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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2017	Jul. 28, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q2
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	0001293971
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		45,589,385

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 686,334	$ 278,887
Marketable securities	409,100	425,491
Tenant improvements receivable	4,334	8,542
Prepaid expenses	15,036	8,209
Restricted cash and other current assets	3,814	3,085
Total current assets	1,118,618	724,214
Marketable securities	101,625	180,452
Property and equipment, net	180,419	156,955
Intangible assets, net	18,812	20,694
Goodwill	13,128	13,128
Restricted cash and other non-current assets	24,528	22,679
Total assets	1,457,130	1,118,122
Current liabilities:
Accounts payable	14,424	13,664
Accrued expenses and other current liabilities	43,748	54,660
Deferred revenue, current portion	19,666	6,209
Total current liabilities	77,838	74,533
Deferred rent, net of current portion	2,937	10,408
Deferred revenue, net of current portion	19,065	40,204
Contingent consideration, net of current portion	3,219	3,277
Financing lease obligation, net of current portion	153,962	120,140
Other non-current liabilities	90	120
Total liabilities	257,111	248,682
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at June 30, 2017 and December 31, 2016
Common stock, $0.01 par value, 125,000 shares authorized; 45,539 and 40,691 shares issued and outstanding at June 30, 2017 and December 31, 2016, respectively	455	407
Additional paid-in capital	1,918,754	1,447,856
Accumulated other comprehensive loss	(1,415)	(1,149)
Accumulated deficit	(717,775)	(577,674)
Total stockholders’ equity	1,200,019	869,440
Total liabilities and stockholders’ equity	$ 1,457,130	$ 1,118,122

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2017	Dec. 31, 2016
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	45,539,000	40,691,000
Common stock, shares outstanding	45,539,000	40,691,000

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues:
License revenue	$ 10,570		$ 10,570
Collaboration revenue	6,146	$ 1,552	12,978	$ 3,051
Total revenues	16,716	1,552	23,548	3,051
Operating expenses:
Research and development	64,311	41,760	119,339	83,671
General and administrative	21,197	18,363	41,481	34,318
Change in fair value of contingent consideration	(970)	1,404	463	2,417
Total operating expenses	84,538	61,527	161,283	120,406
Loss from operations	(67,822)	(59,975)	(137,735)	(117,355)
Interest (expense) income, net	(2,242)	981	(687)	1,932
Other (expense) income, net	(834)	(76)	(1,189)	(66)
Loss before income taxes	(70,898)	(59,070)	(139,611)	(115,489)
Income tax benefit		226		371
Net loss	$ (70,898)	$ (58,844)	$ (139,611)	$ (115,118)
Net loss per share - basic and diluted:	$ (1.73)	$ (1.59)	$ (3.41)	$ (3.12)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	41,035	36,954	40,936	36,937
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities, net of tax of $0.0 million for the three and six months ended June 30, 2017 and $0.2 and $0.9 million for the three and six months ended June 30, 2016, respectively	$ (165)	$ 330	$ (266)	$ 1,649
Comprehensive loss	$ (71,063)	$ (58,514)	$ (139,877)	$ (113,469)

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Unrealized (loss) gain on available-for-sale securities tax expense	$ 0.0	$ 0.2	$ 0.0	$ 0.9

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (139,611)	$ (115,118)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	463	1,001
Depreciation and amortization	6,281	4,747
Stock-based compensation expense	24,971	20,926
Other non-cash items	2,334	1,563
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(9,594)	(1,722)
Accounts payable	2,003	(3,132)
Accrued expenses and other liabilities	(8,509)	2,497
Deferred revenue	(7,682)	7,669
Deferred rent	507	469
Net cash used in operating activities	(128,837)	(81,100)
Investing activities
Purchase of property and equipment	(39,509)	(6,057)
Purchases of marketable securities	(116,740)	(80,845)
Proceeds from maturities of marketable securities	210,810	226,825
Restricted cash		209
Net cash provided by investing activities	54,561	140,132
Financing activities
Proceeds from public offering of common stock, net of issuance costs	436,805
Cash paid for contingent purchase price consideration		(2,025)
Reimbursement of tenant improvements for financing lease obligation	36,399
Payments on financing lease obligation	(120)
Proceeds from issuance of common stock	8,639	1,353
Net cash provided by (used in) financing activities	481,723	(672)
Increase in cash and cash equivalents	407,447	58,360
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	686,334	222,629
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	2,297	24,725
Purchases of property and equipment included in accounts payable and accrued expenses	1,194	$ 909
Offering expenses included in accounts payable and accrued expenses	191
Tenant improvements under financing lease included in tenant improvements receivable	$ 3,635

Description of the business	6 Months Ended
Description of the business	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the business	1. Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company researches, develops, manufactures and plans to commercialize gene therapies for the treatment of severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide general and administrative support for these operations.

Summary of significant accounting policies and basis of presentation

6 Months Ended

Jun. 30, 2017

Accounting Policies [Abstract]

Summary of significant accounting policies and basis of presentation

2. Summary of significant accounting policies and basis of presentation

Basis of presentation and principles of consolidation

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, 2017 and 2016.

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 financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2016, 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 22, 2017.

The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc. (“Pregenen”), bluebird bio France – SARL, bluebird bio Australia Pty Ltd., bluebird bio (UK) Ltd., bluebird bio (Bermuda) Ltd., bluebird bio Securities Corporation, and bluebird bio (Switzerland) GmbH. 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. All material long-lived assets of the Company reside in the United States.

Certain aggregations of prior year amounts have been made to conform to current year presentation. In the prior year balance sheet, tenant improvements receivable, prepaid expenses and restricted cash and other current assets are included within prepaid expenses and other current assets. In the prior period statements of operations and comprehensive loss, interest and other (expense) income were aggregated. In the current period statements of operations and comprehensive loss, interest (expense) income and other (expense) income are disclosed separately.

Summary of accounting policies

The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the six months ended June 30, 2017.

Net loss per share

Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common stock equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan using the treasury stock method.

Property and equipment

Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:

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

Leases

In September 2015, the Company entered into a lease agreement for additional office and laboratory space located at 60 Binney Street, Cambridge, Massachusetts, which was built between 2015 and March 2017, at which time 60 Binney Street became the Company’s corporate headquarters. This lease expires in 2027, subject to the Company’s right to extend the lease for an additional 10 years. Because the Company was involved in the construction project, it was deemed for accounting purposes to be the owner of the building during the construction period. Accordingly, the Company recorded project construction costs incurred by the landlord as an asset in “Property and equipment, net” and a related financing obligation in “Accrued expenses and other current liabilities” and “Financing lease obligation, net of current portion” on its condensed consolidated balance sheets.

Upon completion of the construction of the building in the first quarter of 2017, the Company evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, the Company is depreciating the building and incurring interest expense in its condensed consolidated statement of operations related to the financing obligation recorded on its condensed consolidated balance sheet. The Company bifurcates its lease payments pursuant to the lease into (i) a portion that is allocated to the financing obligation related to the building and (ii) a portion that is allocated to the land on which the building was constructed. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced in September 2015 and is recorded on a straight-line basis over the initial lease term. See Note 7, “Commitments and contingencies,” for additional information.

Stock-based compensation

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

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

The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed using an accelerated attribution model.

The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option pricing model, which requires the input of and subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of company specific historical and implied volatility data of its common stock, the Company uses a weighted-average of expected volatility based on the estimated expected volatilities of a representative group of publicly traded companies; this representative group includes the Company’s data effective January 2017. The other public companies on which the Company has based its expected stock price volatility are companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future.

As a result of the adoption of ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, effective January 1, 2017, the Company accounts for forfeitures as they occur instead of estimating forfeitures at the time of grant and revising those estimates in subsequent periods if actual forfeitures differ from its estimates. Stock-based compensation expense recognized in the financial statements is based on awards for which performance or service conditions are expected to be satisfied.

Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, the fair value of each non-employee stock option is estimated at the date of grant using the Black-Scholes option pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is over the contractual life.

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: subsequent fair value estimates used to assess potential impairment of long-lived assets, including goodwill and intangible assets, financing lease obligation, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes.

Recent accounting pronouncements

Recently adopted

In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, which simplifies share-based payment accounting through a variety of amendments. The amendments are effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods. The Company adopted this standard effective January 1, 2017. The adoption of this standard impacted the income tax footnote disclosure and did not have a material impact on the Company’s condensed consolidated financial statements. Upon adoption of the new standard, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) are recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards are treated as discrete items in the reporting period in which they occur. The Company also recognizes excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. The Company has applied the modified retrospective adoption approach beginning in 2017 and prior periods have not been adjusted. As a result, the Company established a net operating loss deferred tax asset of $76.7 million to account for prior period excess tax benefits through retained earnings, however an offsetting valuation allowance of $76.7 million will also be established through retained earnings because it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there is no impact on the Company’s condensed consolidated financial statements. The Company also elected to account for forfeitures as they occur, and recorded a cumulative catch up of $0.5 million within additional paid-in capital and retained earnings upon adoption in the first quarter of 2017.

Not yet adopted

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“Topic 606”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2018 and earlier application is permitted only for annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. Topic 606 allows for either a full retrospective adoption, in which the standard is applied to all of the periods presented, or a modified retrospective application, in which the standard is applied to the most current period presented in the financial statements. The Company expects to adopt this standard using the modified retrospective approach. The majority of revenue generated in the six months ended June 30, 2017 is from the Company’s collaboration arrangement with Celgene. The Company is continuing to assess the potential impact that Topic 606 may have on its financial position and results of operations as it relates to this arrangement. It is expected that the evaluation of variable consideration, and in particular, milestone payments due from Celgene will require further judgement to assess whether to include them in the transaction price, which could accelerate revenue recognized under ASC 606 compared to ASC 605. The Company has substantially completed its assessment of the ASC 606 impact on its two out-licensing revenue generating arrangements and does not expect the adoption of ASC 606 to have a material impact on its financial position and results of operations.

In February 2016, the FASB issued ASU 2016-02, Leases, (“ASU 2016-02”), which requires a lessee to recognize assets and liabilities on the balance sheet for operating 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 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. 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. The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position and results of operations.

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”). The new standard clarifies certain aspects of the statement of cash flows, including the classification of contingent consideration payments made after a business combination and several other clarifications not currently applicable to the Company. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for the Company on January 1, 2018. The adoption of this standard is not expected to have a material impact on the Company’s condensed consolidated statements of cash flows upon adoption.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”). The amendments in this update require that amounts generally described as restricted cash and restricted cash equivalents be included within cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 will be effective January 1, 2018, with early adoption permitted. As of June 30, 2017, the Company has not elected to early adopt this guidance, but expects the adoption to have an impact on its consolidated statement of cash flows as, upon adoption, it will include the Company’s restricted cash balance in the cash and cash equivalents reconciliation of operating, investing and financing activities.

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 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 with early adoption permitted with measurement dates on or after January 1, 2017. 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.

In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (“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 new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. 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.

In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting. The new standard is intended to reduce the diversity in practice and cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective beginning January 1, 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.

Marketable Securities

6 Months Ended

Jun. 30, 2017

Investments Debt And Equity Securities [Abstract]

Marketable securities

3. Marketable securities

The following table summarizes the available-for-sale securities held at June 30, 2017 and December 31, 2016 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
June 30, 2017
U.S. government agency securities and treasuries	$	508,141	$	—	$	(780	)	$	507,361
Certificates of deposit		3,360		4		—			3,364
Total	$	511,501	$	4	$	(780	)	$	510,725
December 31, 2016
U.S. government agency securities and treasuries	$	600,001	$	34	$	(575	)	$	599,460
Certificates of deposit		6,480		6		(3	)		6,483
Total	$	606,481	$	40	$	(578	)	$	605,943

No available-for-sale securities held as of June 30, 2017 or December 31, 2016 had remaining maturities greater than three years.

Fair value measurements

6 Months Ended

Jun. 30, 2017

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, 2017 and December 31, 2016 (in thousands):

Description	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
June 30, 2017
Assets:
Cash and cash equivalents	$	686,334	$	686,334			$	—
Marketable securities:
U.S. government agency securities and treasuries		507,361		—		507,361		—
Certificates of deposit		3,364		—		3,364		—
Total assets	$	1,197,059	$	686,334	$	510,725	$	—
Liabilities:
Contingent consideration	$	3,219	$	—	$	—	$	3,219
Total liabilities	$	3,219	$	—	$	—	$	3,219
December 31, 2016
Assets:
Cash and cash equivalents	$	278,887	$	278,887	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries		599,460		—		599,460		—
Certificates of deposit		6,483		—		6,483		—
Total assets	$	884,830	$	278,887	$	605,943	$	—
Liabilities:
Contingent consideration	$	7,756	$	—	$	—	$	7,756
Total liabilities	$	7,756	$	—	$	—	$	7,756

Cash and cash equivalents

The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. As of June 30, 2017 and December 31, 2016, cash and cash equivalents comprise funds in cash, money market accounts, and federally insured deposits.

Marketable securities

The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At June 30, 2017 and December 31, 2016, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale marketable securities. There were no realized gains on the sale of available-for-sale securities during the six months ended June 30, 2017, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period.

The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2017 and December 31, 2016 was $481.8 million and $376.1 million, respectively. As of June 30, 2017 and December 31, 2016, there were $15.6 million and $95.5 million in securities held by the Company in an unrealized loss position for more than twelve months. 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, 2017 and December 31, 2016.

Contingent consideration

On June 30, 2014, the Company acquired Pregenen. In connection with the acquisition, the Company recorded contingent consideration pertaining to the amounts potentially payable to Pregenen’s former equityholders pursuant to the Stock Purchase Agreement (the “Stock Purchase Agreement”) by and among the Company, Pregenen and Pregenen’s former equityholders. 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.

Contingent consideration may change significantly as development progresses and additional data are obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. In evaluating the fair value information, considerable judgment is required to interpret the market and internal data used to develop the estimates. The estimates of fair value may not be indicative of the amounts that could be realized in a current market exchange. Accordingly, the use of different market and internal assumptions and/or different valuation techniques could result in materially different fair value estimates.

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 2019 to 2026 and discount rates ranging from 14.2% to 15.5%. 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 these 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, 2017
Fair value, beginning of period	$	7,756
Additions		—
Changes in fair value		463
Reclassification to accrued expenses		(5,000	)
Payments		—
Fair value, end of balance	$	3,219

The Company may be required to make up to $120.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. The clinical and commercial milestone achievement end dates are between 2021 and 2028. During the quarter ended June 30, 2017, a $5.0 million preclinical milestone was achieved and is included as a component of accrued expenses and other current liabilities as of June 30, 2017. The remaining $3.2 million contingent consideration obligations are reflected as a non-current liability as of June 30, 2017.

- Definition

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

+ References + Details

Property and equipment, net

6 Months Ended

Jun. 30, 2017

Property Plant And Equipment [Abstract]

Property and equipment, net

5. Property and equipment, net

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

	As of			As of
	June 30, 2017			December 31, 2016
Building	$	160,403		$	-
Computer equipment and software		4,904			1,655
Office equipment		4,188			1,427
Laboratory equipment		20,946			16,305
Leasehold improvements		65			13,697
Construction-in-progress		476			136,315
Total property and equipment, gross		190,982			169,399
Less accumulated depreciation and amortization		(10,563	)		(12,444	)
Total Property and equipment, net	$	180,419		$	156,955

As of June 30, 2017, total property and equipment, gross, includes $160.4 million related to the Company’s new headquarters at 60 Binney Street in Cambridge, Massachusetts, of which $154.6 million was incurred by the landlord. The majority of the 60 Binney Street building was placed into service during the first quarter of 2017 with the remainder placed into service during the second quarter of 2017. Construction-in-progress as of December 31, 2016 included $126.9 million related to remaining construction costs at 60 Binney Street, of which $120.1 million was incurred by the landlord. Please refer to Note 7, "Commitments and contingencies," for further information.

Accrued expenses and other current liabilities

6 Months Ended

Jun. 30, 2017

Payables And Accruals [Abstract]

Accrued expenses and other current liabilities

6. Accrued expenses and other current liabilities

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

	As of		As of
	June 30, 2017		December 31, 2016
Accrued goods and services	$	26,170	$	34,275
Employee compensation		8,708		11,296
Accrued Pregenen preclinical milestone		5,000		—
Accrued license and milestone fees		724		2,464
Accrued professional fees		1,416		1,492
Deferred rent, current portion		—		11
Contingent consideration, current portion		—		4,479
Other		1,730		643
Total accrued expenses and other current liabilities	$	43,748	$	54,660

Commitments and contingencies	6 Months Ended
Commitments and contingencies	Jun. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	7. Commitments and contingencies The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2017 and December 31, 2016 or royalties on future sales of specified products. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to these agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with claims by any third party with respect to the Company’s products or business activities. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. Operating Lease Commitments On June 3, 2013, the Company entered into a nine-year building lease for approximately 43,600 square feet of space located at 150 Second Street, Cambridge, Massachusetts, which commenced in December 2013. This lease was amended in June 2014 to add approximately 9,900 square feet. The lease originally had monthly lease payments of $0.2 million for the first 12 months, which increased to $0.3 million per month beginning in December 2014 due to the lease amendment, with annual rent escalations thereafter. Rent expense is recognized on a straight-line basis over the term of the lease. The lease provided a contribution from the landlord towards the initial build-out of the space of up to $7.8 million. The Company capitalizes the leasehold improvements as property and equipment and records the landlord incentive payments received as deferred rent and amortizes these amounts as reductions to rent expense over the lease term. In addition, in accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit naming the landlord as beneficiary, which has a balance of $0.6 million as of June 30, 2017. The Company expects the $0.6 million letter of credit to be released in the third quarter of 2017. On September 30, 2016, the Company entered into an Assignment and Assumption of Lease (“Assignment”) relating to its lease at 150 Second Street. Under the Assignment, the Company assigned all of its rights, interests, obligations and responsibilities under the lease effective May 1, 2017. Accordingly, $8.3 million of tenant improvement assets were disposed and $8.0 million of non-current deferred rent was removed from the condensed consolidated balance sheets as of June 30, 2017, with the resulting loss of $0.3 million recorded within the condensed consolidated statement of operations and comprehensive loss during the second quarter of 2017. On June 29, 2015, the Company entered into a lease agreement for additional office space located at 215 First Street, Cambridge, Massachusetts. Under the terms of the lease, the Company leased approximately 15,120 square feet starting on July 13, 2015 for $0.5 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes. The lease term was until August 31, 2020, and included early termination provisions that could allow the Company to terminate the lease without penalty at the end of the 20^th full calendar month following the delivery of the premises if the Company meets certain conditions specified within the lease. Under the terms of the lease, the Company also leased an additional 8,075 square feet of office space in the same premises starting on January 1, 2016 for an additional $0.3 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes. The Company terminated this lease effective April 12, 2017. On June 3, 2016, the Company entered into a strategic manufacturing agreement for the future commercial production of the Company’s Lenti-D and LentiGlobin product candidates 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 is required to pay $12.5 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. The Company paid $5.0 million for the achievement of the first and second contractual milestones during 2016 and paid the third milestone of $3.0 million during the first quarter of 2017. Following construction completion, the Company will pay $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 any time after July 1, 2016 upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company concluded that this agreement contains an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company concluded that it is not the deemed owner during construction nor is it a capital lease under ASC 840-10, Leases - Overall. As a result, the Company will account for the agreement as an operating lease and expense the rental payments on a straight-line basis over the non-cancellable term of the embedded lease. On November 18, 2016, the Company entered into an agreement for future clinical and commercial production of the Company’s LentiGlobin gene therapy drug products with a contract manufacturing organization at an existing facility. The term of the agreement is five years with a three year renewal at the mutual option of each party. Under the agreement, the Company is required to pay an up-front fee of €3.0 million, €2.0 million of which was paid in the fourth quarter of 2016 and €1.0 million of which is expected to be paid in mid-2018, and annual maintenance and production fees of up to €9.8 million, depending on its production needs. The Company may terminate this agreement with six months’ notice and a one-time termination fee prior to July 1, 2018, or twelve months’ notice and a one-time termination fee thereafter. The Company concluded that this agreement contains an embedded lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement, and determined that it is not a capital lease under ASC 840-10, Leases – Overall . As a result, the Company will account for the agreement as an operating lease and expense the rental payments on a straight-line basis over the non-cancellable term of the embedded lease. 60 Binney Street Lease Commitments On September 21, 2015, the Company entered into a lease agreement for additional office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”). Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company will lease 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 also executed a $9.2 million letter of credit upon signing the 60 Binney Street Lease, which was required to be collateralized with a bank account at a financial institution in accordance with the 60 Binney Street Lease agreement. This letter of credit was increased to $13.8 million during the third quarter of 2016 as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease back to $9.2 million over time. The 60 Binney Street Lease will continue until March 31, 2027. Pursuant to a work letter entered into in connection with the 60 Binney Street Lease, the landlord will contribute an aggregate of $42.4 million toward the cost of construction and tenant improvements for the Building. The purpose of the 60 Binney Street Lease was to replace the Company’s previously leased premises at 150 Second Street and 215 First Street in Cambridge, Massachusetts. The Company has the option to extend the 60 Binney Street Lease for two successive five-year terms. The Company occupied 60 Binney Street beginning on March 27, 2017. Because the Company was involved 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. Accordingly, costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance have been recorded as an asset in “Property and equipment, net” with a related financing obligation in “Accrued expenses and other current liabilities” and “Financing lease obligation, net of current portion” on the Company’s condensed consolidated balance sheets. Tenant improvement costs that are reimbursable by the landlord and have not yet been paid to the Company are recorded in “Tenant improvements receivable” on the Company’s condensed consolidated balance sheets. Tenant improvement costs that are not reimbursable by the landlord are only recorded in “Property and equipment, net” on the Company’s condensed 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. 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, which was $3.7 million for the three and six months ended June 30, 2017. The Company bifurcates its lease payments pursuant to the 60 Binney Street Lease into (i) a portion that is allocated to the Building and (ii) a portion that is allocated to the land on which the Building has been constructed, which is recorded as rental expense. The Company began making lease payments pursuant to the 60 Binney Street Lease in March 2017. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced upon execution of the 60 Binney Street Lease in September 2015. During the three and six months ended June 30, 2017, the Company recognized $0.5 million and $0.9 million of non-cash rental expense attributable to the land. As of June 30, 2017, Property and equipment, net, includes $160.4 million related to the Building, of which $154.6 million has been incurred by the landlord. As of June 30, 2017, Tenant improvements receivable of $4.3 million, which consists of tenant improvement costs reimbursable by the landlord that had not yet been received, was recorded in the condensed consolidated balance sheets. As of December 31, 2016, Property and equipment, net, includes $126.9 million related to the Building, of which $120.1 million has been incurred by the landlord. As of December 31, 2016, Tenant improvements receivable was $8.5 million, $7.8 million of which was received by the Company in January 2017.

Significant agreements	6 Months Ended
Significant agreements	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant agreements	8. Significant agreements Celgene Corporation Original Collaboration Agreement On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Collaboration Agreement”) with Celgene Corporation (“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. Under the terms of the 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 I clinical studies, if any, during the initial term of the 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 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. Amended Collaboration Agreement On June 3, 2015, the Company and Celgene amended and restated the Collaboration Agreement (the “Amended Collaboration Agreement”). Under the Amended Collaboration Agreement, the parties will now focus the collaboration exclusively on anti- B-cell maturation antigen (“BCMA”) product candidates for a new three-year term. In connection with the Amended Collaboration Agreement, the Company received an upfront, one-time, non-refundable, non-creditable payment of $25.0 million to fund research and development under the collaboration. The collaboration will continue to be governed by the JSC. Under the terms of the Amended Collaboration Agreement, for up to two product candidates selected for development under the collaboration, the Company is responsible for conducting and funding all research and development activities performed up through completion of the initial Phase I clinical study of such product candidate. On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial Phase I clinical study for such product candidate (the “Option Period”), the Company has granted Celgene an option to obtain an exclusive worldwide license to develop and commercialize such product candidate pursuant to a written agreement, the form of which the Company has already agreed upon. In the event that Celgene exercises its option with respect to any product candidate, the Company may elect to co-develop and co-promote the product candidate in the United States, provided that, if the Company does not exercise its option co-develop and co-promote the first product candidate in-licensed by Celgene under the Amended Collaboration Agreement, then the Company will not be permitted to exercise its option to co-develop and co-promote any future product candidates under the Amended Collaboration Agreement. If Celgene elects to exercise its option to exclusively in-license a product candidate, it must pay the Company an option fee in the amount of $10.0 million for the first product candidate and $15.0 million for any additional product candidates. On February 10, 2016, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb2121, the first product candidate under the Amended Collaboration Agreement, pursuant to an executed license agreement entered into by the parties on February 16, 2016 and paid the associated $10.0 million option fee. The Company expects to initiate an initial Phase I clinical study for bb21217, the second product candidate arising from the collaboration, in 2017. Accordingly, Celgene may exercise its option to obtain an exclusive worldwide license to develop and commercialize bb21217 by paying the associated $15.0 million option fee. The Company may now exercise its option to co-develop and co-promote bb2121 within the United States, which it currently expects to elect. The Company’s election to co-develop and co-promote must be made at the substantial completion of the ongoing Phase 1 trial of bb2121. If elected, the Company will share equally in all costs relating to developing, commercializing and manufacturing the product candidate within the United States. Under this scenario, the Company may receive, per product, up to $10.0 million in clinical milestone payments and, outside of the United States, up to $54.0 million in regulatory milestone payments, and up to $36.0 million in commercial milestone payments. In addition, to the extent bb2121 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 United States, subject to certain reductions. In the event the Company does not exercise its option to co-develop and co-promote bb2121, 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 bb2121 is commercialized, the Company would be entitled to receive a percentage of net sales as a royalty in a range from the mid-single digits to low-teens, subject to certain reductions. Accounting Analysis The Company’s Amended Collaboration Agreement with Celgene contains the following deliverables: (i) research and development services, (ii) participation on the JSC, (iii) participation on the patent committee, (iv) a license to the first product candidate, (v) manufacture of vectors and associated payload for incorporation into the first optioned product candidate under the license, and (vi) participation on the JGC under the co-development and co-promotion agreement for the first optioned product candidate under the license. The license to the first product candidate was considered a deliverable at the inception of the arrangement and therefore the associated option fee was included in allocable arrangement consideration as the Company believed there was minimal risk with regard to whether Celgene will exercise the option based on the successful completion of preclinical activities and proximity of enrollment of the first patient in an initial Phase I clinical study for this product candidate. The Company determined that the obligation within the license to manufacture or have manufactured supplies of vectors and associated payloads for incorporation into the first optioned product candidate is a deliverable, consistent with the option to license the first product candidate. However, the Company determined that the options to license any additional product candidates are substantive options and therefore were not considered deliverables at execution of the Amended Collaboration Agreement. Celgene is not contractually obligated to exercise the options. Additionally, as a result of the uncertain outcome of the discovery, research and development activities, the Company is at risk with regard to whether Celgene will exercise the options to license additional product candidates. Moreover, the Company determined that the options are not priced at a significant and incremental discount. Accordingly, the options to other product candidates are not considered deliverables and the associated option fees are not included in allocable arrangement consideration. Upon execution of the Amended Collaboration Agreement in June 2015, the Company concluded that each of the three delivered elements at the inception of the agreement (research and development services, participation on the JSC and participation on the patent committee) had standalone value from the other undelivered elements. Additionally, the Amended Collaboration Agreement does not include return rights related to the collaboration term. Accordingly, each deliverable qualified as a separate unit of accounting. The Company determined that each of the delivered elements had the same period of performance (the three year term through projected initial Phase I clinical study substantial completion) and the same pattern of revenue recognition, ratably over the period of performance as there was no other discernible pattern of recognition. The Company identified the allocable arrangement consideration as the $25.0 million up-front research and development funding payment, $10.0 million option fee for the first product candidate, $20.0 million related to remaining deferred revenue from the original Collaboration Agreement, and $54.1 million of contingent revenue related to the estimated amounts that will be received from Celgene for manufacturing services. The $109.0 million total allocable arrangement consideration was allocated based on the relative estimated selling price of the separate units of accounting at the inception of the amended agreement, resulting in $17.3 million allocated to the three delivered elements at the inception of the agreement, which will be recognized over an initial three year term. The Company determined that each of the identified deliverables that qualify as a separate unit of accounting continue to have the same period of performance (the three year term through projected initial Phase I clinical study substantial completion) and the same pattern of revenue recognition, ratably over the period of performance as there is no other discernible pattern of recognition, and therefore there is no change in the recognition of $17.3 million allocated to these three elements. As of June 30, 2017, this will continue to be recognized over a three-year term that began in June 2015. However, the Company concluded that the license to bb2121 does not have standalone value from one of the undelivered elements, the post-initial Phase I the manufacture of vectors and associated payload for bb2121 under the license, because the manufacturing is essential to the license agreement. The Company is required to reassess its conclusions on standalone value of deliverables upon delivery, and therefore, upon commencement of manufacturing services in the first quarter of 2017, the Company updated its assessment. The Company determined that there were no changes and that the bb2121 license agreement continues to not have standalone value from the manufacturing services. Accordingly, these two deliverables qualify as a single combined unit of accounting. Revenue recognition for the combined unit of accounting commenced during the first quarter of 2017 after the Company reached agreement with Celgene regarding the budget and timing for such manufacturing services. Accordingly, revenue of $6.0 million and $10.9 million was recognized during the three and six months ended June 30, 2017 related to the combined unit of accounting. The Company recognizes revenue associated with the combined unit of accounting using the proportional performance method. In using this method, the Company estimated, through discussions with Celgene regarding their development plan for bb2121, the proportion of effort it incurred as a percentage of total effort it expects to incur and applied this ratio to the total estimated budget for the post-initial Phase I manufacture of vectors and associated payload for bb2121. In developing the total estimated budget, management assumed that the Company will exercise its option to co-develop and co-promote bb2121 and therefore is currently recognizing revenue related to 67.5% of worldwide development costs incurred, which represents the percentage the Company is contractually entitled to bill Celgene under the cost share provisions of the co-develop and co-promote agreement. Costs incurred and billable by Celgene to the Company under the co-develop and co-promote agreement were $1.6 million for the three and six months ended June 30, 2017 and are recorded as an offset to collaboration revenue in the condensed consolidated statement of operations. In the event that the Company does not exercise its option to co-develop and co-promote bb2121, the Company expects to recognize the remaining 32.5% of worldwide development costs, as Celgene would be responsible for 100% of costs incurred plus a markup. Actual costs could materially differ from these estimates, and management has applied significant judgment in the process of developing its budget estimates. Any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. Assuming the co-develop and co-promote agreement is executed and other revenue recognition criteria have been met, the Company expects to recognize approximately $89.5 million, before any offsetting expenses reimbursable to Celgene, between the commencement of post-Phase 1 development in the first quarter of 2017 and the end of development in 2021. The period of performance and recognition pattern will be revisited as the development plan changes or if other events impacting the deliverables occur. The Company also recognized $0.2 million and $0.6 million for the three and six months ended June 30, 2017, respectively, of revenue related to other development costs incurred, which are billable under the collaboration agreement. During the three and six months ended June 30, 2016, the Company recognized no revenue related to the combined unit of account comprised of the bb2121 license agreement and manufacturing services, and no revenue related to other development costs incurred. During the three and six months ended June 30, 2017, the Company recognized revenue of $1.6 million and $3.1 million related to research and development services, respectively. During the three and six months ended June 30, 2016, the Company recognized revenue of $1.6 million and $3.1 million related to research and development services, respectively. The Company evaluated all of the milestones that may be received in connection with Celgene’s option to license a product candidate resulting from the collaboration. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All clinical and regulatory milestones that may be received under the option to the license agreement are considered substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the milestone as well as the level of effort and investment required. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. During the three months ended June 30, 2017 and 2016, the Company recognized collaboration revenue of $6.1 million and $1.6 million, respectively, related to research and development services, delivery of the license to bb2121 and manufacturing services for bb2121 vector and plasmids under its collaboration with Celgene. During the six months ended June 30, 2017 and 2016, the Company recognized collaboration revenue of $13.0 million and $3.1 million, respectively, related to research and development services, delivery of the license to bb2121 and manufacturing services for bb2121 vector and plasmids under its collaboration with Celgene. As of June 30, 2017 and December 31, 2016, there was $38.7 million and $46.4 million, respectively, of total deferred revenue related to the Company’s collaboration with Celgene, which is classified as current or non-current in the condensed consolidated balance sheets. As of June 30, 2017, Restricted cash and other non-current assets includes a $5.3 million receivable related to cost reimbursement from Celgene for bb2121 development costs incurred to date, net of costs incurred and billable by Celgene to the Company of $1.6 million. There was no receivable from Celgene as of December 31, 2016. Novartis Pharma AG On April 26, 2017, the Company entered into a worldwide license agreement with Novartis Pharma AG (“Novartis”). Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including CTL019, Novartis’s anti-CD19 CAR T investigational therapy. Financial terms of the agreement include a $7.5 million payment upon execution, $7.5 million of potential future milestone payments associated with regulatory approval, and $1.1 million of payments for each subsequently licensed product, as well as low single digit royalty payments on net sales of covered products. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. At the date of contract inception, only one deliverable was identified and accordingly the entire nonrefundable license fee was recognized. Given that there were no further deliverables identified in the contract, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. During the three and six months ended June 30, 2017, the Company recognized revenue $7.5 million upon delivery of the license, as there were no other undelivered elements in the arrangement. GlaxoSmithKline Intellectual Property Development Limited On April 28, 2017, the Company entered into a worldwide license agreement with GlaxoSmithKline Intellectual Property Development Limited (“GSK”). Under the terms of the agreement, GSK non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize gene therapies for Wiscott-Aldrich syndrome and metachromatic leukodystrophy, two rare genetic diseases. Financial terms of the agreement include a nonrefundable upfront payment of $3.0 million as well as $1.3 million of potential milestone payments for each marketing authorization for each indication in any country as well as low single digit royalties on net sales of covered products. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. At the date of contract inception, only one deliverable was identified and accordingly the entire nonrefundable license fee was recognized as revenue. Given that there were no further deliverables identified in the contract, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. During the three and six months ended June 30, 2017, the Company recognized revenue of $3.0 million upon delivery of the license, as there were no other undelivered elements in the arrangement.

Equity	6 Months Ended
Equity	Jun. 30, 2017
Equity [Abstract]
Equity	9. Equity In June 2017, the Company sold 4,831,500 shares of common stock (inclusive of 571,500 shares of common stock sold by the Company pursuant to the full exercise of an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $105.00 per share for aggregate net proceeds of $436.8 million.

Stock-based compensation

6 Months Ended

Jun. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based compensation

10. Stock-based compensation

In January 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 1.6 million shares as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2017, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 1.7 million.

Stock-based compensation expense

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

	For the				For the
	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Stock options	$	10,764	$	9,057	$	20,107	$	17,984
Restricted stock units		2,636		1,626		4,592		2,753
Employee stock purchase plan		89		99		272		189
	$	13,489	$	10,782	$	24,971	$	20,926

As of June 30, 2017, the Company had $128.0 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.7 years.

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

	For the				For the
	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	6,797	$	5,330	$	12,451	$	10,011
General and administrative		6,692		5,452		12,520		10,915
	$	13,489	$	10,782	$	24,971	$	20,926

Stock options

The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average exercise price per share
Outstanding at December 31, 2016		3,735		$	52.17
Granted		943		$	80.41
Exercised		(395	)	$	20.61
Canceled or forfeited		(92	)	$	85.22
Outstanding at June 30, 2017		4,191		$	60.77
Exercisable at June 30, 2017		2,075		$	45.74
Vested and expected to vest at June 30, 2017		4,191		$	60.77

During the six months ended June 30, 2017, 0.4 million shares of common stock were exercised, resulting in total proceeds to the Company of $8.1 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 units

The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2016		263		$	63.07
Granted		266		$	81.83
Vested		(59	)	$	67.54
Forfeited		(10	)	$	58.50
Unvested balance at June 30, 2017		460		$	73.45

Employee stock purchase plan

On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238,000 shares of the Company’s common stock to participating employees. The first offering period under the 2013 ESPP opened on August 1, 2014. During the six months ended June 30, 2017 and 2016, 11,079 shares and 9,758 shares of common stock were issued under the 2013 ESPP, respectively.

Income taxes	6 Months Ended
Income taxes	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income taxes	11. Income taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Net loss per share

6 Months Ended

Jun. 30, 2017

Earnings Per Share [Abstract]

Net loss per share

12. Net loss per share

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

	For the Three and Six Months Ended
	June 30,
	2017		2016
Outstanding stock options		4,191		4,103
Restricted stock units		460		338
ESPP shares		11		10
		4,662		4,451

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

6 Months Ended

Jun. 30, 2017

Accounting Policies [Abstract]

Basis of presentation and principles of consolidation

Summary of accounting policies

Net loss per share

Property and equipment

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

Leases

Stock-based compensation

Use of estimates

Recent accounting pronouncements

Recently adopted

Not yet adopted

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

6 Months Ended

Jun. 30, 2017

Accounting Policies [Abstract]

Estimated Useful Lives of Assets

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

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

Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2017

Investments Debt And Equity Securities [Abstract]

Summary of Available for Sale Securities Held

The following table summarizes the available-for-sale securities held at June 30, 2017 and December 31, 2016 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
June 30, 2017
U.S. government agency securities and treasuries	$	508,141	$	—	$	(780	)	$	507,361
Certificates of deposit		3,360		4		—			3,364
Total	$	511,501	$	4	$	(780	)	$	510,725
December 31, 2016
U.S. government agency securities and treasuries	$	600,001	$	34	$	(575	)	$	599,460
Certificates of deposit		6,480		6		(3	)		6,483
Total	$	606,481	$	40	$	(578	)	$	605,943

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Fair value measurements (Tables)

6 Months Ended

Jun. 30, 2017

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, 2017 and December 31, 2016 (in thousands):

Description	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
June 30, 2017
Assets:
Cash and cash equivalents	$	686,334	$	686,334			$	—
Marketable securities:
U.S. government agency securities and treasuries		507,361		—		507,361		—
Certificates of deposit		3,364		—		3,364		—
Total assets	$	1,197,059	$	686,334	$	510,725	$	—
Liabilities:
Contingent consideration	$	3,219	$	—	$	—	$	3,219
Total liabilities	$	3,219	$	—	$	—	$	3,219
December 31, 2016
Assets:
Cash and cash equivalents	$	278,887	$	278,887	$	—	$	—
Marketable securities:
U.S. government agency securities and treasuries		599,460		—		599,460		—
Certificates of deposit		6,483		—		6,483		—
Total assets	$	884,830	$	278,887	$	605,943	$	—
Liabilities:
Contingent consideration	$	7,756	$	—	$	—	$	7,756
Total liabilities	$	7,756	$	—	$	—	$	7,756

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, 2017
Fair value, beginning of period	$	7,756
Additions		—
Changes in fair value		463
Reclassification to accrued expenses		(5,000	)
Payments		—
Fair value, end of balance	$	3,219

Property and equipment, net (Tables)

6 Months Ended

Jun. 30, 2017

Property Plant And Equipment [Abstract]

Summary of Property and Equipment Net

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

	As of			As of
	June 30, 2017			December 31, 2016
Building	$	160,403		$	-
Computer equipment and software		4,904			1,655
Office equipment		4,188			1,427
Laboratory equipment		20,946			16,305
Leasehold improvements		65			13,697
Construction-in-progress		476			136,315
Total property and equipment, gross		190,982			169,399
Less accumulated depreciation and amortization		(10,563	)		(12,444	)
Total Property and equipment, net	$	180,419		$	156,955

Accrued expenses and other current liabilities (Tables)

6 Months Ended

Jun. 30, 2017

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

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

	As of		As of
	June 30, 2017		December 31, 2016
Accrued goods and services	$	26,170	$	34,275
Employee compensation		8,708		11,296
Accrued Pregenen preclinical milestone		5,000		—
Accrued license and milestone fees		724		2,464
Accrued professional fees		1,416		1,492
Deferred rent, current portion		—		11
Contingent consideration, current portion		—		4,479
Other		1,730		643
Total accrued expenses and other current liabilities	$	43,748	$	54,660

Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2017

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 (in thousands):

	For the				For the
	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Stock options	$	10,764	$	9,057	$	20,107	$	17,984
Restricted stock units		2,636		1,626		4,592		2,753
Employee stock purchase plan		89		99		272		189
	$	13,489	$	10,782	$	24,971	$	20,926

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 (in thousands):

	For the				For the
	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	6,797	$	5,330	$	12,451	$	10,011
General and administrative		6,692		5,452		12,520		10,915
	$	13,489	$	10,782	$	24,971	$	20,926

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):

	Shares			Weighted-average exercise price per share
Outstanding at December 31, 2016		3,735		$	52.17
Granted		943		$	80.41
Exercised		(395	)	$	20.61
Canceled or forfeited		(92	)	$	85.22
Outstanding at June 30, 2017		4,191		$	60.77
Exercisable at June 30, 2017		2,075		$	45.74
Vested and expected to vest at June 30, 2017		4,191		$	60.77

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):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2016		263		$	63.07
Granted		266		$	81.83
Vested		(59	)	$	67.54
Forfeited		(10	)	$	58.50
Unvested balance at June 30, 2017		460		$	73.45

Net loss per share (Tables)

6 Months Ended

Jun. 30, 2017

Earnings Per Share [Abstract]

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

	For the Three and Six Months Ended
	June 30,
	2017		2016
Outstanding stock options		4,191		4,103
Restricted stock units		460		338
ESPP shares		11		10
		4,662		4,451

Description of the business - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992

Summary of significant accounting policies and basis of presentation - Additional Information (Detail) $ in Millions		6 Months Ended
	Sep. 21, 2015	Jun. 30, 2017 USD ($) Segment	Mar. 31, 2017 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment \| Segment		1
ASU 2016-09 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred tax assets, net operating loss		$ 76.7
Deferred tax assets, valuation allowance		$ 76.7
ASU 2016-09 [Member] \| Additional Paid-in Capital and Retained Earnings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cumulative catch up to account for forfeitures			$ 0.5
Lease starting on October 1, 2016 [Member] \| 60 Binney Street Lease [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Lease expiration year	2027
Additional lease period extendable	10 years

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	6 Months Ended
	Jun. 30, 2017
Building [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	40 years
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	3 years
Office and Laboratory Equipment [Member] \| Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	2 years
Office and Laboratory Equipment [Member] \| Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Leasehold improvements	Shorter of the useful life or remaining lease term

Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 511,501	$ 606,481
Unrealized Gains	4	40
Unrealized Losses	(780)	(578)
Fair Value	510,725	605,943
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	508,141	600,001
Unrealized Gains		34
Unrealized Losses	(780)	(575)
Fair Value	507,361	599,460
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,360	6,480
Unrealized Gains	4	6
Unrealized Losses		(3)
Fair Value	$ 3,364	$ 6,483

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Cash and cash equivalents	$ 686,334	$ 278,887
Total assets	1,197,059	884,830
Liabilities:
Contingent consideration	3,219	7,756
Total liabilities	3,219	7,756
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	507,361	599,460
Certificates of Deposit [Member]
Assets:
Marketable securities	3,364	6,483
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	686,334	278,887
Total assets	686,334	278,887
Significant other observable inputs (Level 2) [Member]
Assets:
Total assets	510,725	605,943
Significant other observable inputs (Level 2) [Member] \| U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	507,361	599,460
Significant other observable inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Assets:
Marketable securities	3,364	6,483
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	3,219	7,756
Total liabilities	$ 3,219	$ 7,756

Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities			Three months or less
Realized gain on available-for-sale securities			$ 0
Reclassification out of accumulated other comprehensive loss			0
Unrealized Loss on Securities	$ 481,800,000		481,800,000		$ 376,100,000
Unrealized loss on securities, more than twelve months	15,600,000		15,600,000		95,500,000
Investments with other-than-temporary impairment	0		0		0
Achievement of research milestone	(970,000)	$ 1,404,000	463,000	$ 2,417,000
Contingent consideration, non current	3,219,000		3,219,000		$ 3,277,000
Pregenen [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	120,000,000		120,000,000
Pregenen [Member] \| Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	99,900,000		99,900,000
Pregenen [Member] \| Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Achievement of research milestone	5,000,000
Pregenen [Member] \| Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	$ 20,100,000		$ 20,100,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2019
Milestone discount rates			14.20%
Minimum [Member] \| Pregenen [Member] \| Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2021
Minimum [Member] \| Pregenen [Member] \| Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2021
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2026
Milestone discount rates			15.50%
Maximum [Member] \| Pregenen [Member] \| Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2028
Maximum [Member] \| Pregenen [Member] \| Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2028

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, 2017 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, beginning of period	$ 7,756
Changes in fair value	463
Reclassification to accrued expenses	(5,000)
Fair value, end of balance	$ 3,219

Property and equipment, net - Summary of Property and Equipment Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 190,982	$ 169,399
Less accumulated depreciation and amortization	(10,563)	(12,444)
Total Property and equipment, net	180,419	156,955
Building [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	160,403
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	4,904	1,655
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	4,188	1,427
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	20,946	16,305
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	65	13,697
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 476	$ 136,315

Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 190,982	$ 169,399
60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	160,400
Construction cost incurred by landlord	$ 154,600
Construction In Progress 60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross		126,900
Construction cost incurred by landlord		$ 120,100

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued goods and services	$ 26,170	$ 34,275
Employee compensation	8,708	11,296
Accrued Pregenen preclinical milestone	5,000
Accrued license and milestone fees	724	2,464
Accrued professional fees	1,416	1,492
Deferred rent, current portion		11
Contingent consideration, current portion		4,479
Other	1,730	643
Total accrued expenses and other current liabilities	$ 43,748	$ 54,660

Commitments and contingencies - Additional Information (Detail) € in Millions								1 Months Ended	3 Months Ended					6 Months Ended
	Jun. 30, 2017 USD ($)	Nov. 18, 2016 EUR (€)	Jun. 03, 2016 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Jun. 29, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Jun. 03, 2013 USD ($) ft²	Jan. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 EUR (€)	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Lease period							9 years
Lease building space \| ft²							43,600
Amendment effective date							2014-06
Additional lease building space \| ft²							9,900
Lease payments							$ 200,000
Increase in monthly lease payments						$ 300,000
Contribution from the landlord towards the initial build-out of the space							$ 7,800,000
Cash-collateralized irrevocable standby letter of credit	$ 600,000
Lease starting date							Jun. 03, 2013
Lease termination date														Apr. 12, 2017
Term of agreement		5 years
Renewal period of agreement		3 years
Up-front fee \| €		€ 3.0
Up-front fee paid \| €												€ 2.0
Up-front fee payable in mid-2018 \| €		1.0
Annual maintenance and production fees \| €		€ 9.8
Termination description														The Company may terminate this agreement with six months’ notice and a one-time termination fee prior to July 1, 2018, or twelve months’ notice and a one-time termination fee thereafter.
Property and equipment, gross	190,982,000									$ 190,982,000				$ 190,982,000	$ 169,399,000
Tenant improvements receivable	4,334,000									4,334,000				4,334,000	8,542,000
60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Property and equipment, gross	160,400,000									160,400,000				160,400,000	126,900,000
Construction financing lease obligation	154,600,000									154,600,000				154,600,000	120,100,000
Tenant improvements receivable	4,300,000									4,300,000				$ 4,300,000	$ 8,500,000
Tenant improvement costs received								$ 7,800,000
Building [Member]
Commitments And Contingencies Disclosure [Line Items]
Useful life of asset														40 years
Interest expense related to financing obligation										3,700,000				$ 3,700,000
Property and equipment, gross	160,403,000									160,403,000				160,403,000
Land [Member]
Commitments And Contingencies Disclosure [Line Items]
Non-cash rent expense										500,000				900,000
150 Second Street [Member]
Commitments And Contingencies Disclosure [Line Items]
Tenant improvement assets disposed	8,300,000									8,300,000				8,300,000
Non-current deferred rent disposed	$ 8,000,000									8,000,000				$ 8,000,000
Loss of tenant improvement assets disposed										$ 300,000
Lease starting on July 13, 2015 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period					60 months
Lease building space \| ft²					15,120
Lease payments					$ 500,000
Lease starting date					Jul. 13, 2015
Early termination lease term					20 months
Operating lease description														Under the terms of the lease, the Company leased approximately 15,120 square feet starting on July 13, 2015 for $0.5 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes.
Operating lease, rent increase percentage					3.00%
Lease starting on January 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease building space \| ft²					8,075
Lease payments					$ 300,000
Lease starting date					Jan. 01, 2016
Operating lease description														Under the terms of the lease, the Company also leased an additional 8,075 square feet of office space in the same premises starting on January 1, 2016 for an additional $0.3 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes. The Company terminated this lease effective April 12, 2017.
Operating lease, rent increase percentage					3.00%
Lease expiration date					Aug. 31, 2020
Strategic manufacturing agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period			12 years
Lease payments			$ 5,100,000
Early termination lease term			24 months
Amount payable upon achievement of certain contractual milestones			$ 12,500,000
Milestone payments made			5,000,000								$ 3,000,000
Agreement termination description														Company may terminate this agreement any time after July 1, 2016 upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.
Strategic manufacturing agreement [Member] \| Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Additional contractual payables to milestones			$ 8,000,000
Lease starting on October 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease building space \| ft²				253,108
Contribution from the landlord towards the initial build-out of the space				$ 42,400,000
Cash-collateralized irrevocable standby letter of credit				$ 9,200,000
Lease starting date				Oct. 01, 2016
Lease payments base annual rent				$ 18,400,000
Capital lease description														Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company will lease 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.
Increase in letter of credit under the terms of the lease													$ 13,800,000
Lease starting on October 1, 2016 [Member] \| 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date				Mar. 31, 2027
Annual lease rent per square foot \| $ / ft²				72.50
Lease rent increase percentage				1.75%
Option to extend capital lease				10 years
Scenario, Forecast [Member]
Commitments And Contingencies Disclosure [Line Items]
Cash-collateralized irrevocable standby letter of credit									$ 600,000

Significant agreements - Additional Information (Detail)					3 Months Ended			6 Months Ended
	Apr. 28, 2017 USD ($) Deliverables	Apr. 26, 2017 USD ($) Deliverables	Jun. 03, 2015 USD ($) Deliverables	Mar. 19, 2013 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Feb. 17, 2016 USD ($)	Feb. 16, 2016 USD ($)
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of collaboration agreement				3 years
Celgene Corporation [Member] \| First Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event			$ 10,000,000
Amount paid per product upon achievement of specified event												$ 10,000,000
Celgene Corporation [Member] \| Additional Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event			$ 15,000,000
Celgene Corporation [Member] \| Co-Develop and Co-Promote Options not Exercise [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event											$ 10,000,000
Celgene Corporation [Member] \| Second Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount payable per product upon achievement of specified event					$ 15,000,000			$ 15,000,000
Celgene Corporation [Member] \| Commercial Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					10,000,000			10,000,000
Celgene Corporation [Member] \| Regulatory Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					117,000,000			117,000,000
Celgene Corporation [Member] \| Commercial Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					78,000,000			78,000,000
Celgene Corporation [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event						$ 89,500,000
Number of deliverable \| Deliverables			3
Consideration allocated to agreement								$ 109,000,000
Deferred revenue recognition period								3 years
Deferred revenue recognized					6,100,000		$ 1,600,000	$ 13,000,000	$ 3,100,000
Percentage of revenue related to worldwide development cost						67.50%
Percentage of worldwide development costs for responsible collaborator						100.00%
Revenue recognized related to other development costs incurred					200,000		0	600,000	0
Deferred revenue					38,700,000			38,700,000		$ 46,400,000
Cost reimbursement receivable for product development costs incurred					5,300,000			5,300,000		$ 0
Celgene Corporation [Member] \| Collaborative Arrangement [Member] \| Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized					1,600,000		1,600,000	3,100,000	3,100,000
Celgene Corporation [Member] \| Collaborative Arrangement [Member] \| Co-Develop and Co-Promote Options not Exercise [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of revenue related to worldwide development cost						32.50%
Celgene Corporation [Member] \| Collaborative Arrangement [Member] \| Option Fee [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					10,000,000			10,000,000
Celgene Corporation [Member] \| Collaborative Arrangement [Member] \| Delivered Elements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received			$ 20,000,000
Consideration allocated to agreement								17,300,000
Celgene Corporation [Member] \| Collaborative Arrangement, Co-promotion and Development [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Costs incurred and billable under collaborative agreement					1,600,000			1,600,000
Celgene Corporation [Member] \| Collaborative Arrangement, Co-promotion and Development [Member] \| Commercial Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					10,000,000			10,000,000
Celgene Corporation [Member] \| Collaborative Arrangement, Co-promotion and Development [Member] \| Regulatory Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					54,000,000			54,000,000
Celgene Corporation [Member] \| Collaborative Arrangement, Co-promotion and Development [Member] \| Commercial Milestones Payments [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event					36,000,000			36,000,000
Novartis Pharma AG [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized					7,500,000			7,500,000
License agreement upfront payment		$ 7,500,000
License agreement potential milestone payments associated with regulatory approval		7,500,000
Payments for subsequently licensed products and low single digit royalty payments on net sales of covered products		$ 1,100,000
Number of deliverables identified at the date of contract inception \| Deliverables		1
GlaxoSmithKline Intellectual Property Development Limited [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized					3,000,000			3,000,000
License agreement upfront payment	$ 3,000,000
Number of deliverables identified at the date of contract inception \| Deliverables	1
License agreement, potential future milestone payments	$ 1,300,000
Up-front Payment Arrangement [Member] \| Celgene Corporation [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received				$ 75,000,000				25,000,000
Up-front Payment Arrangement [Member] \| Celgene Corporation [Member] \| Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received			$ 25,000,000
Manufacturing Services [Member] \| Celgene Corporation [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Consideration allocated to agreement								54,100,000
Deferred revenue recognized					$ 6,000,000		$ 0	$ 10,900,000	$ 0

Equity - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	4,831,500
Shares issued, price per share \| $ / shares	$ 105.00
Proceeds from public offering of common stock, net of issuance costs \| $	$ 436.8
Overallotment Option [Member]
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	571,500

Stock-based compensation - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Jan. 31, 2017	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan	1,600,000
Number of shares available for issuance		1,700,000
Stock option share exercised		395,000
Proceed from option share exercised		$ 8.1
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan		$ 128.0
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 12 days
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 awards and employee stock purchase plan		$ 128.0
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 12 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan		$ 128.0
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 12 days
Common shares reserved for future issuance		238,000
Shares of common stock issued under plan		11,079	9,758

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 13,489	$ 10,782	$ 24,971	$ 20,926
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	10,764	9,057	20,107	17,984
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2,636	1,626	4,592	2,753
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 89	$ 99	$ 272	$ 189

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 13,489	$ 10,782	$ 24,971	$ 20,926
Research And Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	6,797	5,330	12,451	10,011
General And Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 6,692	$ 5,452	$ 12,520	$ 10,915

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Shares
Outstanding at beginning of period \| shares	3,735
Granted \| shares	943
Exercised \| shares	(395)
Canceled or forfeited \| shares	(92)
Outstanding at end of period \| shares	4,191
Exercisable at end of period \| shares	2,075
Vested and expected to vest at end of period \| shares	4,191
Weighted-average exercise price per share
Outstanding at beginning of period \| $ / shares	$ 52.17
Granted \| $ / shares	80.41
Exercised \| $ / shares	20.61
Canceled or forfeited \| $ / shares	85.22
Outstanding at end of period \| $ / shares	60.77
Exercisable at end of period \| $ / shares	45.74
Vested and expected to vest at end of period \| $ / shares	$ 60.77

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Shares
Unvested balance at beginning of period \| shares	263
Granted \| shares	266
Vested \| shares	(59)
Forfeited \| shares	(10)
Unvested balance at end of period \| shares	460
Weighted-average grant date fair value
Unvested balance at beginning of period \| $ / shares	$ 63.07
Granted \| $ / shares	81.83
Vested \| $ / shares	67.54
Forfeited \| $ / shares	58.50
Unvested balance at end of period \| $ / shares	$ 73.45

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,662	4,451	4,662	4,451
Outstanding Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,191	4,103	4,191	4,103
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	460	338	460	338
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	11	10	11	10