commercializing and manufacturing bb21217 within the U.S. and to have the right to participate in the development and promotion of bb21217 in the U.S. Under this scenario, the U.S. milestones and royalties payable under the Amended bb21217 License Agreement would be adjusted and the Company would be eligible to receive a $10.0 million development milestone payment related to the development of bb21217 within the U.S. The Company would not be eligible for royalties on U.S. sales of bb21217 under this scenario.

In the event the Company does not exercise its option to co-develop and co-promote bb21217, the Company will receive an additional fee in the amount of $10.0 million. Under this scenario, there would be no change to the U.S. milestones and royalties for U.S. sales of bb21217, as previously described above, for which the Company would be eligible to receive.

Accounting Analysis – Amended Ide-cel CCPS and Amended bb21217 License Agreement

In accordance with the Company’s accounting policies related to variable consideration, as further described in Note 2, Summary of Significant Accounting Policies and Basis of Presentation, if an arrangement includes variable consideration, including milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price of an arrangement. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. The Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied.

Prior to the May 2020 amendments, the Company had constrained all variable consideration related to the remaining ex-U.S. milestones and royalties for ex-U.S. sales under the Ide-cel CCPS and bb21217 License Agreement. As a result of the May 2020 amendments, the uncertainty associated with the previously constrained variable consideration for future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 was resolved in exchange for an up-front, non-refundable, non-creditable payment of $200.0 million.

While the Ide-cel CCPS and bb21217 License Agreement were historically accounted for as separate contracts, the May 2020 amendments to each agreement were negotiated as a package with a single commercial objective and, as such, the Amended Ide-cel CCPS and Amended bb21217 License Agreement were combined for accounting purposes and treated as a single arrangement.

At the time of the May 2020 amendments, there was one remaining performance obligation under each of the Ide-cel CCPS and bb21217 License Agreement, neither of which were fully satisfied: a combined performance obligation of the ide-cel license and ide-cel vector manufacturing through development; and a combined performance obligation of the bb21217 license and bb21217 vector manufacturing through development. Subsequent to the May 2020 amendments, the Company concluded the two performance obligations are distinct from each other as BMS can benefit from each license and associated manufacturing services separately and the respective licenses and manufacturing services do not modify one another and are not interdependent. Accordingly, the Company will continue to account for each performance obligation separately.

The Company allocated the $200.0 million up-front payment received in connection with the May 2020 amendments to the remaining performance obligations described above based on the general allocation principles of Topic 606. In applying these principles, the Company considered the $200.0 million up-front payment is representative of previously constrained variable consideration that has been changed and the related uncertainties resolved by the May 2020 amendments. Moreover, the Company considered that a portion of the $200.0 million was specifically attributable to each remaining performance obligation as the amount represents the aggregate of the probability-weighted, net present value of the future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 and that each respective portion therefore (i) relates specifically to the Company's satisfaction of each of its remaining performance obligations and (ii) is representative of the amount of consideration the Company expects to be entitled to in exchange for satisfying the respective performance obligations. As such, the Company concluded that the portion of the $200.0 million up-front payment specifically attributable to each of ide-cel and bb21217 should be allocated to each respective performance obligation pursuant to the variable consideration allocation exception.

The Amended Ide-cel CCPS and Amended bb21217 License Agreement represent a contract modification to an existing contract under Topic 606 given the May 2020 amendments resulted in a reduction in scope of the Company's responsibilities under each performance obligation described above. Specifically, the May 2020 amendments reduced the scope of the Company's obligation to provide ex-U.S. vector manufacturing services through development for both ide-cel and bb21217 as those activities will transition to BMS over time. In addition, the May 2020 amendments resulted in a change in the overall transaction price under the arrangement. The May 2020 amendments did not include any additional promised goods and services.

The remaining goods and services to be provided in order to fully satisfy each performance obligation described above are not distinct from those previously provided with respect to each performance obligation. Therefore, for each performance obligation, the remaining goods and services are part of a single performance obligation that is partially satisfied at the date of the contract modification. Accordingly, the effect that the contract modification had on the transaction price and the measure of progress toward complete satisfaction of each respective performance obligation has been recognized on a cumulative catch-up basis. The accounting for any previously satisfied performance obligations as of the contract modification date are not affected by the modification.

Ide-cel transaction price

The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement (including those performance obligations that were completed as of the May 2020 contract modification date), and the amount of the transaction price unsatisfied as of December 31, 2020 (in thousands):


	Ide-cel transaction price as of December 31, 2020
Upfront non-refundable payments received prior to May 2020 contract modification (1)	$	120,000
Allocated portion of the upfront non-refundable payment received in connection with the Amended Ide-cel CCPS and bb21217 License Agreement (2)	184,029
Estimated variable consideration (3)	83,900
	$	387,929

(1) Composed of all up-front payments and option fee and milestone payments received under the BMS Collaboration Agreement, Amended BMS Collaboration Agreement, Ide-cel License Agreement, and Ide-cel CCPS. This consideration was allocated to the performance obligations under the Ide-cel CCPS based on a relative standalone selling price (“SSP”) basis. The Company estimated the SSP of the ide-cel license after considering potential future cash flows under the license. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the ide-cel research and development services and ide-cel manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services.

(2) This represents the portion of the $200.0 million up-front payment received under the Amended Ide-cel CCPS and Amended bb21217 License Agreement which was allocated to ide-cel.

(3) Estimated variable consideration represents the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development.


	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of December 31, 2020
Ide-cel research and development services	$	40,912	$	—
Ide-cel license and manufacturing services	347,017		1,082
	$	387,929	$	1,082

Ide-cel research and development services

The Company allocated $40.9 million of the transaction price to the research and development services. The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was through projected initial phase 1 clinical study substantial completion, or through May 2018. The research and development performance obligation was satisfied prior to the May 2020 amendments and, as a result, the accounting for this previously satisfied performance obligation was not affected by the modification. The Company recognized no revenue related to ide-cel research and development services for the year ended December 31, 2020. The Company recognized $2.3 million and $5.8 million related to ide-cel research and development services for the year ended December 31, 2019 and 2018, respectively.

Ide-cel license and manufacturing services

The Company allocated $347.0 million of the transaction price to the combined unit of accounting which consists of the license and manufacture of vectors and associated payload for incorporation into ide-cel.

The Company accounts for its vector manufacturing services for development in the U.S. and BMS’s U.S. development efforts within the scope of ASC 808 given that both parties are active participants in the activities and both parties are exposed to significant risks and rewards dependent on the commercial success of the activities. The Company recognizes collaboration revenue for its U.S. manufacturing services by analogy to Topic 606. The portion of BMS’s U.S. development costs that the Company is responsible for are recognized as a reduction to its collaboration revenues, or, if in excess of such revenues in a given quarter, the excess is recorded as research and development expense.

The Company recognizes revenue associated with the combined performance obligation using the proportional performance method, as the Company will satisfy this performance obligation as the manufacturing services are performed through development. In using this method, the Company estimated its development plan for ide-cel, including expected demand from BMS, and the costs associated with the manufacture of vectors and associated payload for incorporation into ide-cel. On a quarterly basis, the Company determines the proportion of effort incurred as a percentage of total effort it expects to expend. This ratio is applied to the transaction price, which includes variable consideration, allocated to the combined performance obligation consisting of the ide-cel license and manufacturing services. Management has applied significant judgment in the process of developing its budget estimates and any changes to these estimates will be recognized in the period in which they change as a cumulative catch-up.

The following table summarizes the net collaboration revenue recognized or expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808, including revenue or expense related to the combined performance obligation for the license and vector manufacturing of ide-cel in the U.S. for the years ended December 31, 2020, 2019, and 2018 (in thousands):


	For the years ended December 31,
	2020		2019		2018
ASC 808 ide-cel license and manufacturing revenue - U.S. (1)	$	108,196	$	—	$	6,255
ASC 808 ide-cel research and development expense - U.S. (1)	$	41,599	$	32,415	$	8,689

(1)As noted above, the calculation of collaboration revenue or research and development expense to be recognized for joint ide-cel development efforts in the U.S. is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period.

Revenue related to the combined unit of accounting for the non-US license and vector manufacturing services is accounted for in accordance with Topic 606. The following table summarizes the revenue recognized related to the combined unit of accounting for the ide-cel ex-U.S. license and vector manufacturing services for the years ended December 31, 2020, 2019, and 2018 (in thousands):


	For the years ended December 31
	2020		2019		2018
ASC 606 ide-cel license and manufacturing revenue - ex-U.S.	$	99,053	$	25,522	$	35,900

As of December 31, 2020, the aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the ide-cel license and manufacturing services, that is unsatisfied, or partially unsatisfied, is $1.1 million, which the Company expects to recognize as revenue as manufacturing services are provided through the remaining development period. As of December 31, 2020 and 2019, the Company had $0.8 million and $8.5 million, respectively, of deferred revenue associated with the combined performance obligation consisting of the ide-cel license and manufacturing services.

bb21217 transaction price


(in thousands)	bb21217 transaction price as of December 31, 2020
Upfront non-refundable payments received prior to May 2020 contract modification (1)	$	15,000
Allocated portion of the up-front non-refundable payment received in connection with the Amended Ide-cel CCPS and bb21217 License Agreement (2)	15,971
Estimated variable consideration (3)	1,803
	$	32,774

(1) Composed of the up-front non-refundable payment received under the bb21217 License Agreement. This consideration was allocated to the performance obligations under the bb21217 License Agreement based on a relative SSP basis. The Company estimated the SSP of the bb21217 license after considering potential future cash flows under the license. The

Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the bb21217 research and development services and bb21217 manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services.

(2) This represents the portion of the $200.0 million up-front payment received under the Amended Ide-cel CCPS and Amended bb21217 License Agreement which was allocated to bb21217.

(3) Estimated variable consideration represents the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development.


	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of December 31, 2020
bb21217 research and development services	$	5,444	$	—
bb21217 license and manufacturing services	27,330		27,330
	$	32,774	$	27,330

All of the remaining development, regulatory, and commercial milestones under the Amended bb21217 License Agreement are related to U.S. development, regulatory and commercialization activities and are fully constrained and are therefore excluded from the transaction price. As part of its evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestones is outside the control of the Company and contingent upon the future success of its clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to U.S. sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to BMS and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur.

The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, each reporting period and as uncertain events are resolved or other changes in circumstances occur.

bb21217 research and development services

The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was two years through projected substantial completion of the initial phase 1 clinical study, or through September 2019. The research and development performance obligation was satisfied prior to the May 2020 amendments, and as a result, the accounting for this previously satisfied performance obligation was not affected by the modification. As part of performing its initial obligation to complete a phase 1 trial as originally contemplated, the Company recognized no revenue for the year ended December 31, 2020 and revenue of $2.2 million and $2.9 million for the years ended December 31, 2019 and 2018, respectively.

The agreement to expand the bb21217 phase 1 trial that occurred in 2019 was previously treated as a separate contract for accounting purposes, because the trial expansion was for the addition of a promised good or service that is distinct and the associated consideration reflected the standalone selling price of the additional promised good or service. This contract was not affected by the May 2020 amendments and, accordingly, the accounting for this agreement was not impacted by the May 2020 amendments. The transaction price associated with these additional patients consists of variable consideration and is based upon an agreed-upon amount per patient which will be recognized as revenue as the patients are treated. The Company began fulfilling the performance obligation in the fourth quarter of 2019 and it was satisfied in the fourth quarter of 2020. In connection with treating additional patients in the phase 1 trial, the Company recognized revenue of $12.4 million, $0.4 million, and $0.0 million for the years ended December 31, 2020, 2019, and 2018, respectively.

bb21217 license and manufacturing services

The Company will satisfy its performance obligation related to the manufacture of vectors and associated payload for incorporation into bb21217 through development as the bb21217 manufacturing services are performed. As of December 31, 2020, the manufacturing services for bb21217 had not yet commenced. Therefore, no amounts have been recognized for the combined performance obligation in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2020, 2019, and 2018.

The aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the bb21217 license and manufacturing services, is $27.3 million. The Company does not expect that recognition will begin in the next twelve months and has therefore classified deferred revenue associated with the combined performance obligation as deferred revenue, net of current portion on its consolidated balance sheet. The Company had $25.8 million and $9.8 million of remaining deferred revenue as of December 31, 2020 and 2019, respectively, associated with the combined performance obligation consisting of the bb21217 license and manufacturing services.

Contract assets and liabilities – ide-cel and bb21217

The Company receives payments from its collaborative partners based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until such time as the Company satisfies its performance obligations under these arrangements. A contract asset is a conditional right to consideration in exchange for goods or services that the Company has transferred to a customer. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional.

The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the twelve months ended December 31, 2020 (in thousands):


	Balance at December 31, 2019		Additions		Deductions		Balance at December 31, 2020
Receivables	$	400	$	12,400	$	(12,400)	$	400
Contract liabilities:
Deferred revenue	$	18,265	$	200,000	$	(191,683)	$	26,582

The change in the receivables balance for the year ended December 31, 2020 is primarily driven by amounts owed to the Company for bb21217 research and development services provided during the period (expanded phase 1 clinical trial), offset by amounts collected from BMS in the period.

The increase in deferred revenue during the year ended December 31, 2020 is primarily driven by the $200.0 million consideration received in connection with the May 2020 amendments, offset by revenue recognized in the year-to-date period related to the combined unit of accounting for ide-cel license and vector manufacturing services. A total of $191.7 million was released from deferred revenue during the year-to-date period, of which $169.2 million is related to a cumulative catch-up adjustment to revenue recorded in connection with the May 2020 contract modification described further above. As of December 31, 2019, the Company had $8.5 million of deferred revenue associated with the combined performance obligation consisting of the ide-cel license and manufacturing services, of which $8.2 million was released during the year ended December 31, 2020.

Regeneron

Regeneron Collaboration Agreement

In August 2018, the Company entered into a Collaboration Agreement (the “Regeneron Collaboration Agreement”) with Regeneron pursuant to which the parties will apply their respective technology platforms to the discovery, development, and commercialization of novel immune cell therapies for cancer. In August 2018, following the completion of required regulatory reviews, the Regeneron Collaboration Agreement became effective. Under the terms of the agreement, the parties will leverage Regeneron’s proprietary platform technologies for the discovery and characterization of fully human antibodies, as well as T cell receptors directed against tumor-specific proteins and peptides and the Company will contribute its field-leading expertise in gene therapy.

In accordance with the Regeneron Collaboration Agreement, the parties jointly selected six initial targets and intend to equally share the costs of research up to the point of submitting an IND application for a potential gene therapy product directed to a particular target. Additional targets may be selected to add to or replace any of the initial targets during the five-year research collaboration term as agreed to by the parties.

Regeneron will accrue a certain number of option rights exercisable against targets as the parties reach certain milestones under the terms of the agreement. Upon the acceptance of an IND for the first product candidate directed to a target, Regeneron will have the right to exercise an option for co-development/co-commercialization of product candidates directed to such target

on a worldwide or applicable opt-in territory basis, with certain exceptions. Where Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories. Outside of the applicable opt-in territories, the target becomes a licensed target and Regeneron would be eligible to receive, with respect to any resulting product, milestone payments of up to $130.0 million per product and royalties on net sales outside of the applicable opt-in territories at a rate ranging from the mid-single digits to low-double digits. A target would also become a licensed target in the event Regeneron does not have an option to such target, or Regeneron does not exercise its option with respect to such target.

Either party may terminate a given research program directed to a particular target for convenience, and the other party may elect to continue such research program at its expense, receiving applicable cross-licenses. The terminating party will receive licensed product royalties and milestone payments on the potential applicable gene therapy products. Where the Company terminates a given research program for convenience, and Regeneron elects to continue such research program, the parties will enter into a transitional services agreement. Under certain conditions, following its opt-in, Regeneron may terminate a given collaboration program and the Company may elect to continue the development and commercialization of the applicable potential gene therapy products as licensed products.

Regeneron Share Purchase Agreement

A Share Purchase Agreement (“SPA”) was entered into by the parties in August 2018. In August 2018, on the closing date of the transaction, the Company issued Regeneron 0.4 million shares of the Company’s common stock, subject to certain restrictions, for $238.10 per share, or $100.0 million in the aggregate. The purchase price represents $63.0 million worth of common stock plus a $37.0 million premium, which represents a collaboration research advancement, or credit to be applied to Regeneron’s initial 50 percent funding obligation for collaboration research, after which the collaborators will continue to fund ongoing research equally. The collaboration research advancement only applies to pre-IND research activities and is not refundable or creditable against post-IND research activities for any programs where Regeneron exercises their opt-in rights.

Accounting analysis – Regeneron

At the commencement of the arrangement, two units of accounting were identified, which are the issuance of 0.4 million shares of the Company’s common stock and joint research activities during the five year research collaboration term. The Company determined the total transaction price to be $100.0 million, which comprises $54.5 million attributed to the equity sold to Regeneron and $45.5 million attributed to the joint research activities. In determining the fair value of the common stock at closing, the Company considered the closing price of the common stock on the closing date of the transaction and included a lack of marketability discount because Regeneron received shares subject to certain restrictions.

The Company analyzed the joint research activities to assess whether they fall within the scope of ASC 808, and will reassess this throughout the life of the arrangement based on changes in the roles and responsibilities of the parties. Based on the terms of the arrangement as outlined above, for the collaboration research performed prior to submission of an IND application for a potential gene therapy product, both parties are deemed to be active participants in the collaboration. Both parties are performing research and development activities and will share equally in these costs through IND. Additionally, Regeneron and the Company are exposed to significant risks and rewards dependent on the commercial success of any product candidates that may result from the collaboration. As such, the collaboration arrangement is deemed to be within the scope of ASC 808.

The $45.5 million attributed to the joint research activities includes the $37.0 million creditable against amounts owed to the Company by Regeneron. The collaboration research advancement will be reduced over time for amounts due to the Company by Regeneron as a result of the parties agreeing to share in the costs of collaboration research equally. The remainder of the amount attributed to the joint research activities will be recognized over the five-year research collaboration term.

Consistent with its collaboration accounting policy, the Company will recognize collaboration revenue or research and development expense related to the joint research activities in future periods depending on the amounts incurred by each party in a given reporting period. That is, if the Company’s research costs incurred exceed those research costs incurred by Regeneron in a given quarter, the Company will record collaboration revenue and reduce the original $37.0 million advance by the amount due from Regeneron until such advancement is fully utilized, after which the Company would record an amount due from Regeneron. If Regeneron’s research costs incurred exceed those research costs incurred by the Company in a given quarter, the Company will record research and development expense and record a liability for the amount due to Regeneron. As of December 31, 2020 and 2019, the Company has $30.8 million and $38.2 million, respectively, of the amount attributed to the joint research activities remaining to be recognized which is classified as collaboration research advancement, current portion and collaboration research advancement, net of current portion on the consolidated balance sheet.

The Company recognized $7.4 million and $5.7 million of collaboration revenue from the Regeneron Collaboration Agreement during the years ended December 31, 2020 and 2019, respectively.

Royalty and other revenue	12 Months Ended
Royalty and other revenue	Dec. 31, 2020
License And Royalty Revenue [Abstract]
Royalty and other revenue	Royalty and other revenue Novartis Pharma AG In April 2017, the Company entered into a worldwide license agreement with Novartis. Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including Kymriah (formerly known as CTL19), Novartis’s anti-CD19 CAR T therapy. At contract inception, financial terms of the agreement included a $7.5 million payment upon execution, $7.5 million of potential future milestone payments associated with regulatory approvals, and $1.1 million of payments for each subsequently licensed product, as well as low single digit royalty payments on net sales of covered products. In August 2017, Novartis received FDA approval for Kymriah and paid the Company $2.5 million as a result of the achievement of a related milestone. Under Topic 606, the Company identified only one performance obligation, consisting of the license, which was satisfied at contract inception. Accordingly, the nonrefundable license fee of $7.5 million was recognized as revenue upon contract execution in the second quarter of 2017 and a $2.5 million regulatory milestone was recognized as revenue upon milestone achievement, also in the second quarter of 2017, given there were no other unsatisfied performance obligations in the arrangement. Regulatory approvals are not within the Company’s control or the licensee’s control and are generally not considered probable of being achieved until those approvals are received. As such, these milestones are constrained until such time as regulatory approvals are received. Because the single performance obligation was previously satisfied, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved. The Company began recognizing royalty revenue from sales of Kymriah in the fourth quarter of 2017. As the license was deemed to be the predominant item to which the royalties relate, the Company recognizes royalties from the sales of Kymriah when the related sales occur. For the years ended December 31, 2020, 2019, and 2018, the Company recognized royalty and other revenue of $21.1 million, $8.2 million, and $2.2 million, respectively. For the years ended December 31, 2020, 2019, and 2018, the Company recognized cost of royalty and other revenue of $5.4 million, $3.0 million, and $0.9 million, respectively. In December 2020, the Company received notice of termination from Novartis for the license agreement described above. This termination will be effective in March 2021, at which point in time Novartis will no longer be required to pay the Company royalty or other payments on net sales of Kymriah or any future products. Orchard Therapeutics Limited (assigned by GlaxoSmithKline Intellectual Property Development Limited) In April 2017, the Company entered into a worldwide license agreement with GlaxoSmithKline Intellectual Property Development Limited (“GSK”). Under the terms of the agreement, GSK non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize gene therapies for Wiscott-Aldrich syndrome and metachromatic leukodystrophy, two rare genetic diseases. Financial terms of the agreement include a nonrefundable upfront payment of $3.0 million as well as $1.3 million of potential milestone payments related to each marketing authorization for each indication in any country as well as low single digit royalties on net sales of covered products. This license agreement was assigned by GSK to Orchard Therapeutics Limited, effective in April 2018. Under Topic 606, the Company identified only one performance obligation, consisting of the license, which was satisfied at contract inception. Accordingly, the entire nonrefundable license fee of $3.0 million was recognized as revenue upon contract execution in the second quarter of 2017 given there were no other unsatisfied performance obligations in the arrangement. Regulatory approvals are not within the Company’s control or the licensee’s control and are generally not considered probable of being achieved until those approvals are received. As such, these milestones are constrained until such time as after regulatory approvals are received. There was no revenue recognized under this arrangement in the years ended December 31, 2020, 2019, or 2018. Because the single performance obligation was previously satisfied, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved. Given there was no revenue recognized under this arrangement in the years ended December 31, 2020, 2019, or 2018, there was no associated cost of royalty and other revenue.

Intangible assets

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible assets

Intangible assetsIntangible assets, net of accumulated amortization, are summarized as follows (in thousands):


	As of December 31,						As of December 31,
	2020						2019
	Cost		Accumulated amortization		Net		Cost		Accumulated amortization		Net
Developed technology	$	30,100	$	(24,456)	$	5,644	$	30,100	$	(20,694)	$	9,406
In-licensed rights	5,224		(827)		4,397		5,224		(304)		4,920
Total	$	35,324	$	(25,283)	$	10,041	$	35,324	$	(20,998)	$	14,326

Amortization expense for intangible assets was $4.3 million, $4.1 million, and $3.8 million for each of the years ended December 31, 2020, 2019 and 2018, respectively.

Developed technology

The Company's developed technology was obtained through its acquisition of Pregenen, a privately-held biotechnology company in 2014. The Company obtained gene editing and cell signaling technology with a broad range of potential therapeutic applications. The Company considered the intangible asset acquired to be developed technology, as at the date of the acquisition it could be used the way it was intended to be used in certain ongoing research and development activities. The gene editing platform intangible asset is being amortized on a straight-line basis over its expected useful life of approximately eight years from the date of the acquisition. Please refer to Note 4, Fair value measurements, and Note 9, Commitments and contingencies, for further information.

In-licensed rights

In-licensed rights consist of capitalized milestone payments made to third parties upon receiving regulatory approval of ZYNTEGLO in the EU. The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patents of approximately ten years, as the life of the related patents reflects the expected time period that the Company will benefit from the in-licensed rights.

The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands):


	As of December 31, 2020
2021	$	4,285
2022	2,404
2023	522
2024	522
2025	522
2026 and thereafter	1,786
Total	$	10,041

Stock-based compensation

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Stock-based compensation

In June 2013, the Company’s board of directors adopted its 2013 Stock Option and Incentive Plan (“2013 Plan”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO. The 2013 Plan replaces the 2010 Stock Option and Grant Plan (“2010 Plan”).

The 2013 Plan allows for the granting of incentive stock options, non-qualified stock options, restricted stock units and restricted stock awards to the Company’s employees, members of the board of directors, and consultants of the Company. The Company initially reserved 1.0 million shares of its common stock for the issuance of awards under the 2013 Plan. The 2013 Plan provides that the number of shares reserved and available for issuance under the 2013 Plan will automatically increase each January 1, beginning on January 1, 2014, by four percent of the outstanding number of shares of common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. In January 2020 and January 2021, the number of common stock available for issuance under the 2013 Plan was increased by approximately 2.2 million and 2.7 million shares, respectively, as a result of this automatic increase provision.

Any options or awards outstanding under the Company’s previous stock option plans, including both the 2010 Plan and the Second Amended and Restated 2002 Employee, Director and Consultant Stock Plan (“2002 Plan”), at the time of adoption of the 2013 Plan remain outstanding and effective. The shares of common stock underlying any awards that are forfeited, canceled, repurchased, expired or are otherwise terminated (other than by exercise) under the 2002 Plan and 2010 Plan are added to the shares of common stock available for issuance under the 2013 Plan. As of December 31, 2020, the total number of common stock that may be issued under all plans is 2.6 million.

The Company does not currently hold any treasury shares. Upon stock option exercise, the Company issues new shares and delivers them to the participant.

Stock-based compensation expense

The Company recognized stock-based compensation expense totaling $156.6 million, $160.6 million, and $110.8 million during the years ended December 31, 2020, 2019 and 2018, respectively. Stock-based compensation expense recognized by award type is as follows (in thousands):


	Year Ended December 31,
	2020		2019		2018
Stock options	$	93,977	$	95,668	$	83,449
Restricted stock units	49,326		63,580		26,628
Employee stock purchase plan and other	13,328		1,381		759
	$	156,631	$	160,629	$	110,836

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Year Ended December 31,
	2020		2019		2018
Research and development	$	72,239	$	80,139	$	54,422
Selling, general and administrative	84,392		80,490		56,414
	$	156,631	$	160,629	$	110,836

In February 2018, the Company issued restricted stock units with service and performance conditions to employees, less than 0.1 million of which are outstanding as of December 31, 2020. Vesting of these awards is contingent on the occurrence of a certain regulatory milestone event which was achieved in June 2019 and fulfillment of any remaining service condition. The Company began recognizing expense for these awards in the second quarter of 2019 when achievement of the regulatory milestone was deemed probable. The Company recognized $20.1 million of expense related to these awards in the second quarter of 2019 and will continue to recognize stock-based compensation expense related to these awards through June 2021 when the final tranche of the awards vest.

As of December 31, 2020, the Company had $146.0 million, $89.3 million and $0.4 million of unrecognized compensation expense related to unvested stock options, restricted stock units (exclusive of those with service and performance conditions that have not yet been achieved) and the employee stock purchase plan, respectively, that is expected to be recognized over a weighted-average period of 2.1 years, 2.4 years, and 0.3 years, respectively.

Stock options

The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:


	Year Ended December 31,
	2020		2019		2018
Expected volatility	69.5	%	70.7	%	75.5	%
Expected term (in years)	6.0		6.0		6.0
Risk-free interest rate	1.4	%	2.3	%	2.7	%
Expected dividend yield	0.0	%	0.0	%	0.0	%

The following table summarizes the stock option activity under the Company’s equity awards plans:


	Shares (in thousands)	Weighted- average exercise price per share		Weighted- average contractual life (in years)	Aggregate intrinsic value (a) (in thousands)
Outstanding at December 31, 2019	5,483	$	116.30
Granted	1,585	$	69.40
Exercised	(95)	$	19.43
Canceled or forfeited	(711)	$	124.00
Outstanding at December 31, 2020	6,262	$	105.02	7.0	$	15,716
Exercisable at December 31, 2020	3,669	$	107.11	5.8	$	15,659
Vested and expected to vest at December 31, 2020	6,262	$	105.02	7.0	$	15,716

(a)The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2020.

The weighted-average fair values of options granted during 2020, 2019 and 2018 was $43.24, $83.44, and $125.12, respectively. The intrinsic value of options exercised during the years ended December 31, 2020, 2019, and 2018, was $4.0 million, $29.0 million and $73.1 million, respectively.

Restricted stock units

The following table summarizes the restricted stock unit activity under the Company’s equity award plans:


	Shares (in thousands)	Weighted-average grant date fair value
Unvested balance at December 31, 2019	1,127	$	146.10
Granted	1,018	70.18
Vested	(433)	130.10
Forfeited	(217)	123.32
Unvested balance at December 31, 2020	1,495	$	102.34

The intrinsic value of restricted stock units vested during the years ended December 31, 2020, 2019, and 2018 was $30.9 million, $28.4 million and $25.5 million, respectively.

Employee Stock Purchase Plan

In June 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO. The 2013 ESPP authorizes the initial issuance of up to a total of 0.2 million shares of the Company’s common stock to participating employees. During each of the years ended December 31, 2020 and 2019, less than 0.1 million shares of common stock were issued under the 2013 ESPP, respectively.

401(k) Savings plan	12 Months Ended
401(k) Savings plan	Dec. 31, 2020
Retirement Benefits [Abstract]
401(k) Savings plan	401(k) Savings planIn 1997, the Company established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“the 401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. In March 2021, the Company expects to make matching contributions of approximately $5.2 million related to employee contributions made during 2020. In March 2020, the Company made $4.6 million of matching contributions related to employee contributions made during 2019. The match contribution is included in accrued expenses and other current liabilities as of December 31, 2020 and 2019. Expense related to the 401(k) Plan totaled $5.2 million, $4.6 million, $2.4 million for the years ended December 31, 2020, 2019, and 2018, respectively.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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-Section 55
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Income taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income taxes

Income taxesThe components of loss before income taxes were as follows (in thousands):


	Year ended December 31,
	2020		2019		2018
U.S.	$	(489,091)	$	(649,172)	$	(440,473)
Foreign	(128,918)		(140,981)		(114,965)
Total	$	(618,009)	$	(790,153)	$	(555,438)

The provision for (benefit from) income taxes were as follows (in thousands):


	Year ended December 31,
	2020		2019		2018
Current:
Federal	$	—	$	—	$	—
State	2		7		324
Foreign	684		612		222
Deferred:
Federal	—		(966)		(307)
State	—		(198)		(52)
Foreign	—		—		—
Total income tax expense (benefit)	$	686	$	(545)	$	187

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate as reflected in the financial statements is as follows:


	Year ended December 31,
	2020		2019		2018
Federal income tax expense at statutory rate	21.0	%	21.0	%	21.0	%
State income tax, net of federal benefit	3.1	%	3.7	%	5.1	%
Permanent differences	(0.6)	%	(0.8)	%	(0.7)	%
Stock-based compensation	(2.4)	%	(0.7)	%	1.6	%
Research and development credit	6.0	%	5.4	%	6.5	%
Foreign differential	(4.6)	%	(3.7)	%	(4.4)	%
Federal tax rate change	—	%	—	%	0.1	%
Other	(0.3)	%	0.8	%	(0.1)	%
Change in valuation allowance	(22.3)	%	(25.6)	%	(29.1)	%
Effective income tax rate (expense) benefit	(0.1)	%	0.1	%	—	%

For the years ended December 31, 2020, 2019 and 2018, the Company recognized an income tax expense (benefit) of $0.7 million or (0.1)%, $(0.5) million or 0.1%, and $0.2 million or 0.0%, respectively. The Company did not recognize any significant tax expense for the years ended December 31, 2020, 2019, or 2018 as the Company was subject to a full valuation allowance.

Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands):


	Year ended December 31,
	2020		2019
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$	546,098	$	439,839
Tax credit carryforwards (federal and state)	246,742		213,810
Capitalized license fees and research and development expenses	14,402		16,295
Deferred revenue	15,644		15,119
Stock-based compensation	51,828		46,111
Lease liabilities	48,680		52,631
Accruals and other	14,536		15,432
Total deferred tax assets	937,930		799,237
Intangible assets	(1,499)		(2,518)
Right-of-use assets	(45,976)		(49,480)
Fixed assets	(7,576)		(2,670)
Less valuation allowance	(882,879)		(744,569)
Net deferred taxes	$	—	$	—

A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. The valuation allowance increased on a net basis by approximately $138.3 million during the year ended December 31, 2020 due primarily to net operating losses, tax credit carryforwards, and stock-based compensation. Effective January 1, 2019, the Company adopted ASU 2016-02, which resulted in the de-recognition of the 60 Binney Street lease and related fixed assets and the recognition of lease liabilities and right-of-use assets. The Company adjusted its deferred tax balances as a result of the adoption.

As of December 31, 2020, 2019 and 2018, the Company had U.S. federal net operating loss carryforwards of approximately $2.03 billion, $1.62 billion, and $1.10 billion, respectively, which may be available to offset future income tax liabilities. Of the amount as of December 31, 2020, $1.32 billion will carryforward indefinitely while $711.0 million will expire at various dates through 2037. As of December 31, 2020, 2019 and 2018, the Company also had U.S. state net operating loss carryforwards of approximately $1.89 billion, $1.56 billion, and $1.08 billion, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2039.

As of December 31, 2020, 2019 and 2018, the Company had federal research and development and orphan drug tax credit carryforwards of approximately $235.3 million, $203.1 million, and $156.2 million, respectively, available to reduce future tax liabilities which expire at various dates through 2039. As of December 31, 2020, 2019 and 2018, the Company had state credit carryforwards of approximately $14.5 million, $13.6 million, and $14.3 million, respectively, available to reduce future tax liabilities which expire at various dates through 2034. During the fourth quarter of 2018, the Company completed an analysis of prior year estimates of U.S. research and development and orphan drug tax credits for the years 2013 through 2017. The analysis resulted in an immaterial adjustment to the Company's income tax benefit, which was offset by an adjustment to the valuation allowance. An analysis of the U.S. research and development and orphan drug credits has not yet been completed for 2018, 2019, or 2020.

In March 2020, the Coronavirus Aid, Relief and Economic Security Act (“CARES Act”) was enacted. This law temporarily suspends and adjusts certain law changes enacted in the Tax Cuts and Jobs Act in 2017. In December 2020, the Consolidated Appropriations Act was enacted. This law modified the employee retention credit under the CARES Act and created credit extenders for certain credits. The Company has concluded that the provisions in the CARES Act and Consolidated Appropriations Act have an immaterial impact on the Company’s income tax expense due to its cumulative losses and full valuation allowance position.

Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be

utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception and prior to its initial public offering in 2013, which it believes has resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code. The Company completed a study through September 2019 confirming no ownership changes have occurred since the Company's initial public offering in 2013; any ownership shifts occurring after September 2019 could result in an ownership change under Section 382.

The Company files Federal income tax returns in the United States, and various state and foreign jurisdictions. The federal, state and foreign income tax returns are generally subject to tax examinations for the tax years ended December 31, 2017 through December 31, 2019. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, or state or foreign tax authorities to the extent utilized in a future period.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):


	Unrecognized tax benefits
Balance as of December 31, 2018	$	12,095
Increases (decreases) for tax positions related to current period	3,554
Increases (decreases) for tax positions related to prior periods	296
Balance as of December 31, 2019	15,945
Increases (decreases) for tax positions related to current period	3,149
Increases (decreases) for tax positions related to prior periods	(100)
Balance as of December 31, 2020	18,994

The unrecognized tax benefits at December 31, 2020, if recognized, would not affect the Company’s effective tax rate due to its full valuation allowance position. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease within the next 12 months. The Company has elected to include interest and penalties related to uncertain tax positions as a component of its provision for income taxes. For the years ended December 31, 2020, 2019 and 2018, the Company’s accrued interest and penalties related to uncertain tax positions were not material.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Net loss per share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net loss per share

The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):


	Year ended December 31, 2020,
	2020	2019	2018
Outstanding stock options	6,262	5,483	4,643
Restricted stock units	1,495	1,127	931
ESPP shares and other	326	19	10
	8,083	6,629	5,584

Selected quarterly financial data (unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Selected quarterly financial data (Unaudited)

Selected quarterly financial data (unaudited)

The following table contains quarterly financial information for 2020 and 2019. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


	2020
	First quarter		Second quarter		Third quarter		Fourth quarter		Total
	(in thousands, except per share data)
Total revenues	$	21,863	$	198,890	$	19,273	$	10,708	$	250,734
Total operating expenses	225,288		224,835		208,967		214,690		873,780
Loss from operations	(203,425)		(25,945)		(189,694)		(203,982)		(623,046)
Net loss	(202,611)		(21,465)		(194,745)		(199,874)		(618,695)
Net loss per share applicable to common stockholders - basic and diluted	$	(3.64)	$	(0.36)	$	(2.94)	$	(3.01)	$	(9.95)


	2019
	First quarter		Second quarter		Third quarter		Fourth quarter		Total
	(in thousands, except per share data)
Total revenues	$	12,471	$	13,296	$	8,910	$	9,997	$	44,674
Total operating expenses	183,645		215,998		219,326		240,531		859,500
Loss from operations	(171,174)		(202,702)		(210,416)		(230,534)		(814,826)
Net loss	(164,446)		(195,782)		(206,033)		(223,347)		(789,608)
Net loss per share applicable to common stockholders - basic and diluted	$	(2.99)	$	(3.55)	$	(3.73)	$	(4.04)	$	(14.31)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent eventsIn January 2021, the Company announced its intent to separate its severe genetic disease and oncology programs into two separate, independent publicly traded companies, bluebird bio, Inc. and a new company, referred to as Oncology NewCo in these consolidated financial statements. bluebird bio, Inc. intends to retain focus on its severe genetic disease programs and Oncology NewCo is expected to focus on the Company's oncology programs. The transaction is expected to be completed in late 2021 and is anticipated to be tax-free, subject to receipt of a favorable IRS ruling.

Summary of significant accounting policies and basis of presentation (Policies)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Basis of presentation

Amounts reported are computed based on thousands, except percentages, per share amounts or as otherwise noted. As a result, certain totals may not sum due to rounding.

Principles of consolidation

Use of estimates

Foreign currency transaction

Foreign currency translation

Segment information

Cash and cash equivalents

Marketable securities

Concentrations of credit risk and off-balance sheet risk

Fair value of financial instruments

Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable.

Business combinations

Goodwill

GoodwillGoodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. During the fourth quarter of 2019, the Company early adopted ASC 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ("ASU 2017-04"), which removes the second step of the goodwill impairment test. Under this ASU, the Company performs a one-step quantitative test and records the amount of goodwill impairment, if any, as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit.

Intangible assets

Intangible assets, net

Contingent consideration

Property, plant and equipment


Asset						Estimated useful life
Building						40 years
Computer equipment and software						3 years
Furniture and fixtures						2-5 years
Laboratory equipment						2-5 years
Leasehold improvements						Shorter of the useful life or remaining lease term

Impairment of long-lived assets

Leases

ASC 842 transition practical expedients and application of transition provisions to leases at the transition date

Application of ASC 842 policy elections to leases post adoption

The Company has made certain policy elections to apply to its leases executed post adoption, or subsequent to January 1, 2019, as further described below.

Revenue recognition

The Company recognizes revenue within the following financial statement captions:

Service revenue

Collaborative arrangement revenue

Royalty and other revenue

Research and development expenses

Cost of royalty and other revenue

Cost of royalty and other revenue represents expense associated with amounts owed to third parties as a result of revenue recognized under the Company’s out-license arrangements.

Stock-based compensation

Interest income (expense), net

Interest income, netInterest income, net consists primarily of interest income earned on investments, net of amortization of premium and accretion of discount.

Other income (expense), net

Other (expense) income, net

Other (expense) income, net consists primarily of gains and losses on equity securities held by the Company, gains and losses on disposal of assets, and gains and losses on foreign currency.

Net loss per share

Income taxes

Comprehensive loss

Recent accounting pronouncements

ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606

recognized under Topic 606 and outside of Topic 606 had previously otherwise been disclosed in the notes to consolidated financial statements.

ASU No. 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments

Not yet adopted

ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

ASU No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs

ASU No. 2020-10, Codification Improvements

Summary of significant accounting policies and basis of presentation (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Estimated Useful Lives of Assets

Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:


Asset						Estimated useful life
Building						40 years
Computer equipment and software						3 years
Furniture and fixtures						2-5 years
Laboratory equipment						2-5 years
Leasehold improvements						Shorter of the useful life or remaining lease term

Marketable securities (Tables)

12 Months Ended

Dec. 31, 2020

Investments, Debt and Equity Securities [Abstract]

Summary of Marketable Securities Held

The following table summarizes the marketable securities held at December 31, 2020 and 2019 (in thousands):


	Amortized cost / cost		Unrealized gains		Unrealized losses		Fair value
December 31, 2020
U.S. government agency securities and treasuries	$	675,043	$	302	$	(74)	$	675,271
Corporate bonds	197,171		432		(40)		197,563
Commercial paper	77,949		1		—		77,950
Equity securities	20,017		—		(14,364)		5,653
Total	$	970,180	$	735	$	(14,478)	$	956,437
December 31, 2019
U.S. government agency securities and treasuries	$	633,970	$	2,014	$	(48)	$	635,936
Certificates of deposit	960		—		—		960
Corporate bonds	185,827		824		(43)		186,608
Commercial paper	74,378		—		—		74,378
Equity securities	20,017		—		(7,147)		12,870
Total	$	915,152	$	2,838	$	(7,238)	$	910,752

Fair value measurements (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis

The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2020 and 2019 (in thousands):


	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
December 31, 2020
Assets:
Cash and cash equivalents	$	317,705	$	317,705	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries	675,271		—		675,271		—
Corporate bonds	197,563		—		197,563		—
Commercial paper	77,950		—		77,950		—
Equity securities	5,653		5,653		—		—
Total assets	$	1,274,142	$	323,358	$	950,784	$	—
Liabilities:
Contingent consideration	$	1,509	$	—	$	—	$	1,509
Total liabilities	$	1,509	$	—	$	—	$	1,509
December 31, 2019
Assets:
Cash and cash equivalents	$	327,214	$	311,245	$	15,969	$	—
Marketable securities:
U.S. government agency securities and treasuries	635,936		—		635,936		—
Certificates of deposit	960		—		960		—
Corporate bonds	186,608		—		186,608		—
Commercial paper	74,378		—		74,378
Equity securities	12,870		12,870		—		—
Total assets	$	1,237,966	$	324,115	$	913,851	$	—
Liabilities:
Contingent consideration	$	7,977	$	—	$	—	$	7,977
Total liabilities	$	7,977	$	—	$	—	$	7,977

Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value


	Less than 12 months				12 months or greater				Total
Description	Fair value		Unrealized losses		Fair value		Unrealized losses		Fair value		Unrealized losses
December 31, 2020
U.S. government agency securities and treasuries	$	211,384	$	(74)	$	—	$	—	$	211,384	$	(74)
Corporate bonds	76,598		(40)		1,205		—		77,803		(40)
Total	$	287,982	$	(114)	$	1,205	$	—	$	289,187	$	(114)
December 31, 2019
U.S. government agency securities and treasuries	$	13,234	$	(3)	$	79,618	$	(45)	$	92,852	$	(48)
Corporate bonds	53,983		(43)		—		—		53,983		(43)
Total	$	67,217	$	(46)	$	79,618	$	(45)	$	146,835	$	(91)

Property, plant and equipment, net (Tables)

12 Months Ended

Dec. 31, 2020

Property, Plant and Equipment [Abstract]

Summary of Property, Plant and Equipment Net

Property, plant and equipment, net, consists of the following (in thousands):


	As of December 31,
	2020		2019
Land	$	1,210	$	1,210
Building	15,745		15,664
Computer equipment and software	6,950		6,947
Office equipment	7,665		7,599
Laboratory equipment	55,521		44,560
Leasehold improvements	34,286		33,788
Construction-in-progress	92,514		77,981
Total property, plant and equipment	213,891		187,749
Less accumulated depreciation and amortization	(51,060)		(36,573)
Property, plant and equipment, net	$	162,831	$	151,176

Restricted cash (Tables)

12 Months Ended

Dec. 31, 2020

Other Assets, Noncurrent [Abstract]

Schedule of collateralized bank account of financial institution


	As of December 31,
	2020		2019
60 Binney Street lease	$	13,763	$	13,763
50 Binney Street sublease	40,072		40,072
Other	2,188		660
Total restricted cash	$	56,023	$	54,495

Accrued expenses and other current liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Payables and Accruals [Abstract]

Summary of accrued expenses and other current liabilities

Accrued expenses and other current liabilities consist of the following (in thousands):


	As of December 31,
	2020		2019
Employee compensation	$	55,802	$	44,679
Manufacturing costs	22,571		23,126
Clinical and contract research organization costs	23,766		16,799
Collaboration research costs	20,004		27,142
Property, plant, and equipment	789		2,354
License and milestone fees	278		300
Professional fees	1,541		1,827
Other	20,655		25,329
Total accrued expenses and other current liabilities	$	145,406	$	141,556

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Lease, Cost


	For the year ended December 31,
	2020			2019
Lease cost (1)
Operating lease cost	$	35,049		$	35,346
Total lease cost	$	35,049		$	35,346

Other information
Operating cash flows used for operating leases	$	32,097		$	31,026
Weighted average remaining lease term	6.4 years			7.4 years
Weighted average discount rate	6.35		%	6.70		%

(1) Short-term lease costs and variable lease costs incurred by the Company for the twelve months ended December 31, 2020 and 2019 were immaterial.

Summary of Future Minimum Commitments

As of December 31, 2020, future minimum commitments under ASC 842 under the Company’s operating leases were as follows (in thousands):


	As of
Maturity of lease liabilities	December 31, 2020
2021	$	36,430
2022	36,899
2023	37,387
2024	37,398
2025	32,082
2026 and thereafter	56,095
Total lease payments	236,291
Less: imputed interest	(43,270)
Total operating lease liabilities	$	193,021

Commitments and contingencies (Tables)

12 Months Ended

Dec. 31, 2020

Commitments and Contingencies Disclosure [Abstract]

Schedule of Non-cancelable Contractual Obligations

The following table presents non-cancelable contractual obligations arising from these arrangements:


Years ended December 31,	Purchase commitment
2021	$	83,885
2022	12,611
2023	—
2024	—
2025	—
2026 and thereafter	—
Total purchase commitments	$	96,496

Common stock and preferred stock (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Future Issuance of Common Stock Shares

The Company has reserved for future issuance the following number of shares of common stock (in thousands):


	As of December 31,
	2020	2019
Options to purchase common stock	6,262	5,483
Restricted stock units	1,495	1,127
2013 Stock Option and Incentive Plan	2,545	2,007
2013 Employee Stock Purchase Plan	67	147
	10,369	8,764

Collaborative arrangements (Tables)

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied


	Ide-cel transaction price as of December 31, 2020
Upfront non-refundable payments received prior to May 2020 contract modification (1)	$	120,000
Allocated portion of the upfront non-refundable payment received in connection with the Amended Ide-cel CCPS and bb21217 License Agreement (2)	184,029
Estimated variable consideration (3)	83,900
	$	387,929

(2) This represents the portion of the $200.0 million up-front payment received under the Amended Ide-cel CCPS and Amended bb21217 License Agreement which was allocated to ide-cel.

(3) Estimated variable consideration represents the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development.


	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of December 31, 2020
Ide-cel research and development services	$	40,912	$	—
Ide-cel license and manufacturing services	347,017		1,082
	$	387,929	$	1,082


	For the years ended December 31,
	2020		2019		2018
ASC 808 ide-cel license and manufacturing revenue - U.S. (1)	$	108,196	$	—	$	6,255
ASC 808 ide-cel research and development expense - U.S. (1)	$	41,599	$	32,415	$	8,689


	For the years ended December 31
	2020		2019		2018
ASC 606 ide-cel license and manufacturing revenue - ex-U.S.	$	99,053	$	25,522	$	35,900


(in thousands)	bb21217 transaction price as of December 31, 2020
Upfront non-refundable payments received prior to May 2020 contract modification (1)	$	15,000
Allocated portion of the up-front non-refundable payment received in connection with the Amended Ide-cel CCPS and bb21217 License Agreement (2)	15,971
Estimated variable consideration (3)	1,803
	$	32,774

(2) This represents the portion of the $200.0 million up-front payment received under the Amended Ide-cel CCPS and Amended bb21217 License Agreement which was allocated to bb21217.

(3) Estimated variable consideration represents the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development.


	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of December 31, 2020
bb21217 research and development services	$	5,444	$	—
bb21217 license and manufacturing services	27,330		27,330
	$	32,774	$	27,330

Changes in Balances of Company's Receivables and Contract Liabilities

The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the twelve months ended December 31, 2020 (in thousands):


	Balance at December 31, 2019		Additions		Deductions		Balance at December 31, 2020
Receivables	$	400	$	12,400	$	(12,400)	$	400
Contract liabilities:
Deferred revenue	$	18,265	$	200,000	$	(191,683)	$	26,582

Intangible assets (Tables)

12 Months Ended

Dec. 31, 2020

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Finite-Lived Intangible Assets

Intangible assets, net of accumulated amortization, are summarized as follows (in thousands):


	As of December 31,						As of December 31,
	2020						2019
	Cost		Accumulated amortization		Net		Cost		Accumulated amortization		Net
Developed technology	$	30,100	$	(24,456)	$	5,644	$	30,100	$	(20,694)	$	9,406
In-licensed rights	5,224		(827)		4,397		5,224		(304)		4,920
Total	$	35,324	$	(25,283)	$	10,041	$	35,324	$	(20,998)	$	14,326

Schedule of Finite-Lived Intangible Assets, Future Amortization Expense

The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands):


	As of December 31, 2020
2021	$	4,285
2022	2,404
2023	522
2024	522
2025	522
2026 and thereafter	1,786
Total	$	10,041

Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of Stock-Based Compensation Expense by Award Type

Stock-based compensation expense recognized by award type is as follows (in thousands):


	Year Ended December 31,
	2020		2019		2018
Stock options	$	93,977	$	95,668	$	83,449
Restricted stock units	49,326		63,580		26,628
Employee stock purchase plan and other	13,328		1,381		759
	$	156,631	$	160,629	$	110,836

Schedule of Stock-Based Compensation Expense by Classification

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Year Ended December 31,
	2020		2019		2018
Research and development	$	72,239	$	80,139	$	54,422
Selling, general and administrative	84,392		80,490		56,414
	$	156,631	$	160,629	$	110,836

Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted

The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:


	Year Ended December 31,
	2020		2019		2018
Expected volatility	69.5	%	70.7	%	75.5	%
Expected term (in years)	6.0		6.0		6.0
Risk-free interest rate	1.4	%	2.3	%	2.7	%
Expected dividend yield	0.0	%	0.0	%	0.0	%

Summary of Stock Option Activity Under Plan

The following table summarizes the stock option activity under the Company’s equity awards plans:


	Shares (in thousands)	Weighted- average exercise price per share		Weighted- average contractual life (in years)	Aggregate intrinsic value (a) (in thousands)
Outstanding at December 31, 2019	5,483	$	116.30
Granted	1,585	$	69.40
Exercised	(95)	$	19.43
Canceled or forfeited	(711)	$	124.00
Outstanding at December 31, 2020	6,262	$	105.02	7.0	$	15,716
Exercisable at December 31, 2020	3,669	$	107.11	5.8	$	15,659
Vested and expected to vest at December 31, 2020	6,262	$	105.02	7.0	$	15,716

Summary of Restricted Common Stock Awards

The following table summarizes the restricted stock unit activity under the Company’s equity award plans:


	Shares (in thousands)	Weighted-average grant date fair value
Unvested balance at December 31, 2019	1,127	$	146.10
Granted	1,018	70.18
Vested	(433)	130.10
Forfeited	(217)	123.32
Unvested balance at December 31, 2020	1,495	$	102.34

Income taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule Components of Loss Before Income Taxes

The components of loss before income taxes were as follows (in thousands):


	Year ended December 31,
	2020		2019		2018
U.S.	$	(489,091)	$	(649,172)	$	(440,473)
Foreign	(128,918)		(140,981)		(114,965)
Total	$	(618,009)	$	(790,153)	$	(555,438)

Summary of Provision for (Benefit from) Income Taxes

The provision for (benefit from) income taxes were as follows (in thousands):


	Year ended December 31,
	2020		2019		2018
Current:
Federal	$	—	$	—	$	—
State	2		7		324
Foreign	684		612		222
Deferred:
Federal	—		(966)		(307)
State	—		(198)		(52)
Foreign	—		—		—
Total income tax expense (benefit)	$	686	$	(545)	$	187

Reconciliation of Income Tax Provision (Benefit) Computed at the Statutory Federal Income Tax Rate to Effective Income Tax Rate (Provision) Benefit as Reflected in the Financial Statements

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate as reflected in the financial statements is as follows:


	Year ended December 31,
	2020		2019		2018
Federal income tax expense at statutory rate	21.0	%	21.0	%	21.0	%
State income tax, net of federal benefit	3.1	%	3.7	%	5.1	%
Permanent differences	(0.6)	%	(0.8)	%	(0.7)	%
Stock-based compensation	(2.4)	%	(0.7)	%	1.6	%
Research and development credit	6.0	%	5.4	%	6.5	%
Foreign differential	(4.6)	%	(3.7)	%	(4.4)	%
Federal tax rate change	—	%	—	%	0.1	%
Other	(0.3)	%	0.8	%	(0.1)	%
Change in valuation allowance	(22.3)	%	(25.6)	%	(29.1)	%
Effective income tax rate (expense) benefit	(0.1)	%	0.1	%	—	%

Components of Deferred Tax Assets and Liabilities

The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands):


	Year ended December 31,
	2020		2019
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$	546,098	$	439,839
Tax credit carryforwards (federal and state)	246,742		213,810
Capitalized license fees and research and development expenses	14,402		16,295
Deferred revenue	15,644		15,119
Stock-based compensation	51,828		46,111
Lease liabilities	48,680		52,631
Accruals and other	14,536		15,432
Total deferred tax assets	937,930		799,237
Intangible assets	(1,499)		(2,518)
Right-of-use assets	(45,976)		(49,480)
Fixed assets	(7,576)		(2,670)
Less valuation allowance	(882,879)		(744,569)
Net deferred taxes	$	—	$	—

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):


	Unrecognized tax benefits
Balance as of December 31, 2018	$	12,095
Increases (decreases) for tax positions related to current period	3,554
Increases (decreases) for tax positions related to prior periods	296
Balance as of December 31, 2019	15,945
Increases (decreases) for tax positions related to current period	3,149
Increases (decreases) for tax positions related to prior periods	(100)
Balance as of December 31, 2020	18,994

Net loss per share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share


	Year ended December 31, 2020,
	2020	2019	2018
Outstanding stock options	6,262	5,483	4,643
Restricted stock units	1,495	1,127	931
ESPP shares and other	326	19	10
	8,083	6,629	5,584

Selected quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information


	2020
	First quarter		Second quarter		Third quarter		Fourth quarter		Total
	(in thousands, except per share data)
Total revenues	$	21,863	$	198,890	$	19,273	$	10,708	$	250,734
Total operating expenses	225,288		224,835		208,967		214,690		873,780
Loss from operations	(203,425)		(25,945)		(189,694)		(203,982)		(623,046)
Net loss	(202,611)		(21,465)		(194,745)		(199,874)		(618,695)
Net loss per share applicable to common stockholders - basic and diluted	$	(3.64)	$	(0.36)	$	(2.94)	$	(3.01)	$	(9.95)


	2019
	First quarter		Second quarter		Third quarter		Fourth quarter		Total
	(in thousands, except per share data)
Total revenues	$	12,471	$	13,296	$	8,910	$	9,997	$	44,674
Total operating expenses	183,645		215,998		219,326		240,531		859,500
Loss from operations	(171,174)		(202,702)		(210,416)		(230,534)		(814,826)
Net loss	(164,446)		(195,782)		(206,033)		(223,347)		(789,608)
Net loss per share applicable to common stockholders - basic and diluted	$	(2.99)	$	(3.55)	$	(3.73)	$	(4.04)	$	(14.31)

Description of the business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ (2,900,547)				$ (2,281,852)				$ (2,900,547)	$ (2,281,852)
Net loss	(199,874)	$ (194,745)	$ (21,465)	$ (202,611)	$ (223,347)	$ (206,033)	$ (195,782)	$ (164,446)	(618,695)	(789,608)	$ (555,625)
Net cash provided by (used in) operating activities									(470,351)	$ (564,384)	$ (413,426)
Cash, cash equivalents and marketable securities	$ 1,270,000								$ 1,270,000

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669625-108580

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
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-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
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-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22658-107794

+ Details

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2020
Building
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	40 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	3 years
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	2 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	5 years
Laboratory equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	2 years
Laboratory equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of assets	5 years

Summary of significant accounting policies and basis of presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) target	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of operating segments \| target	1
Goodwill, impaired, accumulated impairment loss	$ 0
Interest income net of amortization of premium and accretion of discount	11,500,000	$ 34,800,000	$ 30,100,000
Interest expenses	$ 0.0	$ 0.0	$ 15,500,000

Marketable securities - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, noncurrent	$ 0	$ 0

Marketable securities - Summary of Marketable Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Equity securities, amortized cost / cost	$ 20,017	$ 20,017
Marketable securities, amortized cost / cost	970,180	915,152
Equity securities, unrealized gains	0	0
Marketable securities, unrealized gains	735	2,838
Equity securities, unrealized losses	(14,364)	(7,147)
Marketable securities, unrealized losses	(14,478)	(7,238)
Equity securities	5,653	12,870
Marketable securities, fair value	956,437	910,752
U.S. government agency securities and treasuries
Debt Securities, Available-for-sale [Line Items]
Debt securities, amortized cost / cost	675,043	633,970
Debt securities, unrealized gains	302	2,014
Debt securities, unrealized losses	(74)	(48)
Debt securities, fair value	675,271	635,936
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Debt securities, amortized cost / cost		960
Debt securities, unrealized gains		0
Debt securities, unrealized losses		0
Debt securities, fair value		960
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Debt securities, amortized cost / cost	197,171	185,827
Debt securities, unrealized gains	432	824
Debt securities, unrealized losses	(40)	(43)
Debt securities, fair value	197,563	186,608
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Debt securities, amortized cost / cost	77,949	74,378
Debt securities, unrealized gains	1	0
Debt securities, unrealized losses	0	0
Debt securities, fair value	$ 77,950	$ 74,378

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Equity securities	$ 5,653	$ 12,870
U.S. government agency securities and treasuries
Assets:
Marketable securities	675,271	635,936
Certificates of deposit
Assets:
Marketable securities		960
Corporate bonds
Assets:
Marketable securities	197,563	186,608
Commercial paper
Assets:
Marketable securities	77,950	74,378
Fair value, measurements, recurring
Assets:
Cash and cash equivalents	317,705	327,214
Equity securities	5,653	12,870
Total assets	1,274,142	1,237,966
Liabilities:
Contingent consideration	1,509	7,977
Total liabilities	1,509	7,977
Fair value, measurements, recurring \| U.S. government agency securities and treasuries
Assets:
Marketable securities	675,271
Fair value, measurements, recurring \| U.S. government agency securities and treasuries
Assets:
Marketable securities		635,936
Fair value, measurements, recurring \| Certificates of deposit
Assets:
Marketable securities		960
Fair value, measurements, recurring \| Corporate bonds
Assets:
Marketable securities	197,563	186,608
Fair value, measurements, recurring \| Commercial paper
Assets:
Marketable securities	77,950	74,378
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	317,705	311,245
Equity securities	5,653	12,870
Total assets	323,358	324,115
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| U.S. government agency securities and treasuries
Assets:
Marketable securities	0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| U.S. government agency securities and treasuries
Assets:
Marketable securities		0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| Certificates of deposit
Assets:
Marketable securities		0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| Corporate bonds
Assets:
Marketable securities	0	0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| Commercial paper
Assets:
Marketable securities	0	0
Fair value, measurements, recurring \| Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	0	15,969
Equity securities	0	0
Total assets	950,784	913,851
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| U.S. government agency securities and treasuries
Assets:
Marketable securities	675,271
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| U.S. government agency securities and treasuries
Assets:
Marketable securities		635,936
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| Certificates of deposit
Assets:
Marketable securities		960
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| Corporate bonds
Assets:
Marketable securities	197,563	186,608
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| Commercial paper
Assets:
Marketable securities	77,950	74,378
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Equity securities	0	0
Total assets	0	0
Liabilities:
Contingent consideration	1,509	7,977
Total liabilities	1,509	7,977
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| U.S. government agency securities and treasuries
Assets:
Marketable securities	0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| U.S. government agency securities and treasuries
Assets:
Marketable securities		0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| Certificates of deposit
Assets:
Marketable securities		0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| Corporate bonds
Assets:
Marketable securities	0	0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| Commercial paper
Assets:
Marketable securities	$ 0

Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended	12 Months Ended
Fair value measurements - Narrative (Detail) - USD ($)	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) recognized on sale		$ 0	$ 0
Interest receivable		3,100,000	3,600,000
Interest receivable, write-off		0	0
Investment at fair value		5,653,000	12,870,000
Subsequent Event
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	$ 7,300,000
Proceeds from sale of equity securities	$ 31,300,000
Other Income (Expense), Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on equity securities		$ 7,200,000	$ 9,300,000	$ (2,200,000)

Fair value measurements - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair value
Less than 12 months	$ 287,982	$ 67,217
12 months or greater	1,205	79,618
Total	289,187	146,835
Unrealized losses
Less than 12 months	(114)	(46)
12 months or greater	0	(45)
Total	(114)	(91)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	211,384	13,234
12 months or greater	0	79,618
Total	211,384	92,852
Unrealized losses
Less than 12 months	(74)	(3)
12 months or greater	0	(45)
Total	(74)	(48)
Corporate bonds
Fair value
Less than 12 months	76,598	53,983
12 months or greater	1,205	0
Total	77,803	53,983
Unrealized losses
Less than 12 months	(40)	(43)
12 months or greater	0	0
Total	$ (40)	$ (43)

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 213,891	$ 187,749
Less accumulated depreciation and amortization	(51,060)	(36,573)
Property, plant and equipment, net	162,831	151,176
Land
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	1,210	1,210
Building
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	15,745	15,664
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	6,950	6,947
Office equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	7,665	7,599
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	55,521	44,560
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	34,286	33,788
Construction-in-progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 92,514	$ 77,981

Property, plant and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 15,100	$ 13,400	$ 13,400
Property and equipment, gross	213,891	187,749
Construction In Progress North Carolina Manufacturing Facility
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 91,100	$ 74,200

Restricted cash - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Restricted cash balance	$ 56,023	$ 54,495
Letter of Credit
Debt and Equity Securities, FV-NI [Line Items]
Restricted cash balance	$ 54,500	$ 54,500

Restricted cash - Schedule of Collateralized Bank Account of Financial Institution (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 56,023	$ 54,495
60 Binney Street lease
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	13,763	13,763
50 Binney Street sublease
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	40,072	40,072
Other
Debt and Equity Securities, FV-NI [Line Items]
Total restricted cash	$ 2,188	$ 660

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Employee compensation	$ 55,802	$ 44,679
Manufacturing costs	22,571	23,126
Clinical and contract research organization costs	23,766	16,799
Collaboration research costs	20,004	27,142
Property, plant, and equipment	789	2,354
License and milestone fees	278	300
Professional fees	1,541	1,827
Other	20,655	25,329
Total accrued expenses and other current liabilities	$ 145,406	$ 141,556

Leases - Additional Information (Details) € in Millions		1 Months Ended						12 Months Ended			22 Months Ended
Leases - Additional Information (Details) € in Millions	Sep. 21, 2015 USD ($) ft² $ / ft²	Sep. 30, 2020 ft² $ / ft²	Jul. 31, 2020 USD ($)	Mar. 31, 2020 EUR (€)	Sep. 30, 2019 ft² lease_term $ / ft²	Apr. 30, 2019 USD ($) ft² $ / ft²	Jul. 31, 2018 ft² $ / ft²	Dec. 31, 2020 USD ($) lease_term	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Nov. 30, 2016	Jun. 03, 2016
Lessee, Lease, Description [Line Items]
lease arrangements annual rent increase percentage						3.00%
Operating lease, expense								$ 47,700,000	$ 45,300,000
Rent expense										$ 9,800,000
Seattle, Washington
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) \| ft²		22,188			22,188		36,126
Annual lease rent per square foot (in dollars per sq ft) \| $ / ft²		62.80			62.80		54.00
Lease rent, annual increase percentage		2.50%			2.50%		2.50%
Lease extension terms					5 years
Lessee, operating lease, term of contract extension					16 months
Lessee, operating lease, number of contracts \| lease_term					2
60 Binney Street lease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) \| ft²	253,108
Annual lease rent per square foot (in dollars per sq ft) \| $ / ft²	72.50
Lease payments base annual rent	$ 18,400,000
Lease rent, annual increase percentage	1.75%
Collateralized letter of credit	$ 13,800,000
Decrease In letter of credit under the terms of the lease	9,200,000
Landlord contribution for cost of construction and tenant improvements	$ 42,400,000
Lessee, operating lease, number of terms \| lease_term								2
Lease extension terms								5 years
50 Binney Street sublease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) \| ft²						267,278
Collateralized letter of credit						$ 40,100,000
Rate of area for sublease per square feet (in dollars per sq ft) \| $ / ft²						99.95
Lessee, operating lease, annual expense						$ 26,700,000
Lessee, operating sublease, payment commencement, term						90 days
Upfront payment for purchase of furniture and equipment						$ 7,500,000
Manufacturing Facility
Lessee, Lease, Description [Line Items]
Lease extension terms												3 years
Lessee, operating lease, annual expense								$ 5,100,000
Lease period			5 years									5 years	12 years
Milestone paid											$ 12,000,000.0
Lessee, operating lease, termination fees, term								24 months
Lessee, operating lease, maximum potential annual maintenance and production fees \| €				€ 16.5
Lessee, operating lease, termination notice period			18 months	12 months
Lessee, operating lease, maximum potential annual maintenance fees			$ 5,400,000

Leases - Summary of Lease Costs and Other Information Pertaining to Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 35,049	$ 35,346
Total lease cost	35,049	35,346
Operating cash flows used for operating leases	$ 32,097	$ 31,026
Weighted average remaining lease term	6 years 4 months 24 days	7 years 4 months 24 days
Weighted average discount rate	6.35%	6.70%
Short-term lease, cost	$ 0
Variable lease, cost	$ 0

Leases - Schedule of Future Minimum Commitments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Leases [Abstract]
2021	$ 36,430
2022	36,899
2023	37,387
2024	37,398
2025	32,082
2026 and thereafter	56,095
Total lease payments	236,291
Less: imputed interest	(43,270)
Total operating lease liabilities	$ 193,021

Commitments and contingencies - Additional Information (Detail) - Pregenen $ in Millions	Jun. 30, 2014 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments	$ 120.0
Clinical Milestone Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments	20.1
Commercial Milestones Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments	$ 99.9

Commitments and contingencies - Schedule of Non-cancelable Contractual Obligations (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2021	$ 83,885
2022	12,611
2023	0
2024	0
2025	0
2026	0
Total purchase commitments	$ 96,496

Common stock and preferred stock - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended
	May 31, 2020 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Jul. 31, 2018 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2020 vote shares	Dec. 31, 2019 shares
Equity [Abstract]
Common stock, shares authorized (in shares)					125,000,000	125,000,000
Common stock, number of votes (in votes per share) \| vote					1
Common stock, shares issued (in shares)					66,432,000	55,368,000
Common stock, shares outstanding (in shares)					66,432,000	55,368,000
Number of shares issued in public offering (in shares)	10,500,000	400,000	3,900,000	300,000
Shares issued (in dollars per share) \| $ / shares	$ 55.00	$ 238.10	$ 162.50	$ 185.00
Proceeds from public offering of common stock, net of issuance costs \| $		$ 100.0	$ 600.6	$ 48.7
Common stock amount attributable to prepayment of joint research activities \| $		$ 45.5
Proceeds from issuance of common stock \| $	$ 541.5
Preferred stock, shares authorized (in shares)					5,000,000	5,000,000
Preferred Stock, shares issued (in shares)					0	0
Preferred stock, shares outstanding (in shares)					0	0

Common stock and preferred stock - Summary of Future Issuance of Common Stock Shares (Detail) - shares shares in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Capitalization [Line Items]
Common shares reserved for future issuance (in shares)	10,369	8,764
2013 Stock Option and Incentive Plan
Schedule of Capitalization [Line Items]
Common shares reserved for future issuance (in shares)	2,545	2,007
2013 Employee Stock Purchase Plan
Schedule of Capitalization [Line Items]
Common shares reserved for future issuance (in shares)	67	147
Options to purchase common stock
Schedule of Capitalization [Line Items]
Common shares reserved for future issuance (in shares)	6,262	5,483
Restricted stock units
Schedule of Capitalization [Line Items]
Common shares reserved for future issuance (in shares)	1,495	1,127

Collaborative arrangements - Additional Information (Detail)	1 Months Ended						3 Months Ended	12 Months Ended
	May 31, 2020 USD ($)	Aug. 31, 2018 USD ($) target $ / shares shares	Sep. 30, 2017 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($) product	Mar. 31, 2013 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue								$ (9,817,000)	$ 16,674,000	$ 41,872,000
Investment in common stock	$ 541,500,000
Common shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) \| shares										420,000
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								26,582,000	18,265,000
Term of collaboration agreement						3 years
Deferred revenue								(8,200,000)
Bristol-Myers Squibb \| Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								40,900,000
Bristol-Myers Squibb \| BMS collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability						$ 75,000,000.0
Bristol-Myers Squibb \| Amended collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability					$ 25,000,000.0
Term of collaboration agreement					3 years
Products, number of candidates \| product					2
Bristol-Myers Squibb \| Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								347,017,000
Remaining performance obligation revenue								1,082,000
Bristol-Myers Squibb \| Ide-cel license and manufacturing services \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								800,000	8,500,000
Remaining performance obligation revenue								1,100,000
Bristol-Myers Squibb \| Ide-cel license and manufacturing services \| License and manufacturing services \| Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								99,053,000	25,522,000	$ 35,900,000
Bristol-Myers Squibb \| Ide-cel license agreement \| First product candidates
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received				$ 10,000,000.0
Bristol-Myers Squibb \| Ide-cel license agreement \| First product candidates \| U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				85,000,000.0
Bristol-Myers Squibb \| Ide-cel license agreement \| First product candidates \| Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				$ 55,000,000.0
Bristol-Myers Squibb \| Ide-cel co-development, co-promote and profit share agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable								387,929,000
Contract with customer development milestone payment achieved							$ 10,000,000.0
Bristol-Myers Squibb \| Ide-cel co-development, co-promote and profit share agreement \| Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable											$ 60,000,000.0
Bristol-Myers Squibb \| Ide-cel co-development, co-promote and profit share agreement \| Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable											$ 10,000,000.0
Bristol-Myers Squibb \| bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received			$ 15,000,000.0					15,000,000
Contract with customer development milestone payment achieved								$ 10,000,000.0
Additional fee receivable if option to co-develop and co-promote is not exercised			10,000,000.0
Deferred revenue recognition period								2 years
Milestone and royalty obligation buy-out								$ 15,971,000
Bristol-Myers Squibb \| bb21217 license agreement \| U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable			85,000,000.0
Bristol-Myers Squibb \| bb21217 license agreement \| Outside of U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable			$ 55,000,000.0
Bristol-Myers Squibb \| bb21217 license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								27,330,000
Remaining performance obligation revenue								27,330,000
Bristol-Myers Squibb \| bb21217 license and manufacturing services \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract with customer liability								25,800,000	9,800,000
Remaining performance obligation revenue								27,300,000
Deferred revenue balance recognized as gross revenues								0	0	0
Bristol-Myers Squibb \| Amended Ide Cel Co Development, Co-Promote And Profit Share Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue								191,700,000
Catch-up adjustment to revenue								169,200,000
Milestone and royalty obligation buy-out	$ 200,000,000.0							184,029,000
Bristol-Myers Squibb \| Ide-cel Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								0	2,300,000	5,800,000
Bristol-Myers Squibb \| bb21217 research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								5,444,000
Remaining performance obligation revenue								0
Bristol-Myers Squibb \| bb21217 research and development services \| Phase I, Initial Obligation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								0	2,200,000	2,900,000
Bristol-Myers Squibb \| bb21217 research and development services \| Phase I, Additional Obligation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue								12,400,000	400,000	$ 0.0
Regeneron Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets \| target		6
Research collaboration term		5 years
Joint research activities remaining to be recognized								30,800,000	38,200,000
Regeneron Collaboration Agreement \| Service revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues								7,400,000	$ 5,700,000
Regeneron Collaboration Agreement \| Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable		$ 130,000,000.0
Regeneron Collaboration Agreement \| Research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price								100,000,000.0
Purchase price premium								$ 37,000,000.0
Collaborative arrangement amortization period								5 years
Collaborative arrangement amount attributed to equity sold								$ 54,500,000
Collaborative arrangement amount attributed to joint research activities								$ 45,500,000
Regeneron Collaboration Agreement \| Share purchase agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) \| shares		400,000
Common stock price per share (in dollars per share) \| $ / shares		$ 238.10
Investment in common stock		$ 100,000,000.0
Purchase price premium		$ 37,000,000.0
Collaborative arrangement research initial funding obligation, percentage		50.00%
Regeneron Collaboration Agreement \| Share purchase agreement \| Common shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock shares issued (in shares) \| shares								400,000
Investment in common stock		$ 63,000,000.0

Collaborative arrangements - Summary of Total Transaction Price (Detail) - Bristol-Myers Squibb - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	May 31, 2020
BMS collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment	$ 120,000
Ide-cel co-development, co-promote and profit share agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestone payments receivable	387,929
Ide-cel Research And Development Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	40,912
Transaction price unsatisfied	0
Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	347,017
Transaction price unsatisfied	1,082
Ide-cel Transaction Price
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	387,929
Transaction price unsatisfied	1,082
Amended Ide Cel Co Development, Co-Promote And Profit Share Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone and royalty obligation buy-out	184,029	$ 200,000
Vectors and associated payload
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated variable consideration	83,900
bb21217 transaction price
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	32,774
Transaction price unsatisfied	27,330
bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone and royalty obligation buy-out	15,971
Manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated variable consideration	1,803
bb21217 research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	5,444
Transaction price unsatisfied	0
bb21217 license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price	27,330
Transaction price unsatisfied	$ 27,330

Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 587,956	$ 582,413	$ 448,589
Bristol-Myers Squibb \| U.S. \| Ide Cel Revenue Services \| License and manufacturing services \| ASU 2014-09
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	108,196	0	6,255
Bristol-Myers Squibb \| U.S. \| Ide-cel Research And Development Services \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 41,599	$ 32,415	$ 8,689

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130543-203045

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130545-203045

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details

Collaborative arrangements - Summary of Revenue Recognized Related to Research and Development Services (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Bristol-Myers Squibb \| Ide-cel license and manufacturing services \| Outside of U.S. \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	$ 99,053	$ 25,522	$ 35,900

- Definition

+ References

+ Details

Collaborative arrangements - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Bristol-Myers Squibb $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Receivables
Balance at beginning of period	$ 400
Additions	12,400
Deductions	(12,400)
Balance at end of period	400
Contract liabilities:
Balance at beginning of period	18,265
Additions	200,000
Deductions	(191,683)
Balance at end of period	$ 26,582

Royalty and other revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended	12 Months Ended
	Aug. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 28, 2017 performance_obligation	Apr. 26, 2017 performance_obligation
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				$ 5,396,000	$ 2,978,000	$ 885,000
Royalty and other revenue
License And Royalty Revenue [Line Items]
Revenue				21,076,000	8,205,000	2,226,000
Novartis Pharma AG
License And Royalty Revenue [Line Items]
License agreement upfront payment		$ 7,500,000
Revenue recognized upon achievement of related milestone	$ 2,500,000
Novartis Pharma AG \| Royalty and other revenue
License And Royalty Revenue [Line Items]
Revenue				21,100,000	8,200,000	2,200,000
Cost of license and royalty revenue				5,400,000	3,000,000.0	900,000
Novartis Pharma AG \| Topic 606
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception \| performance_obligation								1
Revenue			$ 7,500,000
Novartis Pharma AG \| Topic 606 \| Regulatory Milestones Payments
License And Royalty Revenue [Line Items]
Revenue			2,500,000
Orchard Therapeutics Limited
License And Royalty Revenue [Line Items]
License agreement upfront payment		3,000,000.0
Orchard Therapeutics Limited \| Royalty and other revenue
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				0	0	0
Orchard Therapeutics Limited \| Topic 606
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception \| performance_obligation							1
Revenue			$ 3,000,000.0
Number of unsatisfied performance obligation identified at date of contract inception \| performance_obligation							0
Orchard Therapeutics Limited \| Topic 606 \| License
License And Royalty Revenue [Line Items]
Revenue				$ 0	$ 0	$ 0
Regulatory Milestones Payments \| Novartis Pharma AG
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		7,500,000
Each Subsequently Licensed Product \| Novartis Pharma AG
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		1,100,000
Potential Milestones Payments \| Orchard Therapeutics Limited
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 1,300,000

Intangible assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 4.3	$ 4.1	$ 3.8
Developed technology
Finite-Lived Intangible Assets [Line Items]
Gene editing platform expected useful life	8 years
In-licensed rights
Finite-Lived Intangible Assets [Line Items]
Gene editing platform expected useful life	10 years

Intangible assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Cost	$ 35,324	$ 35,324
Accumulated amortization	(25,283)	(20,998)
Net	10,041	14,326
Developed technology
Finite-Lived Intangible Assets [Line Items]
Cost	30,100	30,100
Accumulated amortization	(24,456)	(20,694)
Net	5,644	9,406
In-licensed rights
Finite-Lived Intangible Assets [Line Items]
Cost	5,224	5,224
Accumulated amortization	(827)	(304)
Net	$ 4,397	$ 4,920

Intangible assets - Schedule of Finite-Lived Intangible Assets, Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
2021	$ 4,285
2022	2,404
2023	522
2024	522
2025	522
2026 and thereafter	1,786
Net	$ 10,041	$ 14,326

Stock-based compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 24, 2013	Jun. 03, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan (in shares)		2,200,000
Number of shares available for issuance (in shares)				2,600,000
Stock-based compensation expense				$ 156,631	$ 160,629	$ 110,836
Weighted average grant date fair value of options granted (in dollars per share)				$ 43.24	$ 83.44	$ 125.12
Intrinsic value of stock options exercised				$ 4,000	$ 29,000	$ 73,100
Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan (in shares)	2,700,000
Performance and service condition based restricted stock unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 20,100
Performance and service condition based restricted stock unit \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards outstanding (in shares)				100,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 93,977	95,668	83,449
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan				$ 146,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				2 years 1 month 6 days
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 49,326	$ 63,580	26,628
Unvested restricted stock awards outstanding (in shares)				1,495,000	1,127,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan				$ 89,300
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				2 years 4 months 24 days
Intrinsic value of restricted stock units vested				$ 30,900	$ 28,400	25,500
Employee stock purchase plan and other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for future issuance (in shares)								200,000
Stock-based compensation expense				13,328	$ 1,381	$ 759
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan				$ 400
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				3 months 18 days
Employee stock purchase plan and other \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares issued in period (less than) (in shares)				100,000	100,000
2013 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for future issuance (in shares)							1,000,000.0
Percentage of automatic increase in shares reserved and available for issuance determined based on shares outstanding				4.00%

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 156,631	$ 160,629	$ 110,836
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	93,977	95,668	83,449
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	49,326	63,580	26,628
Employee stock purchase plan and other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 13,328	$ 1,381	$ 759

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 156,631	$ 160,629	$ 110,836
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	72,239	80,139	54,422
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 84,392	$ 80,490	$ 56,414

Stock-based compensation - Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Expected volatility	69.50%	70.70%	75.50%
Expected term (in years)	6 years	6 years	6 years
Risk-free interest rate	1.40%	2.30%	2.70%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares
Shares (in thousands)
Outstanding at beginning of period (in shares) \| shares	5,483
Granted (in shares) \| shares	1,585
Exercised (in shares) \| shares	(95)
Canceled or forfeited (in shares) \| shares	(711)
Outstanding at end of period (in shares) \| shares	6,262
Exercisable at end of period (in shares) \| shares	3,669
Vested and expected to vest at end of period (in shares) \| shares	6,262
Weighted- average exercise price per share
Outstanding at beginning of period (in dollars per share) \| $ / shares	$ 116.30
Granted (in dollars per share) \| $ / shares	69.40
Exercised (in dollars per share) \| $ / shares	19.43
Canceled or forfeited (in dollars per share) \| $ / shares	124.00
Outstanding at end of period (in dollars per share) \| $ / shares	105.02
Exercisable at end of period (in dollars per share) \| $ / shares	107.11
Vested and expected to vest at end of period (in dollars per share) \| $ / shares	$ 105.02
Weighted- average contractual life (in years)
Outstanding at end of period	7 years
Exercisable at end of period	5 years 9 months 18 days
Vested and expected to vest at end of period	7 years
Aggregate intrinsic value (in thousands)
Outstanding at end of period \| $	$ 15,716
Exercisable at end of period \| $	15,659
Vested and expected to vest at end of period \| $	$ 15,716

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted stock units shares in Thousands	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Shares (in thousands)
Unvested balance at beginning of period (in shares) \| shares	1,127
Granted (in shares) \| shares	1,018
Vested (in shares) \| shares	(433)
Forfeited (in shares) \| shares	(217)
Unvested balance at end of period (in shares) \| shares	1,495
Weighted-average grant date fair value
Unvested balance at beginning of period (in dollars per share) \| $ / shares	$ 146.10
Granted (in dollars per share) \| $ / shares	70.18
Vested (in dollars per share) \| $ / shares	130.10
Forfeited (in dollars per share) \| $ / shares	123.32
Unvested balance at end of period (in dollars per share) \| $ / shares	$ 102.34

401(k) Savings plan - Additional Information (Detail) - 401 (k) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) plan		$ 4.6
Expenses related to 401(k) plan			$ 5.2	$ 4.6	$ 2.4
Scenario, Forecast
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) plan	$ 5.2

Income taxes - Schedule of Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S.	$ (489,091)	$ (649,172)	$ (440,473)
Foreign	(128,918)	(140,981)	(114,965)
Loss before income taxes	$ (618,009)	$ (790,153)	$ (555,438)

- Definition

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Income taxes - Summary of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 0	$ 0	$ 0
State	2	7	324
Foreign	684	612	222
Deferred:
Federal	0	(966)	(307)
State	0	(198)	(52)
Foreign	0	0	0
Total income tax expense (benefit)	$ 686	$ (545)	$ 187

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

Income taxes - Reconciliation of Income Tax Provision (Benefit) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	21.00%	21.00%
State income tax, net of federal benefit	3.10%	3.70%	5.10%
Permanent differences	(0.60%)	(0.80%)	(0.70%)
Stock-based compensation	(2.40%)	(0.70%)	1.60%
Research and development credit	6.00%	5.40%	6.50%
Foreign differential	(4.60%)	(3.70%)	(4.40%)
Federal tax rate change	0	0	0.001
Other	(0.30%)	0.80%	(0.10%)
Change in valuation allowance	(22.30%)	(25.60%)	(29.10%)
Effective income tax rate (expense) benefit	(0.10%)	0.10%	0.00%

Income taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components Of Income Tax Expense Benefit [Line Items]
Income tax expense (benefit)	$ 686,000	$ (545,000)	$ 187,000
Effective income tax rate	(0.10%)	0.10%	0.00%
Approximately valuation allowance increased	$ 138,300,000
Operating loss carryforwards, without expiration dates	1,320,000,000
Operating loss carryforwards, with expiration dates	711,000,000.0
Cumulative-effect adjustment to retained earnings	546,098,000	$ 439,839,000
Stockholders' equity	1,355,056,000	1,284,993,000	$ 1,885,070,000	$ 1,623,432,000
Income tax examination, penalties and interest accrued	0	0	0
Cumulative Effect, Period of Adoption, Adjustment
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity			6,564,000	(29,375,000)
Accumulated Deficit
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity	$ (2,900,547,000)	(2,281,852,000)	(1,498,808,000)	(913,808,000)
Accumulated Deficit \| Cumulative Effect, Period of Adoption, Adjustment
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity			6,564,000	$ (29,375,000)
Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Limitations on use of net operating losses and tax credit carryforwards, period	3 years
Limitations on use of net operating losses and tax credit carryforwards, percentage	50.00%
U.S.
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 2,030,000,000.00	1,620,000,000	1,100,000,000
U.S. \| Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	235,300,000	203,100,000	156,200,000
State and Local Jurisdiction
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	1,890,000,000	1,560,000,000	1,080,000,000.00
State and Local Jurisdiction \| Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 14,500,000	$ 13,600,000	$ 14,300,000

Income taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$ 546,098	$ 439,839
Tax credit carryforwards (federal and state)	246,742	213,810
Capitalized license fees and research and development expenses	14,402	16,295
Deferred revenue	15,644	15,119
Stock-based compensation	51,828	46,111
Lease liabilities	48,680	52,631
Accruals and other	14,536	15,432
Total deferred tax assets	937,930	799,237
Intangible assets	(1,499)	(2,518)
Right-of-use assets	(45,976)	(49,480)
Fixed assets	(7,576)	(2,670)
Less valuation allowance	(882,879)	(744,569)
Net deferred taxes	$ 0	$ 0

Income taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of beginning of period	$ 15,945	$ 12,095
Increases (decreases) for tax positions related to current period	3,149	3,554
Increases (decreases) for tax positions related to prior periods	(100)	(296)
Balance as of end of period	$ 18,994	$ 15,945

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	8,083	6,629	5,584
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	6,262	5,483	4,643
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	1,495	1,127	931
ESPP shares and other
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	326	19	10

Selected quarterly financial data (unaudited) - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 10,708	$ 19,273	$ 198,890	$ 21,863	$ 9,997	$ 8,910	$ 13,296	$ 12,471	$ 250,734	$ 44,674	$ 54,579
Total operating expenses	214,690	208,967	224,835	225,288	240,531	219,326	215,998	183,645	873,780	859,500	626,602
Loss from operations	(203,982)	(189,694)	(25,945)	(203,425)	(230,534)	(210,416)	(202,702)	(171,174)	(623,046)	(814,826)	(572,023)
Net loss	$ (199,874)	$ (194,745)	$ (21,465)	$ (202,611)	$ (223,347)	$ (206,033)	$ (195,782)	$ (164,446)	$ (618,695)	$ (789,608)	$ (555,625)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (3.01)	$ (2.94)	$ (0.36)	$ (3.64)	$ (4.04)	$ (3.73)	$ (3.55)	$ (2.99)	$ (9.95)	$ (14.31)	$ (10.68)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Subsequent Events (Details)	1 Months Ended
Subsequent Events (Details)	Jan. 31, 2021 company
Subsequent Event
Subsequent Event [Line Items]
Number of publicly traded companies	2