bluebird bio (BLUE) 10-Q 1 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 27, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	1,293,971
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		45,830,942

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 237,962	$ 278,887
Marketable securities	490,812	425,491
Tenant improvements receivable	2,712	8,542
Prepaid expenses	13,893	8,209
Other current assets and receivables	13,328	3,085
Total current assets	758,707	724,214
Marketable securities	413,856	180,452
Property and equipment, net	182,358	156,955
Intangible assets, net	17,872	20,694
Goodwill	13,128	13,128
Restricted cash and other non-current assets	19,835	22,679
Total assets	1,405,756	1,118,122
Current liabilities:
Accounts payable	12,309	13,664
Accrued expenses and other current liabilities	42,595	54,660
Deferred revenue, current portion	21,897	6,209
Total current liabilities	76,801	74,533
Deferred rent, net of current portion	2,761	10,408
Deferred revenue, net of current portion	27,600	40,204
Contingent consideration, net of current portion	2,961	3,277
Financing lease obligation, net of current portion	154,240	120,140
Other non-current liabilities	75	120
Total liabilities	264,438	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 September 30, 2017 and December 31, 2016
Common stock, $0.01 par value, 125,000 shares authorized; 45,785 and 40,691 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	458	407
Additional paid-in capital	1,939,537	1,447,856
Accumulated other comprehensive loss	(2,097)	(1,149)
Accumulated deficit	(796,580)	(577,674)
Total stockholders’ equity	1,141,318	869,440
Total liabilities and stockholders’ equity	$ 1,405,756	$ 1,118,122

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 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	45,785,000	40,691,000
Common stock, shares outstanding	45,785,000	40,691,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
License revenue	$ 2,500		$ 13,070
Collaboration revenue	5,211	$ 1,552	18,189	$ 4,603
Total revenues	7,711	1,552	31,259	4,603
Operating expenses:
Research and development	61,545	63,971	180,464	147,642
General and administrative	22,982	14,623	64,463	48,941
Cost of license revenue	1,100		1,520
Change in fair value of contingent consideration	(258)	1,098	205	3,515
Total operating expenses	85,369	79,692	246,652	200,098
Loss from operations	(77,658)	(78,140)	(215,393)	(195,495)
Interest (expense) income, net	(1,155)	937	(1,842)	2,871
Other (expense) income, net	8		(1,180)	(68)
Loss before income taxes	(78,805)	(77,203)	(218,415)	(192,692)
Income tax benefit		178		549
Net loss	$ (78,805)	$ (77,025)	$ (218,415)	$ (192,143)
Net loss per share - basic and diluted:	$ (1.73)	$ (2.07)	$ (5.14)	$ (5.19)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	45,648	37,201	42,524	37,026
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities, net of tax (benefit) expense of $0.0 million for the three and nine months ended September 30, 2017 and $(0.1) and $0.8 million for the three and nine months ended September 30, 2016, respectively	$ (683)	$ (194)	$ (949)	$ 1,455
Comprehensive loss	$ (79,488)	$ (77,219)	$ (219,364)	$ (190,688)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Unrealized (loss) gain on available-for-sale securities, tax (benefit) expense	$ 0	$ (0.1)	$ 0	$ 0.8

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (218,415)	$ (192,143)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(1,460)	2,099
Depreciation and amortization	9,899	7,132
Stock-based compensation expense	38,953	30,831
Other non-cash items	2,545	2,166
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(12,865)	(3,857)
Accounts payable	(447)	1,210
Accrued expenses and other liabilities	(6,943)	24,099
Deferred revenue	3,084	6,117
Deferred rent	475	1,805
Net cash used in operating activities	(185,174)	(120,541)
Investing activities
Purchase of property and equipment	(43,408)	(15,005)
Purchases of marketable securities	(621,004)	(145,135)
Proceeds from maturities of marketable securities	320,190	356,684
Proceeds from sales of marketable securities		7,500
Release of restricted cash	628
Increase in restricted cash		(4,379)
Net cash (used in) provided by investing activities	(343,594)	199,665
Financing activities
Proceeds from public offering of common stock, net of issuance costs	436,805
Cash paid for contingent purchase price consideration	(1,074)	(2,025)
Reimbursement of tenant improvements for financing lease obligation	38,021
Payments on financing lease obligation	(315)
Proceeds from issuance of common stock	14,406	3,786
Net cash provided by financing activities	487,843	1,761
Increase (decrease) in cash and cash equivalents	(40,925)	80,885
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	237,962	245,154
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	2,467
Tenant improvements under financing lease included in tenant improvements receivable	2,013
Purchases of property and equipment included in accounts payable and accrued expenses	1,740	2,479
Construction financing lease obligation		38,090
Stock option exercise proceeds receivable	$ 845	$ 374

Description of the business

Description of the business	9 Months Ended
	Sep. 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 accounti

Summary of significant accounting policies and basis of presentation	9 Months Ended
	Sep. 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 September 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., Certain aggregations of prior period 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 year 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. Additionally, in the prior quarter statement of operation and comprehensive loss, cost of license revenue is included within research and development expenses. In the current period statements of operations and comprehensive loss, cost of license revenue is 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 nine months ended September 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 units, and employee stock purchase plan shares 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 and comprehensive loss 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 or has been achieved. 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 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, 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. Cost of license revenue Cost of license revenue represents expense associated with amounts owed to third party licensors as a result of revenue recognized under the Company’s out-license arrangements with Novartis Pharma AG (“Novartis”) and GlaxoSmithKline Intellectual Property Development Limited (“GSK”). 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 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 approach, 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 revenue generated in the nine months ended September 30, 2017 relates to the Company’s collaboration arrangement with Celgene Corporation (“Celgene”) and the Company’s out-licensing arrangements. The Company is continuing to assess . The Company expects that certain of its accounting conclusions will require further judgment, including, but not limited to, the evaluation of variable consideration, and in particular, milestone payments due from Celgene as the inclusion of milestone payments in the transaction price 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 arrangements and does not expect the adoption of ASC 606 to have a material impact on . 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 and early adoption is permitted. 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 and early adoption is permitted. As of September 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 and early adoption is 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 . The new standard will be effective beginning January 1, 2019 and early adoption is permitted for public entities. 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

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Marketable securities	3. Marketable securities The following table summarizes the available-for-sale securities held at September 30, 2017 and December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value September 30, 2017 U.S. government agency securities and treasuries $ 887,586 $ 5 $ (1,405 ) $ 886,186 Certificates of deposit 18,480 2 — 18,482 Total $ 906,066 $ 7 $ (1,405 ) $ 904,668 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 September 30, 2017 or December 31, 2016 had remaining maturities greater than three years.

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 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 September 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) September 30, 2017 Assets: Cash and cash equivalents $ 237,962 $ 227,245 $ 10,717 $ — Marketable securities: U.S. government agency securities and treasuries 886,186 — 886,186 — Certificates of deposit 18,482 — 18,482 — Total assets $ 1,142,630 $ 227,245 $ 915,385 $ — Liabilities: Contingent consideration $ 2,961 $ — $ — $ 2,961 Total liabilities $ 2,961 $ — $ — $ 2,961 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 September 30, 2017 and December 31, 2016, cash and cash equivalents is composed of funds in cash, money market accounts, U.S. Treasury securities, certificates of deposit 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 September 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 The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of September 30, 2017 and December 31, 2016 was $775.2 million and $376.1 million, respectively. As of September 30, 2017 and December 31, 2016, there were $79.1 million and $95.5 million in securities held by the Company in an unrealized loss position for more than twelve months. 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 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 Nine Months Ended September 30, 2017 Fair value, beginning of period $ 7,756 Additions — Changes in fair value 205 Payments (5,000 ) Fair value, end of period $ 2,961 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 nine months ended September 30, 2017, a $5.0 million preclinical milestone was achieved and paid to the former equityholders of Pregenen. The remaining $3.0 million contingent consideration obligation is reflected as a non-current liability in the condensed consolidated balance sheet as of September 30, 2017.

Property and equipment, net

Property and equipment, net	9 Months Ended
	Sep. 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 September 30, 2017 December 31, 2016 Building $ 161,817 $ — Computer equipment and software 5,065 1,655 Office equipment 4,196 1,427 Laboratory equipment 23,191 16,305 Leasehold improvements 116 13,697 Construction-in-progress 1,212 136,315 Total property and equipment, gross 195,597 169,399 Less accumulated depreciation and amortization (13,239 ) (12,444 ) Total property and equipment, net $ 182,358 $ 156,955 As of September 30, 2017, total property and equipment, gross, includes $161.8 million related to the Company’s headquarters at 60 Binney Street in Cambridge, Massachusetts, of which $154.8 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

Accrued expenses and other curr

Accrued expenses and other current liabilities	9 Months Ended
	Sep. 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 September 30, 2017 December 31, 2016 Accrued goods and services $ 24,193 $ 34,275 Employee compensation 12,339 11,296 Accrued license and milestone fees 3,367 2,464 Accrued professional service fees 1,391 1,492 Financing lease obligation, current portion 815 — Contingent consideration, current portion — 4,479 Other 490 654 Total accrued expenses and other current liabilities $ 42,595 $ 54,660

Commitments and contingencies

Commitments and contingencies	9 Months Ended
	Sep. 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 September 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 capitalized the leasehold improvements as property and equipment and recorded the landlord incentive payments received as deferred rent and amortized these amounts as reductions to rent expense over the lease term. In addition, i 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 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 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. under ASC 840-10, Leases – Overall 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 non-cancellable 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 leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. The Company 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, both of which were fully exited in the first half of 2017. The Company has the option to extend the 60 Binney Street Lease for two successive five-year terms. The Company occupied the Building 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 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 of $4.1 million and $7.8 million for the three and nine months ended September 30, 2017, respectively. 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 nine months ended September 30, 2017 and 2016, the Company recognized As of September 30, 2017, Property and equipment, net, includes $161.8 As of December 31, 2016, Property and equipment, net, included $126.9 million related to the Building, of which $120.1 million had been incurred by the landlord. As of December 31, 2016, Tenant improvements receivable was $8.5 million.

Significant agreements

Significant agreements	9 Months Ended
	Sep. 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 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 to 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. bb2121 License Agreement 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 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 by the substantial completion of the ongoing Phase I 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 tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales, subject to certain reductions. bb21217 License Agreement On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the Amended Collaboration Agreement, pursuant to an executed license agreement entered into by the parties on September 28, 2017 and paid the associated $15.0 million option fee. The Company may also be permitted to exercise its option to co-develop and co-promote bb21217. 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 bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the United States, subject to certain reductions. In the event the Company does not exercise its option to co-develop and co-promote bb21217, the Company will receive an additional fee in the amount of $10.0 million. Under this scenario, the Company may be eligible to receive up to $10.0 million in clinical milestone payments, up to $117.0 million in regulatory milestone payments, and up to $78.0 million in commercial milestone payments. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales, subject to certain reductions. Accounting Analysis – bb2121 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, bb2121, (v) manufacture of vectors and associated payload for incorporation into the first optioned product candidate, bb2121, under the license, and (vi) participation on the Joint Governance Committee (“JGC”) under the co-development and co-promotion agreement for the first optioned product candidate, bb2121, under the license. The license to the first product candidate, bb2121, 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 bb2121 is a deliverable. However, the Company determined that the options to license any additional product candidates were substantive options and therefore were not considered deliverables at execution of the Amended Collaboration Agreement. Celgene was not contractually obligated to exercise the options. Additionally, as a result of the uncertain outcome of the discovery, research and development activities, the Company was at risk with regard to whether Celgene would exercise the options to license additional product candidates. Moreover, the Company determined that the options were not priced at a significant and incremental discount. Accordingly, the options to other product candidates were not considered deliverables and the associated option fees were 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 September 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 the post-initial Phase I 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. As of September 30, 2017, the Company determined that there were no changes to its initial conclusion and that the bb2121 license agreement continues to not have standalone value from the manufacturing services. Accordingly, these two deliverables continue to qualify as a single combined unit of accounting. 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. Research and development services The Company recognized revenue related to research and development services of $1.6 million and $4.7 million in the three and nine months ended September 30, 2017, respectively. The Company recognized revenue related to research and development services of $1.6 million and $4.7 million in the three and nine months ended September 30, 2016, respectively. bb2121 license and manufacturing services 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. 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, upon its execution. In the event 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 $89.8 million, before any offsetting expenses reimbursable to Celgene, between the commencement of post-Phase I 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 recognized revenue related to the combined unit of account of $7.7 million and $18.6 million in the three and nine months ended September 30, 2017, respectively. As noted above, revenue recognition for the combined unit of accounting commenced during the first quarter of 2017 and as such there was no revenue recognized related to the combined unit of account in the comparative periods in 2016. Other The Company recognized revenue of $0.6 million for the nine months ended September 30, 2017 related to other development costs incurred, which are billable to Celgene under the collaboration agreement. Costs incurred and billable by Celgene to the Company under the co-develop and co-promote agreement were $4.0 million and $5.6 million for the three and nine months ended September 30, 2017, respectively, and are recorded as an offset to collaboration revenue in the condensed consolidated statements of operations. Accounting Analysis – bb21217 On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217 pursuant to an executed license agreement (the “bb21217 License Agreement”) entered into by the parties on September 28, 2017, and paid the associated $15.0 million option fee. The Company’s bb21217 License Agreement with Celgene contains the following deliverables: (i) research and development services, (ii) a license to bb21217, (iii) manufacture of vectors and associated payload for incorporation into the optioned product candidate, bb21217, under the license, and (iv) participation on the JGC under the co-development and co-promotion agreement for the optioned product candidate, bb21217, under the license. Upon execution of the bb21217 License Agreement in September 2017, the Company concluded that the research and development services, which were delivered at the inception of the arrangement, have standalone value from the license to bb21217 and manufacture of vectors and associated payload for incorporation into the optioned product candidate under the license. However, the Company concluded that the license to bb21217 does not have standalone value from one of the undelivered elements, the post-initial Phase I manufacture of vectors and associated payload for bb21217 under the license, because the manufacturing is essential to the license agreement. Accordingly, these two deliverables qualify as a single combined unit of accounting. 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 and nine months ended September 30, 2017, the Company did not recognize any revenue under the bb21217 License Agreement. As of September 30, 2017 and December 31, 2016, there was $49.5 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 September 30, 2017, Deferred revenue, net of current portion includes $14.5 million related to bb2121 and $13.1 million related to bb21217. As of September 30, 2017, Other current assets and receivables includes a $5.7 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 $5.6 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. Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including KYMRIAH™ (formerly known as CTL019), Novartis’s anti-CD19 CAR T therapy. At contract inception, financial terms of the agreement included a $7.5 million payment upon execution, $7.5 million of potential future milestone payments associated with regulatory 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. In August 2017, Novartis received FDA approval for KYMRIAH™ and as a result the Company recognized revenue of $2.5 million as a result of the achievement of a related milestone. During the three and nine months ended September 30, 2017, the Company recognized license revenue of $2.5 million and $10.0 million, respectively, in connection with this arrangement, as there were no other undelivered elements in the arrangement. The cost of Novartis license revenue was $1.1 million and $1.4 million for the three and nine months ended September 30, 2017, respectively. GlaxoSmithKline Intellectual Property Development Limited On April 28, 2017, the Company entered into a worldwide license agreement with 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 nine months ended September 30, 2017, the Company recognized revenue of $3.0 million associated with the delivery of the license, as there were no other undelivered elements in the arrangement. The cost of GSK license revenue was $0.1 million for the three and nine months ended September 30, 2017.

Equity

Equity	9 Months Ended
	Sep. 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

Stock-based compensation	9 Months Ended
	Sep. 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 September 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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Stock options $ 10,786 $ 8,294 $ 30,892 $ 26,278 Restricted stock units 3,067 1,495 7,659 4,248 Employee stock purchase plan 130 116 402 305 $ 13,983 $ 9,905 $ 38,953 $ 30,831 As of September 30, 2017, the Company had $122.5 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 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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 7,049 $ 5,399 $ 19,500 $ 15,410 General and administrative 6,934 4,506 19,453 15,421 $ 13,983 $ 9,905 $ 38,953 $ 30,831 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 1,066 $ 83.88 Exercised (613 ) $ 23.59 Canceled or forfeited (172 ) $ 83.09 Outstanding at September 30, 2017 4,016 $ 63.62 Exercisable at September 30, 2017 2,037 $ 49.05 Vested and expected to vest at September 30, 2017 4,016 $ 63.62 During the nine months ended September 30, 2017, 0.6 million shares of common stock were exercised, resulting in total proceeds to the Company of $14.5 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans. Restricted stock 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 297 $ 84.79 Vested (78 ) $ 73.58 Forfeited (26 ) $ 63.18 Unvested balance at September 30, 2017 456 $ 75.41 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 initial public offering on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238,000 shares 2016, 20,773 and shares

Income taxes

Income taxes	9 Months Ended
	Sep. 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

Net loss per share	9 Months Ended
	Sep. 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 Nine Months Ended September 30, 2017 2016 Outstanding stock options 4,016 3,867 Restricted stock units 456 254 ESPP shares 10 12 4,482 4,133

Summary of significant accoun19

Summary of significant accounting policies and basis of presentation (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of presentation and principles of consolidation	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 September 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., Certain aggregations of prior period 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 year 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. Additionally, in the prior quarter statement of operation and comprehensive loss, cost of license revenue is included within research and development expenses. In the current period statements of operations and comprehensive loss, cost of license revenue is disclosed separately.
Summary of accounting policies	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 nine months ended September 30, 2017.
Net loss per share	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 units, and employee stock purchase plan shares using the treasury stock method.
Property and equipment	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	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 and comprehensive loss 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	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 or has been achieved. 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 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, 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.
Cost of license revenue	Cost of license revenue Cost of license revenue represents expense associated with amounts owed to third party licensors as a result of revenue recognized under the Company’s out-license arrangements with Novartis Pharma AG (“Novartis”) and GlaxoSmithKline Intellectual Property Development Limited (“GSK”).
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially
Recent accounting pronouncements	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 approach, 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 revenue generated in the nine months ended September 30, 2017 relates to the Company’s collaboration arrangement with Celgene Corporation (“Celgene”) and the Company’s out-licensing arrangements. The Company is continuing to assess . The Company expects that certain of its accounting conclusions will require further judgment, including, but not limited to, the evaluation of variable consideration, and in particular, milestone payments due from Celgene as the inclusion of milestone payments in the transaction price 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 arrangements and does not expect the adoption of ASC 606 to have a material impact on . 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 and early adoption is permitted. 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 and early adoption is permitted. As of September 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 and early adoption is 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 . The new standard will be effective beginning January 1, 2019 and early adoption is permitted for public entities. 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

Summary of significant accoun20

Summary of significant accounting policies and basis of presentation (Tables)	9 Months Ended
	Sep. 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)

Marketable Securities (Tables)	9 Months Ended
	Sep. 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 September 30, 2017 and December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value September 30, 2017 U.S. government agency securities and treasuries $ 887,586 $ 5 $ (1,405 ) $ 886,186 Certificates of deposit 18,480 2 — 18,482 Total $ 906,066 $ 7 $ (1,405 ) $ 904,668 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

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 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 September 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) September 30, 2017 Assets: Cash and cash equivalents $ 237,962 $ 227,245 $ 10,717 $ — Marketable securities: U.S. government agency securities and treasuries 886,186 — 886,186 — Certificates of deposit 18,482 — 18,482 — Total assets $ 1,142,630 $ 227,245 $ 915,385 $ — Liabilities: Contingent consideration $ 2,961 $ — $ — $ 2,961 Total liabilities $ 2,961 $ — $ — $ 2,961 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 Nine Months Ended September 30, 2017 Fair value, beginning of period $ 7,756 Additions — Changes in fair value 205 Payments (5,000 ) Fair value, end of period $ 2,961

Property and equipment, net (Ta

Property and equipment, net (Tables)	9 Months Ended
	Sep. 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 September 30, 2017 December 31, 2016 Building $ 161,817 $ — Computer equipment and software 5,065 1,655 Office equipment 4,196 1,427 Laboratory equipment 23,191 16,305 Leasehold improvements 116 13,697 Construction-in-progress 1,212 136,315 Total property and equipment, gross 195,597 169,399 Less accumulated depreciation and amortization (13,239 ) (12,444 ) Total property and equipment, net $ 182,358 $ 156,955

Accrued expenses and other cu24

Accrued expenses and other current liabilities (Tables)	9 Months Ended
	Sep. 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 September 30, 2017 December 31, 2016 Accrued goods and services $ 24,193 $ 34,275 Employee compensation 12,339 11,296 Accrued license and milestone fees 3,367 2,464 Accrued professional service fees 1,391 1,492 Financing lease obligation, current portion 815 — Contingent consideration, current portion — 4,479 Other 490 654 Total accrued expenses and other current liabilities $ 42,595 $ 54,660

Stock-based compensation (Table

Stock-based compensation (Tables)	9 Months Ended
	Sep. 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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Stock options $ 10,786 $ 8,294 $ 30,892 $ 26,278 Restricted stock units 3,067 1,495 7,659 4,248 Employee stock purchase plan 130 116 402 305 $ 13,983 $ 9,905 $ 38,953 $ 30,831
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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 7,049 $ 5,399 $ 19,500 $ 15,410 General and administrative 6,934 4,506 19,453 15,421 $ 13,983 $ 9,905 $ 38,953 $ 30,831
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 1,066 $ 83.88 Exercised (613 ) $ 23.59 Canceled or forfeited (172 ) $ 83.09 Outstanding at September 30, 2017 4,016 $ 63.62 Exercisable at September 30, 2017 2,037 $ 49.05 Vested and expected to vest at September 30, 2017 4,016 $ 63.62
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 297 $ 84.79 Vested (78 ) $ 73.58 Forfeited (26 ) $ 63.18 Unvested balance at September 30, 2017 456 $ 75.41

Net loss per share (Tables)

Net loss per share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Common Stock Equivalents Excluded from Calculation of Diluted 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 Nine Months Ended September 30, 2017 2016 Outstanding stock options 4,016 3,867 Restricted stock units 456 254 ESPP shares 10 12 4,482 4,133

Description of the business - A

Description of the business - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992

Summary of significant accoun28

Summary of significant accounting policies and basis of presentation - Additional Information (Detail) $ in Millions	Sep. 21, 2015	Sep. 30, 2017USD ($)Segment	Mar. 31, 2017USD ($)
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	2,027
Additional lease period extendable	10 years

Summary of significant accoun29

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	9 Months Ended
	Sep. 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

Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 906,066	$ 606,481
Unrealized Gains	7	40
Unrealized Losses	(1,405)	(578)
Fair Value	904,668	605,943
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	887,586	600,001
Unrealized Gains	5	34
Unrealized Losses	(1,405)	(575)
Fair Value	886,186	599,460
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	18,480	6,480
Unrealized Gains	2	6
Unrealized Losses		(3)
Fair Value	$ 18,482	$ 6,483

Fair value measurements - Recor

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	Sep. 30, 2017	Dec. 31, 2016
Assets:
Cash and cash equivalents	$ 237,962	$ 278,887
Total assets	1,142,630	884,830
Liabilities:
Contingent consideration	2,961	7,756
Total liabilities	2,961	7,756
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	886,186	599,460
Certificates of Deposit [Member]
Assets:
Marketable securities	18,482	6,483
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	227,245	278,887
Total assets	227,245	278,887
Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	10,717
Total assets	915,385	605,943
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	886,186	599,460
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable securities	18,482	6,483
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	2,961	7,756
Total liabilities	$ 2,961	$ 7,756

Fair value measurements - Addit

Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ 775,200,000		775,200,000		$ 376,100,000
Unrealized loss on securities, more than twelve months	79,100,000		79,100,000		95,500,000
Investments with other-than-temporary impairment	0		0		0
Achievement of research milestone	(258,000)	$ 1,098,000	205,000	$ 3,515,000
Contingent consideration, non current	2,961,000		2,961,000		$ 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			2,021
Milestone discount rates			14.60%
Minimum [Member] | Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2,021
Minimum [Member] | Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2,021
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2,028
Milestone discount rates			15.80%
Maximum [Member] | Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2,028
Maximum [Member] | Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2,028

Fair value measurements - Roll-

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	9 Months Ended
	Sep. 30, 2017USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value, beginning of period	$ 7,756
Changes in fair value	205
Payments	(5,000)
Fair value, end of period	$ 2,961

Property and equipment, net - S

Property and equipment, net - Summary of Property and Equipment Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 195,597	$ 169,399
Less accumulated depreciation and amortization	(13,239)	(12,444)
Total property and equipment, net	182,358	156,955
Building [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	161,817
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	5,065	1,655
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	4,196	1,427
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	23,191	16,305
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	116	13,697
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 1,212	$ 136,315

Property and equipment, net - A

Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 195,597	$ 169,399
60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	161,800
Construction cost incurred by landlord	$ 154,800
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 cu36

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued goods and services	$ 24,193	$ 34,275
Employee compensation	12,339	11,296
Accrued license and milestone fees	3,367	2,464
Accrued professional service fees	1,391	1,492
Financing lease obligation, current portion	815
Contingent consideration, current portion		4,479
Other	490	654
Total accrued expenses and other current liabilities	$ 42,595	$ 54,660

Commitments and contingencies -

Commitments and contingencies - Additional Information (Detail) € in Millions

Nov. 18, 2016EUR (€)

Jun. 03, 2016USD ($)

Sep. 21, 2015USD ($)ft²$ / ft²

Jun. 29, 2015USD ($)ft²

Dec. 31, 2014USD ($)

Jun. 03, 2013USD ($)ft²

Sep. 30, 2017USD ($)

Jun. 30, 2017USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2016EUR (€)

Sep. 30, 2016USD ($)

Sep. 30, 2017USD ($)

Sep. 30, 2016USD ($)

Dec. 31, 2016USD ($)

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. 3, 2013

Lease termination date

Apr. 12, 2017

Term of agreement

5 years

Renewal period of agreement

3 years

Up-front fee | €

€ 3

Up-front fee paid | €

€ 2

Up-front fee payable in mid-2018 | €

1

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

195,597,000

$ 195,597,000

$ 169,399,000

Tenant improvements receivable

2,712,000

2,712,000

8,542,000

60 Binney Street Lease [Member]

Commitments And Contingencies Disclosure [Line Items]

Property and equipment, gross

161,800,000

161,800,000

126,900,000

Construction financing lease obligation

154,800,000

154,800,000

120,100,000

Tenant improvements receivable

2,700,000

$ 2,700,000

$ 8,500,000

Building [Member]

Commitments And Contingencies Disclosure [Line Items]

Useful life of asset

40 years

Interest expense related to financing obligation

4,100,000

$ 7,800,000

Property and equipment, gross

161,817,000

161,817,000

Land [Member]

Commitments And Contingencies Disclosure [Line Items]

Non-cash rent expense

$ 500,000

$ 500,000

$ 1,400,000

$ 1,400,000

150 Second Street [Member]

Commitments And Contingencies Disclosure [Line Items]

Tenant improvement assets disposed

$ 8,300,000

Non-current deferred rent disposed

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. 1, 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. 1, 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 leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes.

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

Significant agreements - Additi

Significant agreements - Additional Information (Detail)	Apr. 28, 2017USD ($)Deliverables	Apr. 26, 2017USD ($)Deliverables	Jun. 03, 2015USD ($)Deliverables	Mar. 19, 2013USD ($)	Aug. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 28, 2017USD ($)	Dec. 31, 2016USD ($)	Feb. 17, 2016USD ($)	Feb. 16, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue						$ 5,211,000	$ 1,552,000	$ 18,189,000	$ 4,603,000
Deferred revenue, net of current portion						27,600,000		27,600,000			$ 40,204,000
License revenue						2,500,000		13,070,000
Cost of license revenue						1,100,000		1,520,000
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of collaboration agreement				3 years
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event						89,800,000		89,800,000
Number of deliverable | Deliverables			3
Consideration allocated to agreement								$ 109,000,000
Deferred revenue recognition period								3 years
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								$ 600,000
Deferred revenue						49,500,000		49,500,000			46,400,000
Celgene Corporation [Member] | Amended 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	$ 4,700,000	4,700,000
Celgene Corporation [Member] | Amended Collaborative Arrangement [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
Celgene Corporation [Member] | Amended Collaborative Arrangement [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] | Amended 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] | Amended 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] | Amended 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] | bb2121 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, net of current portion						14,500,000		14,500,000
Cost reimbursement receivable for product development costs incurred						5,700,000		5,700,000			$ 0
Celgene Corporation [Member] | bb2121 License Agreement [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
Celgene Corporation [Member] | bb2121 License Agreement [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] | bb2121 License Agreement [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] | bb2121 License Agreement [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] | bb2121 License Agreement [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] | bb2121 License Agreement, 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] | bb2121 License Agreement, 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] | bb2121 License Agreement, 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
Celgene Corporation [Member] | bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event										$ 15,000,000
Deferred revenue recognized						0		0
Deferred revenue, net of current portion						13,100,000		13,100,000
Celgene Corporation [Member] | bb21217 License Agreement [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] | bb21217 License Agreement [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] | bb21217 License Agreement [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] | bb21217 License Agreement [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] | bb21217 License Agreement, 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] | bb21217 License Agreement, 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] | bb21217 License Agreement, 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
Celgene Corporation [Member] | Collaborative Arrangement, Co-promotion
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Costs incurred and billable under collaborative agreement						4,000,000		5,600,000
Novartis Pharma AG [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement upfront payment		$ 7,500,000
Number of deliverables identified at the date of contract inception | Deliverables		1
Revenue recognized upon achievement of a related milestone					$ 2,500,000
License revenue						2,500,000		10,000,000
Cost of license revenue						1,100,000		1,400,000
Novartis Pharma AG [Member] | Regulatory Milestones Payments [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 7,500,000
Novartis Pharma AG [Member] | Each Subsequently Licensed Product [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 1,100,000
GlaxoSmithKline Intellectual Property Development Limited [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement upfront payment	$ 3,000,000
Number of deliverables identified at the date of contract inception | Deliverables	1
License revenue						3,000,000		3,000,000
Cost of license revenue						100,000		100,000
GlaxoSmithKline Intellectual Property Development Limited [Member] | Potential Milestones Payments [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 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
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					25,000,000
Manufacturing Services [Member] | Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Consideration allocated to agreement								54,100,000
Collaboration revenue						$ 7,700,000	$ 0	$ 18,600,000	$ 0

Equity - Additional Information

Equity - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended
	Jun. 30, 2017USD ($)$ / sharesshares
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	4,831,500
Shares issued, price per share | $ / shares	$ 105
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 - Addi

Stock-based compensation - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Jan. 31, 2017	Sep. 30, 2017	Sep. 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		613,000
Proceed from option share exercised		$ 14.5
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		$ 122.5
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 13 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		$ 122.5
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 13 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		$ 122.5
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 8 months 13 days
Common shares reserved for future issuance		238,000
Shares of common stock issued under plan		20,773	18,338

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 13,983	$ 9,905	$ 38,953	$ 30,831
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	10,786	8,294	30,892	26,278
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	3,067	1,495	7,659	4,248
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 130	$ 116	$ 402	$ 305

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 13,983	$ 9,905	$ 38,953	$ 30,831
Research And Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	7,049	5,399	19,500	15,410
General And Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 6,934	$ 4,506	$ 19,453	$ 15,421

Stock-based compensation - Su43

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Shares
Outstanding at beginning of period | shares	3,735
Granted | shares	1,066
Exercised | shares	(613)
Canceled or forfeited | shares	(172)
Outstanding at end of period | shares	4,016
Exercisable at end of period | shares	2,037
Vested and expected to vest at end of period | shares	4,016
Weighted-average exercise price per share
Outstanding at beginning of period | $ / shares	$ 52.17
Granted | $ / shares	83.88
Exercised | $ / shares	23.59
Canceled or forfeited | $ / shares	83.09
Outstanding at end of period | $ / shares	63.62
Exercisable at end of period | $ / shares	49.05
Vested and expected to vest at end of period | $ / shares	$ 63.62

Stock-based compensation - Su44

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Shares
Unvested balance at beginning of period | shares	263
Granted | shares	297
Vested | shares	(78)
Forfeited | shares	(26)
Unvested balance at end of period | shares	456
Weighted-average grant date fair value
Unvested balance at beginning of period | $ / shares	$ 63.07
Granted | $ / shares	84.79
Vested | $ / shares	73.58
Forfeited | $ / shares	63.18
Unvested balance at end of period | $ / shares	$ 75.41

Net loss per share - Common Sto

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	4,482	4,133	4,482	4,133
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	4,016	3,867	4,016	3,867
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	456	254	456	254
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	10	12	10	12