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Document and Entity Information - shares	6 Months Ended
Jun. 30, 2019	Jul. 25, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	0001293971
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		55,269,239
Entity File Number	001-35966
Entity Tax Identification Number	133680878
Entity Address, Address Line One	60 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	Massachusetts
Entity Address, Postal Zip Code	02142
City Area Code	339
Local Phone Number	499-9300

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 280,995	$ 402,579
Marketable securities	972,885	982,725
Prepaid expenses	25,427	19,762
Receivables and other current assets	21,473	13,931
Total current assets	1,300,780	1,418,997
Marketable securities	287,922	506,123
Property, plant and equipment, net	129,135	246,622
Intangible assets, net	16,480	13,169
Goodwill	13,128	13,128
Operating lease right-of-use assets	190,979
Restricted cash and other non-current assets	84,920	44,805
Total assets	2,023,344	2,242,844
Current Liabilities:
Accounts payable	29,458	17,831
Accrued expenses and other current liabilities	91,105	99,393
Operating lease liability, current portion	18,872
Deferred revenue, current portion	9,484	18,602
Collaboration research advancement, current portion	13,190	10,605
Total current liabilities	162,109	146,431
Deferred revenue, net of current portion	13,739	16,338
Collaboration research advancement, net of current portion	28,333	33,349
Contingent consideration	5,740	5,230
Operating lease liability, net of current portion	175,350
Financing lease obligation, net of current portion		153,319
Other non-current liabilities	1,699	3,107
Total liabilities	386,970	357,774
Commitments and contingencies (Note 8)
Stockholders’ Equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.01 par value, 125,000 shares authorized; 55,228 and 54,738 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	553	547
Additional paid-in capital	3,489,112	3,386,958
Accumulated other comprehensive loss	(819)	(3,627)
Accumulated deficit	(1,852,472)	(1,498,808)
Total stockholders’ equity	1,636,374	1,885,070
Total liabilities and stockholders’ equity	$ 2,023,344	$ 2,242,844

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue:
Total revenues	$ 13,296	$ 7,851	$ 25,767	$ 23,808
Operating expenses:
Research and development	146,540	115,014	269,180	212,123
General and administrative	68,631	41,168	128,910	76,094
Cost of license and royalty revenue	$ 613	$ 21	$ 1,043	$ 36
Type of cost, good or service [Extensible List]	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember
Change in fair value of contingent consideration	$ 214	$ 262	$ 510	$ 796
Total operating expenses	215,998	156,465	399,643	289,049
Loss from operations	(202,702)	(148,614)	(373,876)	(265,241)
Interest income, net	9,387	2,436	19,489	3,824
Other (expense) income, net	(2,936)	182	(6,325)	297
Loss before income taxes	(196,251)	(145,996)	(360,712)	(261,120)
Income tax benefit	469		484
Net loss	$ (195,782)	$ (145,996)	$ (360,228)	$ (261,120)
Net loss per share - basic and diluted:	$ (3.55)	$ (2.91)	$ (6.54)	$ (5.22)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	55,165	50,153	55,062	50,038
Other comprehensive income (loss):
Other comprehensive income (loss), net of tax expense of $0.8 million and $0.0 million for the three months ended June 30, 2019 and 2018, respectively, and $1.3 million and $0.0 million for the six months ended June 30, 2019 and 2018, respectively	$ 973	$ (345)	$ 2,808	$ (1,189)
Total other comprehensive income (loss)	973	(345)	2,808	(1,189)
Comprehensive loss	(194,809)	(146,341)	(357,420)	(262,309)
Collaboration [Member]
Revenue:
Total revenues	11,558	7,437	22,735	23,045
License and Royalty [Member]
Revenue:
Total revenues	$ 1,738	$ 414	$ 3,032	$ 763

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 1,623,432	$ 494	$ 2,540,951	$ (4,205)	$ (913,808)
Beginning balance, shares at Dec. 31, 2017		49,406
Adjustment to beginning accumulated deficit from adoption of accounting standard update \| ASU 2014-09 [Member]	(29,375)				(29,375)
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		74
Issuance of common stock upon public offering, net of issuance costs	48,701	$ 3	48,698
Issuance of common stock upon public offering, net of issuance costs, shares		277
Exercise of stock options	19,730	$ 3	19,727
Exercise of stock options, shares		301
Purchase of common stock under ESPP	687		687
Purchase of common stock under ESPP, shares		9
Stock-based compensation	22,995		22,995
Other comprehensive income (loss)	(844)			(844)
Net loss	(115,126)				(115,126)
Ending balance at Mar. 31, 2018	1,570,201	$ 501	2,633,057	(5,049)	(1,058,308)
Ending balance, shares at Mar. 31, 2018		50,067
Beginning balance at Dec. 31, 2017	1,623,432	$ 494	2,540,951	(4,205)	(913,808)
Beginning balance, shares at Dec. 31, 2017		49,406
Other comprehensive income (loss)	(1,189)
Net loss	(261,120)
Ending balance at Jun. 30, 2018	1,457,534	$ 502	2,666,729	(5,394)	(1,204,303)
Ending balance, shares at Jun. 30, 2018		50,225
Beginning balance at Mar. 31, 2018	1,570,201	$ 501	2,633,057	(5,049)	(1,058,308)
Beginning balance, shares at Mar. 31, 2018		50,067
Vesting of restricted stock units, shares		33
Exercise of stock options	5,617	$ 1	5,616
Exercise of stock options, shares		125
Stock-based compensation	28,056		28,056
Other comprehensive income (loss)	(345)			(345)
Net loss	(145,996)				(145,996)
Ending balance at Jun. 30, 2018	1,457,534	$ 502	2,666,729	(5,394)	(1,204,303)
Ending balance, shares at Jun. 30, 2018		50,225
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	3,386,958	(3,627)	(1,498,808)
Beginning balance, shares at Dec. 31, 2018	54,738	54,738
Adjustment to beginning accumulated deficit from adoption of accounting standard update \| ASU 2016-02 [Member]	$ 6,564				6,564
Vesting of restricted stock units		$ 2	(2)
Vesting of restricted stock units, shares		131
Exercise of stock options	9,504	$ 2	9,502
Exercise of stock options, shares		189
Purchase of common stock under ESPP	1,231		1,231
Purchase of common stock under ESPP, shares		11
Stock-based compensation	32,341		32,341
Other comprehensive income (loss)	1,835			1,835
Net loss	(164,446)				(164,446)
Ending balance at Mar. 31, 2019	1,772,099	$ 551	3,430,030	(1,792)	(1,656,690)
Ending balance, shares at Mar. 31, 2019		55,069
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	3,386,958	(3,627)	(1,498,808)
Beginning balance, shares at Dec. 31, 2018	54,738	54,738
Exercise of stock options, shares	282
Other comprehensive income (loss)	$ 2,808
Net loss	(360,228)
Ending balance at Jun. 30, 2019	$ 1,636,374	$ 553	3,489,112	(819)	(1,852,472)
Ending balance, shares at Jun. 30, 2019	55,228	55,228
Beginning balance at Mar. 31, 2019	$ 1,772,099	$ 551	3,430,030	(1,792)	(1,656,690)
Beginning balance, shares at Mar. 31, 2019		55,069
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		66
Exercise of stock options	3,973	$ 1	3,972
Exercise of stock options, shares		93
Stock-based compensation	55,111		55,111
Other comprehensive income (loss)	973			973
Net loss	(195,782)				(195,782)
Ending balance at Jun. 30, 2019	$ 1,636,374	$ 553	$ 3,489,112	$ (819)	$ (1,852,472)
Ending balance, shares at Jun. 30, 2019	55,228	55,228

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (360,228)	$ (261,120)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	510	796
Depreciation and amortization	7,831	8,199
Stock-based compensation expense	87,452	51,051
Unrealized loss on equity securities	6,184
Other non-cash items	(7,064)	2,834
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(20,694)	(3,279)
Operating lease right-of-use assets	11,037
Accounts payable	6,652	4,097
Accrued expenses and other liabilities	(9,301)	9,789
Operating lease liabilities	(259)
Deferred revenue	(11,716)	(21,589)
Collaboration research advancement	(2,431)
Net cash used in operating activities	(292,027)	(209,222)
Cash flows from investing activities:
Purchase of property, plant and equipment	(37,925)	(20,689)
Purchases of marketable securities	(471,365)	(689,163)
Proceeds from maturities of marketable securities	704,803	417,640
Net cash provided by (used in) investing activities	195,513	(292,212)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs		48,702
Reimbursement of assets under financing lease obligation		3,098
Payments on financing lease obligation		(446)
Proceeds from exercise of stock options and ESPP contributions	15,004	25,624
Net cash provided by financing activities	15,004	76,978
Decrease in cash, cash equivalents and restricted cash	(81,510)	(424,456)
Cash, cash equivalents and restricted cash at beginning of period	417,099	772,268
Cash, cash equivalents and restricted cash at end of period	335,589	347,812
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents			$ 280,995	$ 333,949
Restricted cash included in receivables and other current assets			$ 100	$ 100
Restricted Cash, Current, Asset, Statement of Financial Position [Extensible List]			blue:ReceivablesAndOtherCurrentAssets	blue:ReceivablesAndOtherCurrentAssets
Restricted cash included in restricted cash and other non-current assets			$ 54,494	$ 13,763
Restricted Cash, Noncurrent, Asset, Statement of Financial Position [Extensible List]			us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents and restricted cash	417,099	772,268	$ 335,589	$ 347,812
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	8,869	$ 7,815
Assets acquired under operating lease obligation	$ 17,489

Basis of presentation, principles of consolidation and significant accounting policies

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Basis of presentation, principles of consolidation and significant accounting policies

2. Basis of presentation, principles of consolidation and significant accounting policies

Basis of presentation

The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2019 and 2018.

The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 21, 2019.

Certain items in the prior year’s condensed consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year condensed consolidated financial statements were impacted.

Amounts reported are computed based on thousands. As a result, certain totals may not sum due to rounding.

The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment.

Significant accounting policies

The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2019 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s 2018 Annual Report on Form 10-K, except as noted below with respect to the Company’s lease accounting policies and as noted within the “Recent accounting pronouncements – Recently adopted” section below.

Leases

Effective January 1, 2019, the Company adopted ASU 2016-02, Leases (Topic 842), (“ASU 2016-02” or “ASC 842”), using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods 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 short-term and long-term 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.

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.

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 are used in the following areas, among others: future undiscounted cash flows and subsequent fair value estimates, as appropriate, 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, revenue and income taxes.

Recent accounting pronouncements

Recently adopted

ASU No. 2016-02, Leases (Topic 842), ASU No. 2018-10 Codification Improvements to Topic 842, Leases, ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, and ASU No. 2019-01 Leases (Topic 842): Codification Improvements

In February 2016, the FASB issued ASU 2016-02, as amended, which superseded the lease accounting requirements in ASC 840 and created ASC 842. ASC 842 requires a lessee to recognize assets and liabilities on the balance sheet for most leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of one year or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities for those leases. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance.

Effective January 1, 2019, the Company adopted ASU 2016-02, using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840.

The adoption of this standard resulted in the recognition of operating lease right-of-use assets and operating lease liabilities of $184.4 million and $177.0 million, respectively, on the Company’s condensed consolidated balance sheet at adoption relating to its leases for its corporate headquarters at 60 Binney Street in Cambridge, Massachusetts (the “60 Binney Street Lease”), its office and laboratory space in Seattle, Washington, its office space in Zug, Switzerland, and its embedded leases associated with certain of the Company’s contract manufacturing agreements. The application of the standard’s transition guidance required the de-recognition of the 60 Binney Street Lease building asset, financing lease obligation, current portion, and financing lease obligation, net of current portion in the amounts of $149.3 million, $1.4 million, and $153.3 million, respectively, as well as certain other adjustments to related account balances. In adopting ASU 2016-02, the Company recorded a total one-time adjustment of $6.6 million to the opening balance of accumulated deficit as of January 1, 2019 primarily relating to the de-recognition of the 60 Binney Street Lease building asset and related finance lease obligation.

As a result of adopting ASU 2016-02, the Company recorded an increase to deferred tax assets and deferred tax liabilities of $5.3 million and $7.1 million, respectively. The $1.8 million net increase to deferred tax liabilities and an offsetting valuation allowance adjustment was recorded through the accumulated deficit as of January 1, 2019, such that there was no tax impact on the Company’s condensed consolidated financial statements as a result of adoption.

ASU No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Topic 310-20): Premium Amortization on Purchased Callable Debt Securities

In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (Topic 310-20): Premium Amortization on Purchased Callable Debt Securities (“Subtopic 310-20”). The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. Subtopic 310-20 calls for a modified retrospective application under which a cumulative-effect adjustment will be made to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. The Company adopted this standard on January 1, 2019 and it did not have a material impact on the Company’s financial position or results of operations upon adoption.

ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income

In February 2018, the FASB issued ASU 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The new standard allows for a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The Company adopted this standard on January 1, 2019 and it did not have a material impact on the Company’s financial position and results of operations upon adoption.

Not yet adopted

ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements, ASU No. 2019-05 Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief

In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements. The new standard 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 new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2016-13, and related updates, will have on its financial position and results of operations upon adoption.

ASU No. 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment

In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. To address concerns over the cost and complexity of the two-step goodwill impairment test, the amendments in this ASU remove the second step of the test. An entity will instead apply a one-step quantitative test and record the amount of goodwill impairment 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 new guidance does not amend the optional qualitative assessment of goodwill impairment. The new standard will be effective beginning January 1, 2020 and the Company plans to early adopt the provisions of this ASU for purposes of performing its annual goodwill impairment test for 2019 during the fourth quarter of 2019. The adoption of this standard is not expected to have a material impact on the Company’s financial position or results of operations upon adoption.

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, (“ASU 2018-13”). The new standard removes certain disclosures, modifies certain disclosures and adds additional disclosures related to fair value measurement. The new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2018-13 may have on its disclosures 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, (“ASU 2018-15”). 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 new standard will be effective beginning January 1, 2020. The amendments in this update should be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the potential impact ASU 2018-15 may have 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 when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2018-18 may have on its financial position and results of operations upon adoption.

ASU No. 2019-04, 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 2019-04, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, (“ASU 2019-04”). This update provides clarifications for three topics related to financial instruments accounting, some of which apply to the Company. The amendments in this update will be effective beginning January 1, 2020. The adoption of ASU 2019-04 is not expected to have a material impact on the Company’s financial position or results of operations upon adoption.

Marketable Securities

6 Months Ended

Jun. 30, 2019

Investments Debt And Equity Securities [Abstract]

Marketable securities

3. Marketable securities

The following table summarizes the marketable securities held at June 30, 2019 and December 31, 2018 (in thousands):

Description	Amortized cost / Cost		Unrealized gains		Unrealized losses			Fair value
June 30, 2019
U.S. government agency securities and treasuries	$	1,060,606	$	3,084	$	(519	)	$	1,063,171
Certificates of deposit		4,520		—		—			4,520
Corporate bonds		113,820		633		(12	)		114,441
Commercial paper		62,692		—		—			62,692
Equity securities		20,017		—		(4,034	)		15,983
Total	$	1,261,655	$	3,717	$	(4,565	)	$	1,260,807
December 31, 2018
U.S. government agency securities and treasuries	$	1,459,649	$	963	$	(3,011	)	$	1,457,601
Certificates of deposit		9,080		—		—			9,080
Equity securities		20,017		2,150		—			22,167
Total	$	1,488,746	$	3,113	$	(3,011	)	$	1,488,848

No available-for-sale debt securities held as of June 30, 2019 or December 31, 2018 had remaining maturities greater than three years.

Fair value measurements

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair value measurements

4. 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 June 30, 2019 and December 31, 2018 (in thousands):

Description	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
June 30, 2019
Assets:
Cash and cash equivalents	$	280,995	$	280,995	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries		1,063,171		—		1,063,171		—
Certificates of deposit		4,520		—		4,520		—
Commercial paper		62,692		—		62,692		—
Corporate bonds		114,441		—		114,441		—
Equity securities		15,983		15,983		—		—
Total assets	$	1,541,802	$	296,978	$	1,244,824	$	—
Liabilities:
Contingent consideration	$	5,740	$	—	$	—	$	5,740
Total liabilities	$	5,740	$	—	$	—	$	5,740
December 31, 2018
Assets:
Cash and cash equivalents	$	402,579	$	348,638	$	53,941	$	—
Marketable securities:
U.S. government agency securities and treasuries		1,457,601		—		1,457,601		—
Certificates of deposit		9,080		—		9,080		—
Equity securities		22,167		22,167		—		—
Total assets	$	1,891,427	$	370,805	$	1,520,622	$	—
Liabilities:
Contingent consideration	$	5,230	$	—	$	—	$	5,230
Total liabilities	$	5,230	$	—	$	—	$	5,230

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 June 30, 2019, cash and cash equivalents comprise funds in cash and money market accounts. As of December 31, 2018, cash and cash equivalents comprise funds in cash, U.S. treasury securities, U.S. government agency securities, and money market accounts.

Marketable securities

Marketable securities classified as Level 2 within the valuation hierarchy generally consist of certificates of deposit, U.S. treasury securities and government agency securities, 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 June 30, 2019 and December 31, 2018, 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 three and six months ended June 30, 2019 or 2018, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same periods.

The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2019 and December 31, 2018 was $52.4 million and $787.5 million, respectively. As of June 30, 2019 and December 31, 2018, there were $229.2 million and $315.3 million in securities held by the Company in an unrealized loss position for more than twelve months, respectively. The aggregate unrealized loss on securities held by the Company for less than twelve months as of June 30, 2019 and December 31, 2018 was $0.1 million and $0.9 million, respectively. The aggregate unrealized loss on securities held by the Company for more than twelve months as of June 30, 2019 and December 31, 2018 was $0.5 million and $2.1 million, respectively. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of June 30, 2019 and December 31, 2018.

The Company holds equity securities with an aggregate fair value of $16.0 million and $22.2 million as of June 30, 2019 and December 31, 2018, respectively, within short-term marketable securities on its condensed consolidated balance sheet. The Company has recorded a $3.1 million and $6.2 million unrealized loss during the three and six months ended June 30, 2019, respectively, related to its equity securities, which is included in other (expense) income, net on the condensed consolidated statements of operations and comprehensive loss.

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 condensed consolidated statements of operations and comprehensive loss. In the absence of new information, changes in fair value will reflect changing discount rates and the passage of time.

The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from 2021 to 2028, and discount rates ranging from 14.2% to 15.0%. Significant increases or decreases in any of the probabilities of success would result in a significantly higher or lower fair value measurement, respectively. Significant increases or decreases in the other inputs would result in a significantly lower or higher fair value measurement, respectively.

The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands):

	For the six months ended
	June 30, 2019
Beginning balance	$	5,230
Additions		—
Changes in fair value		510
Payments		—
Ending balance	$	5,740

Please refer to Note 8, “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

Leases

6 Months Ended

Jun. 30, 2019

Leases [Abstract]

Leases

7. Leases

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

Embedded operating leases

On June 3, 2016, the Company entered into a manufacturing agreement for the future commercial production of the Company’s ZYNTEGLO, LentiGlobin, and Lenti-D 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, and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. Construction was completed in March 2018 and beginning in April 2018 the Company pays $5.1 million per year in fixed suite fees as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services, and may pay additional suite fees if it elects its option to reserve or lease additional suites.

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 under ASC 840 as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company concluded that it was not the deemed owner during construction nor was it a capital lease under ASC 840. As a result, in prior periods the Company accounted for the agreement as an operating lease under ASC 840 and recognized straight-line rent expense over the non-cancellable term of the embedded lease. As part of its adoption of ASC 842, effective January 1, 2019, the Company carried forward its existing lease classification under ASC 840. Additionally, the Company recorded a right-of-use asset and lease liability for this operating lease on the effective date and is recognizing rent expense on a straight-line basis throughout the remaining term of the embedded lease.

The Company’s other embedded leases are not material to the condensed consolidated financial statements.

60 Binney Street Lease

On September 21, 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. 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. The Company is accounting for this lease under ASC 842 using its initial 10-year term through March 31, 2027 and will reassess the lease term on a quarterly basis.

Due to the Company’s involvement in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building, among other items, the Company was deemed for accounting purposes to be the owner of the Building during the construction period, per ASC 840. Accordingly, under ASC 840, construction costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance were recorded as an asset in Property, plant and equipment, net on the Company’s consolidated balance sheets.

The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment under ASC 840. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the portion of the building in service over a useful life of 40 years and incurred interest expense related to the financing obligation.

As part of its adoption of ASC 842, the Company de-recognized the building asset and corresponding financing obligation recorded on the Company’s consolidated balance sheets as of December 31, 2018, in accordance with the ASC 842 transition guidance. In applying the ASC 842 transition guidance, the Company classified this lease as an operating lease and recorded a right-of-use asset of $127.3 million and lease liability of $125.8 million 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 July 1, 2021, and end on December 31, 2030, unless the Company earlier occupies the premises or other conditions specified in the 50 Binney Street Sublease occur. The sublessor has the right to postpone the commencement date until January 1, 2022 by providing not less than nine months’ prior written notice to the Company. 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 condensed 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 condensed consolidated balance sheets as of June 30, 2019.

The Company will assess the lease classification of the 50 Binney Street Sublease 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 three and six months ended June 30, 2019:

Operating leases	For the three months ended		For the six months ended
(in thousands)	June 30, 2019		June 30, 2019
Lease cost (1)
Operating lease cost	$	8,917	$	17,400
Total lease cost	$	8,917	$	17,400

Other information
Operating cash flows used for operating leases			$	13,237
Weighted average remaining lease term			7.7 years
Weighted average discount rate				6.17	%

(1)	Short-term lease costs and variable lease costs incurred by the Company for the three and six months ended June 30, 2019 were immaterial.

As of June 30, 2019, future minimum commitments under ASC 842 under the Company’s operating leases were as follows:

Maturity of lease liabilities	As of
(in thousands)	June 30, 2019
2019 (excluding the six months ended June 30, 2019)	$	17,031
2020		33,780
2021		34,215
2022		29,417
2023		29,869
2024 and thereafter		108,687
Total lease payments		252,999
Less: imputed interest		(58,777	)
Total operating lease liabilities	$	194,222

The above table excludes legally binding minimum lease payments for leases executed but not yet commenced as of June 30, 2019.

Collaborative Arrangements

6 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaborative Arrangements

9. Collaborative arrangements

To date, the Company’s collaboration revenue has been exclusively generated from its collaboration arrangements with Celgene Corporation and Regeneron, each as further described below.

Celgene

Celgene Original Collaboration Agreement

On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Celgene Collaboration Agreement”) with Celgene 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, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine, for use in the collaboration.

Under the terms of the Celgene Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. 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 Celgene Collaboration Agreement, or three years. The collaboration is governed by a joint steering committee (“JSC”) formed by an equal number of representatives from the Company and Celgene. The JSC, among other activities, reviews the collaboration program, reviews and evaluates product candidates and approves regulatory plans. In addition to the JSC, the Celgene Collaboration Agreement provides that the Company and Celgene each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration.

Celgene Amended Collaboration Agreement

On June 3, 2015, the Company and Celgene amended and restated the Celgene Collaboration Agreement (the “Amended Celgene Collaboration Agreement”). Under the Amended Celgene 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 that ended in June 2018. In connection with the Amended Celgene 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. The collaboration is governed by the JSC. Under the terms of the Amended Celgene 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 “Option Period”), the Company had granted Celgene an option to obtain an exclusive worldwide license to develop and commercialize such product. Following Celgene’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.

Celgene Ide-cel License Agreement

On February 10, 2016, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize ide-cel, the first product candidate under the Amended Celgene Collaboration Agreement, pursuant to an executed license agreement (“Ide-cel License Agreement”) entered into by the parties on February 16, 2016 and paid to the Company the associated $10.0 million option fee. Pursuant to the Ide-cel License Agreement, Celgene 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 Celgene’s request, throughout commercialization, the costs of which were reimbursable by Celgene in accordance with the terms of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement, as further described below. Celgene was responsible for the manufacture of drug product throughout development and commercialization.

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

On March 28, 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. 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. Celgene is responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup.

Under the Ide-cel CCPS, the Company may receive up to a total of $70.0 million in development milestone payments for the first indication to be addressed by ide-cel, with the ability to obtain additional milestone payments for a second indication and modified licensed products. In the second quarter of 2019, a $10.0 million development milestone was achieved such that as of June 30, 2019, the total remaining potential development milestones on the first indication of ide-cel is $60.0 million. In addition, to the extent ide-cel is commercialized, the Company is entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions.

Celgene bb21217 License Agreement

On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the Amended Celgene Collaboration Agreement, pursuant to an executed license agreement (“bb21217 License Agreement”) entered into by the parties on September 28, 2017 and paid the Company an option fee of $15.0 million. Pursuant to the bb21217 License Agreement, Celgene is responsible for development and related funding of bb21217 after the substantial completion of the on-going phase 1 clinical trial. The Company is responsible for the manufacture of vector and associated payload throughout development and upon Celgene’s request, throughout commercialization. Expenses incurred by the Company associated with these activities are fully reimbursable by Celgene at cost plus a mark-up. Throughout both development and commercialization, Celgene is responsible for the manufacture of drug product.

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. Celgene would be responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. Under this scenario, the Company expects to receive, per product, up to $70.0 million in development milestone payments for the first indication to be addressed by the bb21217 product candidate, with the ability to obtain additional milestone payments for a second indication and modified licensed products. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions.

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, the Company may be eligible to receive up to $10.0 million in clinical milestone payments, up to $117.0 million in regulatory milestone payments, and up to $78.0 million in commercial milestone payments. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales, subject to certain reductions.

Accounting Analysis – Ide-cel

ASC Topic 606, Revenue From Contracts With Customers (“Topic 606”), allows entities to reflect the aggregate effect of all contract modifications when identifying the satisfied and unsatisfied performance obligations for contracts that were modified prior to Topic 606 adoption. Celgene’s option to in-license the first product candidate, ide-cel, under the arrangement was considered a material right at the time the Amended Celgene Collaboration Agreement was executed in June 2015 given the product candidate had been formally nominated by the JSC and that substantially all investigational new drug application, or IND, enabling activities had been completed by that time. In making this determination, the Company also considered the option price relative to the value of the underlying license. Celgene’s exercise of this material right in February 2016 was determined to represent a contract modification and represents the last contract modification prior to the adoption of Topic 606. As a result, the Celgene Collaboration Agreement, Amended Celgene Collaboration Agreement, and Ide-cel CCPS are combined for accounting purposes and treated as a single arrangement. As of February 2016, Celgene’s option to license an additional product candidate under the collaboration did not represent a material right due primarily to the significant uncertainty regarding whether any additional product candidates would be identified under the Amended Celgene Collaboration Agreement. Therefore, the license to the Company’s second product candidate, bb21217, which was executed in September 2017, is accounted for as a separate contract. Refer below for discussion of the bb21217 accounting analysis.

As of the February 2016 contract modification date, the Company concluded the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to ide-cel, and (iii) manufacture of vectors and associated payload for incorporation into ide-cel through development. The Company determined that the manufacture of commercial vector represented an option to acquire additional goods and services that is not representative of a material right. In addition, as of the February 2016 contract modification date, Celgene had not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector was not considered to be a performance obligation at that time.

The Company concluded that the research and development services are distinct from the other promised goods and services under the arrangement given that Celgene can benefit from the research and development services on their own and such services are distinct within the context of the contract. Thus, such services are considered to be a separate performance obligation. The Company concluded that the license to ide-cel is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation.

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, and the amount of the transaction price unsatisfied as of June 30, 2019:

(in thousands)	Ide-cel transaction price as of June 30, 2019
Up-front non-refundable payment - Celgene Collaboration Agreement	$	75,000
Up-front non-refundable payment - Amended Celgene Collaboration Agreement		25,000
Ide-cel license fee - Ide-cel License Agreement		10,000
Ide-cel development milestone		10,000
Estimated variable consideration		87,189
	$	207,189

(in thousands)	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of June 30, 2019
Ide-cel research and development services	$	40,912	$	—
Ide-cel license and manufacturing services		166,277		26,723
	$	207,189	$	26,723

The estimated variable consideration of $87.2 million relates to the estimated reimbursement from Celgene for the manufacture of vectors and associated payload through development. The total transaction price has been allocated to the performance obligations identified based on a relative standalone selling price (“SSP”) basis. The Company estimated the SSP of the 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 research and development services and 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.

All of the clinical and regulatory milestones which have not been met as of period end are fully constrained and are 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 clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to 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 Celgene 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, at each reporting period and as uncertain events are resolved or other changes in circumstances occur.

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 three years through projected initial phase 1 clinical study substantial completion, or through May 2018. The Company recognized revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. Although the Company fully satisfied this performance obligation during the second quarter of 2018, any changes to the total transaction price following the completion of this performance obligation in May 2018 will be allocated to the performance obligations under the arrangement based on a relative SSP basis and therefore the allocation of any changes to the total transaction price may impact the revenue recognized for this performance obligation in the period of change.

The following table summarizes the revenue recognized, or revenue adjustment recorded, related to the ide-cel research and development services for the three and six months ended June 30, 2019 and 2018:

	For the three months ended June 30,				For the six months ended June 30,
(in thousands)	2019		2018		2019		2018
Ide-cel research and development services revenue	$	2,473	$	953	$	2,264	$	3,136
	$	2,473	$	953	$	2,264	$	3,136

Ide-cel license and manufacturing services

The Company allocated $166.3 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 Celgene’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 Celgene’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.

Revenue recognition for the combined unit of accounting commenced during the first quarter of 2017. The Company recognizes revenue associated with the combined unit of accounting 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 Celgene, 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 for development in the U.S., for the three and six months ended June 30, 2019, and 2018:

	For the three months ended June 30,						For the six months ended June 30,
(in thousands)	2019			2018			2019			2018
ASC 808 ide-cel revenue - U.S. (1)	$	—		$	—		$	—		$	3,761
ASC 808 ide-cel research and development expense - U.S. (1)	$	(1,065	)	$	(3,349	)	$	(4,309	)	$	(3,349	)

(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 non-US license and vector manufacturing services for the three and six months ended June 30, 2019, and 2018:

For the three months ended

June 30,

For the six months ended

June 30,

(in thousands)

2019

2018

2019

2018

ASC 606 ide-cel license and

manufacturing revenue - outside of U.S.

7,899

5,764

16,963

14,706

As of June 30, 2019, 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 $26.7 million, which the Company expects to recognize as revenue as manufacturing services are provided through the remaining development period which is estimated to be through 2020. As of June 30, 2019 and December 31, 2018, the Company had $12.7 million and $23.0 million, respectively, of deferred revenue associated with the combined performance obligation consisting of the ide-cel license and manufacturing services.

Accounting Analysis – bb21217

On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second optioned product candidate, pursuant to the bb21217 License Agreement entered into by the parties on September 28, 2017. The bb21217 License Agreement is considered a separate contract for accounting purposes as the option to obtain an exclusive worldwide license to develop and commercialize bb21217, or any other product candidate, was not considered a material right to Celgene at the time the practical expedient was applied. The Company made this evaluation after considering the significant uncertainty at that time regarding whether any additional product candidates would be identified under the Amended Celgene Collaboration Agreement. In particular, the Company considered that bb21217 had not been formally nominated as a product candidate under the collaboration at that time, primarily due to a lack of pre-clinical data as well as uncertainty surrounding the ability to successfully complete various IND-enabling activities.

At contract inception, the Company concluded that the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to the second product candidate, bb21217, and (iii) manufacture of vectors and associated payload for incorporation into bb21217 through development. The Company determined that the manufacture of commercial vector represents an option to acquire additional goods and services that is not representative of a material right. In addition, at this time Celgene has not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector is not considered to be a performance obligation at this time.

The Company concluded that the research and development services are distinct from the other promised goods and services under the arrangement given that Celgene can benefit from the research and development services on their own and such services are distinct within the context of the contract. Thus, such services are considered to be a separate performance obligation. Similar to ide-cel, the Company concluded that the license to bb21217 is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation.

bb21217 transaction price

(in thousands)	bb21217 transaction price as of June 30, 2019
bb21217 license fee - bb21217 License Agreement	$	15,000
Estimated variable consideration		26,687
	$	41,687

(in thousands)	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of June 30, 2019
bb21217 research and development services	$	5,444	$	721
bb21217 license and manufacturing services		36,243		36,243
	$	41,687	$	36,964

The estimated variable consideration of $26.7 million relates to reimbursement from Celgene for the manufacturing services during development. The total transaction price has been allocated to the performance obligations identified based on a relative SSP basis. The Company estimated the SSP of the 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 research and development services and 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.

All of the clinical and regulatory milestones are fully constrained and are 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 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 Celgene 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 allocated $5.4 million of the transaction price to the research and development services. The Company will satisfy this performance obligation as the research and development services are performed. The Company determined that the period of performance of the research and development services was two years through projected initial phase 1 clinical study substantial completion, or through September 2019. The Company recognizes revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred.

The following table summarizes the revenue recognized related to the bb21217 research and development services for the three and six months ended June 30, 2019, and 2018:

	For the three months ended June 30,				For the six months ended June 30,
(in thousands)	2019		2018		2019		2018
bb21217 research and development services revenue	$	721	$	721	$	1,442	$	1,442
	$	721	$	721	$	1,442	$	1,442

As of June 30, 2019, and December 31, 2018, the aggregate amount of the transaction price allocated to the bb21217 research and development services performance obligation that are unsatisfied, or partially unsatisfied, and deferred is $0.7 million and $1.8 million, respectively, which the Company expects to recognize through September 2019 as research and development services are performed.

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 June 30, 2019, the manufacturing services for bb21217 had not yet commenced. Therefore, no amounts have been recognized for the combined performance obligation in the consolidate statement of operations for the three and six months ended June 30, 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 $36.2 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 $9.8 million of remaining deferred revenue as of June 30, 2019 and December 31, 2018 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 Celgene receivables and contract liabilities during the six months ended June 30, 2019:

(in thousands)	Balance at beginning of period		Additions		Deductions			Balance at end of period
Receivables	$	6,528	$	10,450	$	(6,528	)	$	10,450
Contract liabilities:
Payable included in accrued expenses	$	—	$	3,781	$	—		$	3,781
Deferred revenue	$	34,939	$	10,000	$	(21,716	)	$	23,223

The increase in the receivables balance for the six months ended June 30, 2019 is primarily driven by amounts owed to the Company for a development milestone that was achieved in the second quarter of 2019, offset by amounts collected from Celgene in the period and amounts owed to Celgene under the Ide-cel CCPS.

The decrease in deferred revenue during the six months ended June 30, 2019 is primarily driven by amounts recognized for the combined performance obligation consisting of the ide-cel license and manufacturing services. During the six months ended June 30, 2019, $21.7 million of the deferred revenue balance at the beginning of the period was released from deferred revenue, of which $10.0 million was recognized as collaboration revenue and $11.7 million was recorded as contra-research and development expense.

Regeneron

Regeneron Collaboration Agreement

On August 3, 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. On August 24, 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 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 on August 3, 2018. On August 24, 2018, 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 June 30, 2019 and December 31, 2018, the Company has $41.5 million and $44.0 million, respectively, of the collaboration research advancement remaining to be recognized.

The Company recognized $0.5 million and $2.5 million of collaboration revenue from the Regeneron Collaboration Agreement during the three and six months ended June 30, 2019, respectively. The Company did not recognize any collaboration revenue, or research and development expense, from the Regeneron Collaboration Agreement during the three and six months ended June 30, 2018.

Stock-based compensation

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based compensation

11. Stock-based compensation

In January 2019 and 2018, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.2 million and 2.0 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2019, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.2 million.

Stock-based compensation expense

The Company recognized stock-based compensation expense totaling $55.1 million and $87.5 million for the three and six months ended June 30, 2019, respectively. The Company recognized stock-based compensation expense totaling $28.1 million and $51.1 million for the three and six months ended June 30, 2018, respectively. Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows:

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Stock options	$	24,800	$	20,932	$	47,983	$	38,227
Restricted stock units		30,012		6,953		38,893		12,494
Employee stock purchase plan		299		171		576		330
	$	55,111	$	28,056	$	87,452	$	51,051

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

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Research and development	$	29,694	$	14,196	$	45,210	$	25,820
General and administrative		25,417		13,860		42,242		25,231
	$	55,111	$	28,056	$	87,452	$	51,051

In February 2018, the Company issued restricted stock units with service and performance conditions to employees, approximately 0.2 million of which are outstanding as of June 30, 2019. 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 June 30, 2019, the Company had $372.4 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units, and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.8 years.

Stock option activity

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

	Shares (in thousands)			Weighted- average exercise price per share
Outstanding at December 31, 2018		4,643		$	108.56
Granted		1,266		$	136.17
Exercised		(282	)	$	47.80
Canceled, forfeited, or expired		(234	)	$	129.85
Outstanding at June 30, 2019		5,393		$	117.30
Exercisable at June 30, 2019		2,520		$	88.94
Vested and expected to vest at June 30, 2019		5,393		$	117.30

During the six months ended June 30, 2019, 0.3 million shares of common stock were exercised, resulting in total proceeds to the Company of $13.5 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans.

Restricted stock unit activity

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, 2018		931		$	155.99
Granted		438		$	136.32
Vested		(197	)	$	142.82
Forfeited		(69	)	$	153.94
Unvested balance at June 30, 2019		1,103		$	150.67

Employee stock purchase plan

On June 3, 2013, the Company adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized 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 six months ended June 30, 2019 and 2018, less than 0.1 million shares of common stock were issued under the 2013 ESPP.

Basis of presentation, principles of consolidation and significant accounting policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2019 and 2018. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 21, 2019. Certain items in the prior year’s condensed consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year condensed consolidated financial statements were impacted. Amounts reported are computed based on thousands. As a result, certain totals may not sum due to rounding. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment.
Significant accounting policies	Significant accounting policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2019 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s 2018 Annual Report on Form 10-K, except as noted below with respect to the Company’s lease accounting policies and as noted within the “Recent accounting pronouncements – Recently adopted” section below.
Leases	Leases Effective January 1, 2019, the Company adopted ASU 2016-02, Leases (Topic 842), (“ASU 2016-02” or “ASC 842”), using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods 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 short-term and long-term 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. 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.
Use of estimates	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 are used in the following areas, among others: future undiscounted cash flows and subsequent fair value estimates, as appropriate, 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, revenue and income taxes.
Recent accounting pronouncements	Recent accounting pronouncements Recently adopted ASU No. 2016-02, Leases (Topic 842), ASU No. 2018-10 Codification Improvements to Topic 842, Leases, ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, and ASU No. 2019-01 Leases (Topic 842): Codification Improvements In February 2016, the FASB issued ASU 2016-02, as amended, which superseded the lease accounting requirements in ASC 840 and created ASC 842. ASC 842 requires a lessee to recognize assets and liabilities on the balance sheet for most leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of one year or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities for those leases. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. Effective January 1, 2019, the Company adopted ASU 2016-02, using the required modified retrospective approach and utilizing the effective date as its date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840. The adoption of this standard resulted in the recognition of operating lease right-of-use assets and operating lease liabilities of $184.4 million and $177.0 million, respectively, on the Company’s condensed consolidated balance sheet at adoption relating to its leases for its corporate headquarters at 60 Binney Street in Cambridge, Massachusetts (the “60 Binney Street Lease”), its office and laboratory space in Seattle, Washington, its office space in Zug, Switzerland, and its embedded leases associated with certain of the Company’s contract manufacturing agreements. The application of the standard’s transition guidance required the de-recognition of the 60 Binney Street Lease building asset, financing lease obligation, current portion, and financing lease obligation, net of current portion in the amounts of $149.3 million, $1.4 million, and $153.3 million, respectively, as well as certain other adjustments to related account balances. In adopting ASU 2016-02, the Company recorded a total one-time adjustment of $6.6 million to the opening balance of accumulated deficit as of January 1, 2019 primarily relating to the de-recognition of the 60 Binney Street Lease building asset and related finance lease obligation. As a result of adopting ASU 2016-02, the Company recorded an increase to deferred tax assets and deferred tax liabilities of $5.3 million and $7.1 million, respectively. The $1.8 million net increase to deferred tax liabilities and an offsetting valuation allowance adjustment was recorded through the accumulated deficit as of January 1, 2019, such that there was no tax impact on the Company’s condensed consolidated financial statements as a result of adoption. ASU No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Topic 310-20): Premium Amortization on Purchased Callable Debt Securities In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (Topic 310-20): Premium Amortization on Purchased Callable Debt Securities (“Subtopic 310-20”). The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. Subtopic 310-20 calls for a modified retrospective application under which a cumulative-effect adjustment will be made to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. The Company adopted this standard on January 1, 2019 and it did not have a material impact on the Company’s financial position or results of operations upon adoption. ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued ASU 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The new standard allows for a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The Company adopted this standard on January 1, 2019 and it did not have a material impact on the Company’s financial position and results of operations upon adoption. Not yet adopted ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements, ASU No. 2019-05 Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements. The new standard 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 new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2016-13, and related updates, will have on its financial position and results of operations upon adoption. ASU No. 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. To address concerns over the cost and complexity of the two-step goodwill impairment test, the amendments in this ASU remove the second step of the test. An entity will instead apply a one-step quantitative test and record the amount of goodwill impairment 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 new guidance does not amend the optional qualitative assessment of goodwill impairment. The new standard will be effective beginning January 1, 2020 and the Company plans to early adopt the provisions of this ASU for purposes of performing its annual goodwill impairment test for 2019 during the fourth quarter of 2019. The adoption of this standard is not expected to have a material impact on the Company’s financial position or results of operations upon adoption. 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, (“ASU 2018-13”). The new standard removes certain disclosures, modifies certain disclosures and adds additional disclosures related to fair value measurement. The new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2018-13 may have on its disclosures 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, (“ASU 2018-15”). 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 new standard will be effective beginning January 1, 2020. The amendments in this update should be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the potential impact ASU 2018-15 may have 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 when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The new standard will be effective beginning January 1, 2020. The Company is currently evaluating the potential impact ASU 2018-18 may have on its financial position and results of operations upon adoption. ASU No. 2019-04, 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 2019-04, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, (“ASU 2019-04”). This update provides clarifications for three topics related to financial instruments accounting, some of which apply to the Company. The amendments in this update will be effective beginning January 1, 2020. The adoption of ASU 2019-04 is not expected to have a material impact on the Company’s financial position or results of operations upon adoption.

Fair value measurements (Tables)

6 Months Ended

Jun. 30, 2019

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 June 30, 2019 and December 31, 2018 (in thousands):

Description	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
June 30, 2019
Assets:
Cash and cash equivalents	$	280,995	$	280,995	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries		1,063,171		—		1,063,171		—
Certificates of deposit		4,520		—		4,520		—
Commercial paper		62,692		—		62,692		—
Corporate bonds		114,441		—		114,441		—
Equity securities		15,983		15,983		—		—
Total assets	$	1,541,802	$	296,978	$	1,244,824	$	—
Liabilities:
Contingent consideration	$	5,740	$	—	$	—	$	5,740
Total liabilities	$	5,740	$	—	$	—	$	5,740
December 31, 2018
Assets:
Cash and cash equivalents	$	402,579	$	348,638	$	53,941	$	—
Marketable securities:
U.S. government agency securities and treasuries		1,457,601		—		1,457,601		—
Certificates of deposit		9,080		—		9,080		—
Equity securities		22,167		22,167		—		—
Total assets	$	1,891,427	$	370,805	$	1,520,622	$	—
Liabilities:
Contingent consideration	$	5,230	$	—	$	—	$	5,230
Total liabilities	$	5,230	$	—	$	—	$	5,230

Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations

The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands):

	For the six months ended
	June 30, 2019
Beginning balance	$	5,230
Additions		—
Changes in fair value		510
Payments		—
Ending balance	$	5,740

Collaborative arrangements (Tables) - Celgene Corporation [Member]

6 Months Ended

Jun. 30, 2019

Changes in Balances of Company's Receivables and Contract Liabilities

The following table presents changes in the balances of the Company’s Celgene receivables and contract liabilities during the six months ended June 30, 2019:

(in thousands)	Balance at beginning of period		Additions		Deductions			Balance at end of period
Receivables	$	6,528	$	10,450	$	(6,528	)	$	10,450
Contract liabilities:
Payable included in accrued expenses	$	—	$	3,781	$	—		$	3,781
Deferred revenue	$	34,939	$	10,000	$	(21,716	)	$	23,223

U.S. [Member]

Summary of Revenue Recognized or Expense Incurred for Joint Ide-cel Development Efforts Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121

	For the three months ended June 30,						For the six months ended June 30,
(in thousands)	2019			2018			2019			2018
ASC 808 ide-cel revenue - U.S. (1)	$	—		$	—		$	—		$	3,761
ASC 808 ide-cel research and development expense - U.S. (1)	$	(1,065	)	$	(3,349	)	$	(4,309	)	$	(3,349	)

(1)

Outside of U.S. [Member]

Summary of Revenue Recognized or Expense Incurred for Joint Ide-cel Development Efforts Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121

The following table summarizes the revenue recognized related to the combined unit of accounting for the ide-cel non-US license and vector manufacturing services for the three and six months ended June 30, 2019, and 2018:

For the three months ended

June 30,

For the six months ended

June 30,

(in thousands)

2019

2018

2019

2018

ASC 606 ide-cel license and

manufacturing revenue - outside of U.S.

7,899

5,764

16,963

14,706

Ide-cel Transaction Price [Member]

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

(in thousands)	Ide-cel transaction price as of June 30, 2019
Up-front non-refundable payment - Celgene Collaboration Agreement	$	75,000
Up-front non-refundable payment - Amended Celgene Collaboration Agreement		25,000
Ide-cel license fee - Ide-cel License Agreement		10,000
Ide-cel development milestone		10,000
Estimated variable consideration		87,189
	$	207,189

(in thousands)	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of June 30, 2019
Ide-cel research and development services	$	40,912	$	—
Ide-cel license and manufacturing services		166,277		26,723
	$	207,189	$	26,723

Ide-cel Research and Development Services [Member]

Summary of Revenue Recognized or Revenue Adjustment Recorded Related to Research and Development Services

	For the three months ended June 30,				For the six months ended June 30,
(in thousands)	2019		2018		2019		2018
Ide-cel research and development services revenue	$	2,473	$	953	$	2,264	$	3,136
	$	2,473	$	953	$	2,264	$	3,136

bb21217 Transaction Price [Member]

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

(in thousands)	bb21217 transaction price as of June 30, 2019
bb21217 license fee - bb21217 License Agreement	$	15,000
Estimated variable consideration		26,687
	$	41,687

(in thousands)	Allocation of transaction price to performance obligations		Transaction price unsatisfied as of June 30, 2019
bb21217 research and development services	$	5,444	$	721
bb21217 license and manufacturing services		36,243		36,243
	$	41,687	$	36,964

bb21217 Research and Development Services [Member]

Summary of Revenue Recognized or Revenue Adjustment Recorded Related to Research and Development Services

The following table summarizes the revenue recognized related to the bb21217 research and development services for the three and six months ended June 30, 2019, and 2018:

	For the three months ended June 30,				For the six months ended June 30,
(in thousands)	2019		2018		2019		2018
bb21217 research and development services revenue	$	721	$	721	$	1,442	$	1,442
	$	721	$	721	$	1,442	$	1,442

Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2019

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Stock-Based Compensation Expense by Award Type

Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows:

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Stock options	$	24,800	$	20,932	$	47,983	$	38,227
Restricted stock units		30,012		6,953		38,893		12,494
Employee stock purchase plan		299		171		576		330
	$	55,111	$	28,056	$	87,452	$	51,051

Schedule of Stock-Based Compensation Expense by Classification

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

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Research and development	$	29,694	$	14,196	$	45,210	$	25,820
General and administrative		25,417		13,860		42,242		25,231
	$	55,111	$	28,056	$	87,452	$	51,051

Summary of Stock Option Activity Under Plan

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

	Shares (in thousands)			Weighted- average exercise price per share
Outstanding at December 31, 2018		4,643		$	108.56
Granted		1,266		$	136.17
Exercised		(282	)	$	47.80
Canceled, forfeited, or expired		(234	)	$	129.85
Outstanding at June 30, 2019		5,393		$	117.30
Exercisable at June 30, 2019		2,520		$	88.94
Vested and expected to vest at June 30, 2019		5,393		$	117.30

Restricted Stock Units [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

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, 2018		931		$	155.99
Granted		438		$	136.32
Vested		(197	)	$	142.82
Forfeited		(69	)	$	153.94
Unvested balance at June 30, 2019		1,103		$	150.67

Leases - Additional Information (Detail)			1 Months Ended	6 Months Ended
Leases - Additional Information (Detail)	Jun. 03, 2016 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Apr. 30, 2019 USD ($) ft² $ / ft²	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee Lease Description [Line Items]
Operating lease, right-of-use asset				$ 190,979,000	$ 127,300,000
Operating lease, liability				$ 194,222,000	$ 125,800,000
Building [Member]
Lessee Lease Description [Line Items]
Useful life of asset				40 years
50 Binney Street Sublease [Member]
Lessee Lease Description [Line Items]
Lease rent, annual increase percentage			3.00%
Sublease rent description				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 July 1, 2021, and end on December 31, 2030, unless the Company earlier occupies the premises or other conditions specified in the 50 Binney Street Sublease occur. The sublessor has the right to postpone the commencement date until January 1, 2022 by providing not less than nine months’ prior written notice to the Company.
Area of office space for sublease \| ft²			267,278
Rate of area for sublease per square feet \| $ / ft²			99.95
Sublease annual base rent			$ 26,700,000
Sublease rent payments				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
50 Binney Street Sublease [Member] \| Cash-Collateralized Letter of Credit [Member]
Lessee Lease Description [Line Items]
Collateralized letter of credit			40,100,000
50 Binney Street Sublease [Member] \| Cash-Collateralized Letter of Credit [Member] \| Restricted Cash and Other Non-current Assets [Member]
Lessee Lease Description [Line Items]
Collateralized letter of credit			$ 40,100,000
Furniture Purchase Agreement [Member] \| Restricted Cash and Other Non-current Assets [Member]
Lessee Lease Description [Line Items]
Upfront payment for purchase of furniture and equipment				$ 7,500,000
Manufacturing Agreement [Member]
Lessee Lease Description [Line Items]
Lease period	12 years
Milestone paid	$ 12,000,000.0
Lease payments	$ 5,100,000
Agreement termination description				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.
Early termination lease term	24 months
Manufacturing Agreement [Member] \| Maximum [Member]
Lessee Lease Description [Line Items]
Additional contractual payables to milestones	$ 8,000,000.0
Lease starting on October 1, 2016 [Member] \| 60 Binney Street Lease [Member]
Lessee Lease Description [Line Items]
Lease period		10 years
Lease starting date		Oct. 01, 2016
Lease building space \| ft²		253,108
Annual lease rent per square foot \| $ / ft²		72.50
Lease payments base annual rent		$ 18,400,000
Operating lease description				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.
Lease rent, annual increase percentage		1.75%
Collateralized letter of credit		$ 13,800,000
Landlord contribution for cost of construction and tenant improvements		42,400,000
Decrease in letter of credit under the terms of the lease		$ 9,200,000
Lease expiration date		Mar. 31, 2027
Lease extension terms		two successive five-year terms

Collaborative arrangements - Additional Information (Detail)					3 Months Ended		6 Months Ended
	Aug. 24, 2018 USD ($) Target $ / shares shares	Sep. 28, 2017 USD ($)	Feb. 16, 2016 USD ($)	Mar. 19, 2013 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Aug. 03, 2018 USD ($)	Mar. 28, 2018 USD ($)	Jun. 03, 2015 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue released							$ 21,700,000
Investment in common stock								$ 48,702,000
Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues							11,700,000
Research and Development Services [Member] \| Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues							10,000,000.0
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					$ 23,223,000		23,223,000		$ 34,939,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues							21,716,000
Celgene Corporation [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability				$ 75,000,000.0
Celgene Corporation [Member] \| Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability												$ 25,000,000.0
Estimated variable consideration					87,200,000		$ 87,200,000
Deferred revenue recognition period							3 years
Celgene Corporation [Member] \| Amended Collaborative Arrangement [Member] \| Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					40,900,000		$ 40,900,000
Celgene Corporation [Member] \| Ide-cel License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received							10,000,000
Celgene Corporation [Member] \| Ide-cel License Agreement [Member] \| First Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received			$ 10,000,000.0
Celgene Corporation [Member] \| Ide-cel Co-Development, Co-Promote and Profit Share Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments achieved					10,000,000.0
Remaining potential development milestone payment receivable					60,000,000.0		60,000,000.0
Celgene Corporation [Member] \| Ide-cel Co-Development, Co-Promote and Profit Share Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable											$ 70,000,000.0
Celgene Corporation [Member] \| bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received		$ 15,000,000.0					15,000,000
Additional fee receivable if option to co-develop and co-promote is not exercised		$ 10,000,000.0
Estimated variable consideration					26,700,000		$ 26,700,000
Deferred revenue recognition period							2 years
Celgene Corporation [Member] \| bb21217 License Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical milestone payments receivable					10,000,000.0		$ 10,000,000.0
Regulatory milestone payments receivable					117,000,000.0		117,000,000.0
Commercial milestone payments receivable					78,000,000.0		78,000,000.0
Celgene Corporation [Member] \| bb21217 License Agreement [Member] \| Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					700,000		700,000		1,800,000
Collaboration agreement, transaction price					5,400,000		5,400,000
Remaining performance obligation revenue					700,000		700,000		1,800,000
Celgene Corporation [Member] \| bb21217 License Agreement [Member] \| License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					9,800,000		9,800,000		9,800,000
Remaining performance obligation revenue					36,200,000		36,200,000
Deferred revenue balance recognized as gross revenues					0	$ 0	0	$ 0
Celgene Corporation [Member] \| bb21217 License Agreement, Co-promotion and Development [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable					70,000,000.0		70,000,000.0
Celgene Corporation [Member] \| Ide-cel License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					166,277,000		166,277,000
Remaining performance obligation revenue					26,723,000		26,723,000
Celgene Corporation [Member] \| Ide-cel License and Manufacturing Services [Member] \| License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					12,700,000		12,700,000		23,000,000.0
Remaining performance obligation revenue					$ 26,700,000		$ 26,700,000
Remaining performance obligation expected to be recognize as revenue, year					2020		2020
Regeneron Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of initial collaboration targets \| Target	6
Research collaboration term	5 years
Collaboration agreements costs and profits share description								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.
Purchase price premium	$ 37,000,000.0
Collaboration research advancement remaining to be recognized					$ 41,500,000		$ 41,500,000		$ 44,000,000.0
Regeneron Collaboration Agreement [Member] \| Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues					500,000	$ 0	2,500,000	$ 0
Regeneron Collaboration Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments receivable										$ 130,000,000.0
Regeneron Collaboration Agreement [Member] \| Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					100,000,000.0		100,000,000.0
Purchase price premium							37,000,000.0
Collaborative arrangement amount attributed to equity sold					54,500,000		54,500,000
Collaborative arrangement amount attributed to joint research activities					$ 45,500,000		$ 45,500,000
Collaborative arrangement amortization period							5 years
Regeneron Collaboration Agreement [Member] \| Share Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock shares issued \| shares	400,000						0.4
Investment in common stock	$ 100,000,000.0
Common stock price per share \| $ / shares	$ 238.10
Purchase price premium	$ 37,000,000.0
Collaborative arrangement research initial funding obligation, percentage	50.00%
Regeneron Collaboration Agreement [Member] \| Share Purchase Agreement [Member] \| Common Shares [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Investment in common stock	$ 63,000,000.0

Stock-based compensation - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Jun. 03, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan	2,200,000	2,000,000.0
Number of shares available for issuance			2,200,000		2,200,000
Stock-based compensation expense			$ 55,111	$ 28,056	$ 87,452	$ 51,051
Stock option share exercised					282,000
Proceed from option share exercised					$ 13,500
Performance And Service Condition Based Restricted Stock Unit [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 20,100
Unvested restricted stock awards outstanding			200,000		200,000
Performance And Service Condition Based Restricted Stock Unit [Member] \| Tranche One [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vest, month and year					2021-06
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 24,800	20,932	$ 47,983	38,227
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			372,400		$ 372,400
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan					2 years 9 months 18 days
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 30,012	6,953	$ 38,893	12,494
Unvested restricted stock awards outstanding			1,103,000		1,103,000		931,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			$ 372,400		$ 372,400
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan					2 years 9 months 18 days
Performance-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			372,400		$ 372,400
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan					2 years 9 months 18 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			299	$ 171	$ 576	$ 330
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			$ 372,400		$ 372,400
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan					2 years 9 months 18 days
Common shares reserved for future issuance								200,000
Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock issued under plan					100,000	100,000

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	55,228,000	54,738,000
Common stock, shares outstanding	55,228,000	54,738,000

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Other comprehensive income (loss) tax expense	$ 0.8	$ 0.0	$ 1.3	$ 0.0

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Statement Of Stockholders Equity [Abstract]
Costs from IPO	$ 2,563

Description of the business - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992
Cash, cash equivalents and marketable securities	$ 1,540

Basis of presentation, principles of consolidation and significant accounting policies - Additional Information (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) Segment	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment \| Segment	1
Operating lease liabilities	$ 194,222	$ 125,800
Operating lease right-of-use assets	190,979	$ 127,300
Financing lease obligation, current portion			$ 1,424
ASU 2016-02 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase to deferred tax assets	5,300
Increase to deferred tax liabilities	7,100
Net increase to deferred tax liabilities	1,800
ASU 2016-02 [Member] \| 60 Binney Street [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Operating lease liabilities	177,000
Operating lease right-of-use assets	184,400
Financing lease obligation	149,300
Financing lease obligation, current portion	1,400
Financing lease obligation, net of current portion	153,300
Adjustment on accumulated deficit	$ 6,600

Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Equity Securities, Amortized cost / Cost	$ 20,017	$ 20,017
Equity Securities, Unrealized gains		2,150
Equity Securities, Unrealized losses	(4,034)
Equity Securities, Fair value	15,983	22,167
Marketable Securities, Amortized cost / Cost	1,261,655	1,488,746
Marketable Securities, Unrealized gains	3,717	3,113
Marketable Securities, Unrealized losses	(4,565)	(3,011)
Marketable Securities, Fair value	1,260,807	1,488,848
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	1,060,606	1,459,649
Debt Securities, Unrealized gains	3,084	963
Debt Securities, Unrealized losses	(519)	(3,011)
Debt Securities, Fair value	1,063,171	1,457,601
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	4,520	9,080
Debt Securities, Fair value	4,520	$ 9,080
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	113,820
Debt Securities, Unrealized gains	633
Debt Securities, Unrealized losses	(12)
Debt Securities, Fair value	114,441
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	62,692
Debt Securities, Fair value	$ 62,692

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Marketable Securities, Fair value	$ 1,260,807	$ 1,488,848
Equity Securities, Fair value	15,983	22,167
Liabilities:
Contingent consideration	5,740	5,230
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	280,995	402,579
Equity Securities, Fair value	15,983	22,167
Total assets	1,541,802	1,891,427
Liabilities:
Contingent consideration	5,740	5,230
Total liabilities	5,740	5,230
Fair Value, Measurements, Recurring [Member] \| U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,063,171	1,457,601
Fair Value, Measurements, Recurring [Member] \| Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	4,520	9,080
Fair Value, Measurements, Recurring [Member] \| Commercial Paper [Member]
Assets:
Marketable Securities, Fair value	62,692
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Assets:
Marketable Securities, Fair value	114,441
Fair Value, Measurements, Recurring [Member] \| Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	280,995	348,638
Equity Securities, Fair value	15,983	22,167
Total assets	296,978	370,805
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents		53,941
Total assets	1,244,824	1,520,622
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,063,171	1,457,601
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	4,520	9,080
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Commercial Paper [Member]
Assets:
Marketable Securities, Fair value	62,692
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Corporate Bonds [Member]
Assets:
Marketable Securities, Fair value	114,441
Fair Value, Measurements, Recurring [Member] \| Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	5,740	5,230
Total liabilities	$ 5,740	$ 5,230

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) - Significant unobservable inputs (Level 3) [Member] - Contingent consideration obligations [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 5,230
Changes in fair value	510
Ending balance	$ 5,740

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 158,970	$ 275,798
Less accumulated depreciation and amortization	(29,835)	(29,176)
Property, plant and equipment, net	129,135	246,622
Land [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	1,210	1,210
Building [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	14,913	180,094
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	6,518	6,365
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	6,308	5,584
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	40,953	35,693
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	21,598	183
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 67,470	$ 46,669

Property, plant and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 158,970	$ 275,798
Accumulated depreciation	29,835	29,176
Construction In Progress North Carolina Manufacturing Facility [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	63,700	$ 40,400
60 Binney Street [Member] \| ASU 2016-02 [Member]
Property Plant And Equipment [Line Items]
De-recognized of building asset	156,000
Accumulated depreciation	$ 6,700

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Employee compensation	$ 22,638	$ 28,567
Accrued manufacturing costs	20,497	21,618
Accrued clinical and contract research organization costs	16,035	11,891
Accrued property, plant, and equipment	6,113	5,451
Accrued license and milestone fees	1,690	7,739
Accrued professional fees	1,497	1,830
Financing lease obligation, current portion		1,424
Other	22,635	20,873
Total accrued expenses and other current liabilities	$ 91,105	$ 99,393

Leases - Summary of Lease Costs and Other Information Pertaining to Operating Leases (Detail) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Lease cost
Operating lease cost	$ 8,917	$ 17,400
Total lease cost	$ 8,917	17,400
Operating cash flows used for operating leases		$ 13,237
Weighted average remaining lease term	7 years 8 months 12 days	7 years 8 months 12 days
Weighted average discount rate	6.17%	6.17%

Leases - Schedule of Future Minimum Commitments Under Operating Leases (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Leases [Abstract]
2019 (excluding the six months ended June 30, 2019)	$ 17,031
2020	33,780
2021	34,215
2022	29,417
2023	29,869
2024 and thereafter	108,687
Total lease payments	252,999
Less: imputed interest	(58,777)
Total operating lease liabilities	$ 194,222	$ 125,800

Commitments and contingencies - Additional Information (Detail) - Pregenen [Member] $ in Millions	Jun. 30, 2019 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 120.0
Clinical Milestone Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	20.1
Commercial Milestones Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 99.9

Collaborative arrangements - Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied (Detail) - Celgene Corporation [Member] - USD ($) $ in Thousands		6 Months Ended
	Sep. 28, 2017	Jun. 30, 2019
Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		$ 75,000
Amended Celgene Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		25,000
Ide-cel License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee		10,000
Ide-cel Co-Development, Co-Promote and Profit Share Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, development milestone		10,000
Vectors and Associated Payload [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		87,189
Ide-cel Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		207,189
Transaction price unsatisfied		26,723
Ide-cel Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		40,912
Ide-cel License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		166,277
Transaction price unsatisfied		26,723
bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee	$ 15,000	15,000
Estimated variable consideration		26,700
Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		26,687
bb21217 Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		41,687
Transaction price unsatisfied		36,964
bb21217 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		5,444
Transaction price unsatisfied		721
bb21217 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		36,243
Transaction price unsatisfied		$ 36,243

Collaborative arrangements - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Celgene Corporation [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Receivables
Balance at beginning of period	$ 6,528
Additions	10,450
Deductions	(6,528)
Balance at end of period	10,450
Contract liabilities:
Balance at beginning of period	34,939
Additions	10,000
Deductions	(21,716)
Balance at end of period	23,223
Accrued Expenses [Member]
Contract liabilities:
Additions	3,781
Balance at end of period	$ 3,781

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	Jul. 31, 2018	Jan. 31, 2018
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering		0.3
Shares issued, price per share		$ 185.00
Proceeds from public offering of common stock, net of issuance costs		$ 48.7
Inclusive of Shares Pursuant to Over Allotment Option Granted [Member]
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	3.9
Shares issued, price per share	$ 162.50
Proceeds from public offering of common stock, net of issuance costs	$ 600.6

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) - $ / shares shares in Thousands	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding at beginning of period	4,643
Granted	1,266
Exercised	(282)
Canceled, forfeited, or expired	(234)
Outstanding at end of period	5,393
Exercisable at end of period	2,520
Vested and expected to vest at end of period	5,393
Outstanding at beginning of period	$ 108.56
Granted	136.17
Exercised	47.80
Canceled, forfeited, or expired	129.85
Outstanding at end of period	117.30
Exercisable at end of period	88.94
Vested and expected to vest at end of period	$ 117.30

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Shares
Unvested balance at beginning of period \| shares	931
Granted \| shares	438
Vested \| shares	(197)
Forfeited \| shares	(69)
Unvested balance at end of period \| shares	1,103
Weighted-average grant date fair value
Unvested balance at beginning of period \| $ / shares	$ 155.99
Granted \| $ / shares	136.32
Vested \| $ / shares	142.82
Forfeited \| $ / shares	153.94
Unvested balance at end of period \| $ / shares	$ 150.67