bluebird bio (BLUE) 10-Q 11 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 05, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	001-35966
Entity Registrant Name	bluebird bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	13-3680878
Entity Address, Address Line One	60 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	339
Local Phone Number	499-9300
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	BLUE
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		55,652,110
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001293971
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 346,629	$ 327,214
Marketable securities	564,054	779,246
Prepaid expenses	40,209	32,888
Receivables and other current assets	16,575	12,826
Total current assets	967,467	1,152,174
Marketable securities	107,674	131,506
Property, plant and equipment, net	153,920	151,176
Intangible assets, net	13,254	14,326
Goodwill	13,128	13,128
Operating lease right-of-use assets	194,469	185,885
Restricted cash and other non-current assets	79,192	79,229
Total assets	1,529,104	1,727,424
Current liabilities:
Accounts payable	30,527	42,995
Accrued expenses and other current liabilities	118,091	141,556
Operating lease liability, current portion	20,380	20,175
Deferred revenue, current portion	6,355	8,474
Collaboration research advancement, current portion	9,069	10,380
Total current liabilities	184,422	223,580
Deferred revenue, net of current portion	9,791	9,791
Collaboration research advancement, net of current portion	26,843	27,834
Contingent consideration	4,869	7,977
Operating lease liability, net of current portion	179,962	170,812
Other non-current liabilities	2,784	2,437
Total liabilities	408,671	442,431
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at March 31, 2020 and December 31, 2019	0	0
Common stock, $0.01 par value, 125,000 shares authorized; 55,620 and 55,368 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	556	554
Additional paid-in capital	3,607,139	3,568,184
Accumulated other comprehensive loss	(2,799)	(1,893)
Accumulated deficit	(2,484,463)	(2,281,852)
Total stockholders’ equity	1,120,433	1,284,993
Total liabilities and stockholders’ equity	$ 1,529,104	$ 1,727,424

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 10	$ 10
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par or stated value per share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	125,000,000	125,000,000
Common stock, shares, issued (in shares)	55,620,000	55,368,000
Common stock, shares, outstanding (in shares)	55,620,000	55,368,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue:
Collaborative arrangement revenue	$ 2,302	$ 1,966
Total revenues	21,863	12,471
Operating expenses:
Research and development	154,123	122,640
Selling, general and administrative	73,248	60,279
Cost of royalty revenue	1,025	430
Change in fair value of contingent consideration	(3,108)	296
Total operating expenses	225,288	183,645
Loss from operations	(203,425)	(171,174)
Interest income, net	5,355	10,102
Other expense, net	(4,447)	(3,389)
Loss before income taxes	(202,517)	(164,461)
Income tax (expense) benefit	(94)	15
Net loss	$ (202,611)	$ (164,446)
Net loss per share - basic and diluted:	$ (3.64)	$ (2.99)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	55,590	54,957
Other comprehensive (loss) income:
Other comprehensive (loss) income, net of tax expense of $0.0 million and $0.4 million for the three months ended March 31, 2020 and 2019, respectively	$ (906)	$ 1,835
Total other comprehensive (loss) income	(906)	1,835
Comprehensive loss	(203,517)	(162,611)
Service revenue
Revenue:
Revenue	16,833	9,211
Royalty revenue
Revenue:
Revenue	$ 2,728	$ 1,294

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Other comprehensive income (loss) tax expense	$ 0	$ 0.4

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2018		54,738
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 547	$ 3,386,958	$ (3,627)	$ (1,498,808)
Vesting of restricted stock units (in shares)		131
Vesting of restricted stock units	0	$ 2	(2)
Exercise of stock options, shares (in shares)		189
Exercise of stock options	9,504	$ 2	9,502
Purchase of common stock under ESPP (in shares)		11
Purchase of common stock under ESPP	1,231		1,231
Stock-based compensation	32,341		32,341
Other comprehensive loss	1,835			1,835
Net loss	(164,446)				(164,446)
Ending balance, shares (in shares) at Mar. 31, 2019		55,069
Ending balance at Mar. 31, 2019	$ 1,772,099	$ 551	3,430,030	(1,792)	(1,656,690)
Beginning balance (in shares) at Dec. 31, 2019	55,368	55,368
Beginning balance at Dec. 31, 2019	$ 1,284,993	$ 554	3,568,184	(1,893)	(2,281,852)
Vesting of restricted stock units (in shares)		204
Vesting of restricted stock units	$ 0	$ 2	(2)
Exercise of stock options, shares (in shares)	20	20
Exercise of stock options	$ 750		750
Purchase of common stock under ESPP (in shares)		28
Purchase of common stock under ESPP	1,872		1,872
Stock-based compensation	36,335		36,335
Other comprehensive loss	(906)			(906)
Net loss	$ (202,611)				(202,611)
Ending balance, shares (in shares) at Mar. 31, 2020	55,620	55,620
Ending balance at Mar. 31, 2020	$ 1,120,433	$ 556	$ 3,607,139	$ (2,799)	$ (2,484,463)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (202,611)	$ (164,446)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(3,108)	296
Depreciation and amortization	4,880	3,783
Stock-based compensation expense	36,293	32,341
Unrealized loss on equity securities	4,520	3,085
Other non-cash items	(1,387)	(3,456)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(10,984)	(12,793)
Operating lease right-of-use assets	5,842	5,374
Accounts payable	(9,519)	10,590
Accrued expenses and other liabilities	(20,557)	(21,514)
Operating lease liabilities	(5,070)	3,224
Deferred revenue	(2,118)	(8,672)
Collaboration research advancement	(2,302)	(1,966)
Net cash used in operating activities	(206,121)	(154,154)
Cash flows from investing activities:
Purchase of property, plant and equipment	(10,676)	(19,321)
Purchases of marketable securities	(101,421)	(381,735)
Proceeds from maturities of marketable securities	336,675	364,143
Net cash provided by (used in) investing activities	224,578	(36,913)
Cash flows from financing activities:
Proceeds from exercise of stock options and ESPP contributions	963	10,223
Net cash provided by financing activities	963	10,223
Increase (decrease) in cash, cash equivalents and restricted cash	19,420	(180,844)
Cash, cash equivalents and restricted cash at beginning of period	381,709	417,099
Cash, cash equivalents and restricted cash at end of period	401,129	236,255
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	346,629	221,738
Restricted cash included in receivables and other current assets	0	100
Restricted cash included in restricted cash and other non-current assets	54,500	14,417
Cash, cash equivalents and restricted cash at end of period	401,129	236,255
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	1,125	9,302
Right-of-use assets obtained in exchange for operating lease liabilities	14,425	5,500
Tenant improvements included in receivables and other current assets	$ 1,516	$ 8,009

Income taxes

Income taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income taxes	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. The tax expense recognized during the first quarter of 2020 is due to income taxes on foreign earnings. On March 27, 2020, the Coronavirus Aid, Relief and Economic Security Act (“CARES Act”) was enacted. This law temporarily suspended and adjusted certain law changes enacted in the Tax Cuts and Jobs Act in 2017. The Company has concluded that the provisions in the CARES Act have an immaterial impact on the Company’s income tax expense, net deferred tax assets and associated valuation allowance.

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
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	8,063	6,527
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 (in shares)	6,478	5,384
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	1,585	1,128
ESPP Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	0	15

Subsequent events

Subsequent events	3 Months Ended
	Mar. 31, 2020
Subsequent Events [Abstract]
Subsequent events	Subsequent events On May 8, 2020, the Company and BMS entered into an amendment and restatement of the Ide-cel CCPS, an amendment and restatement of the bb21217 License Agreement, and a non-exclusive license agreement to certain patent rights controlled by the Company and related to lentiviral vector technology for BMS to develop and commercialize CD19-directed CAR T cell therapies.

Description of the business

Description of the business	3 Months Ended
	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the business	Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company is a biotechnology company committed to researching, developing and commercializing potentially transformative gene therapies for severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide selling, general and administrative support for these operations, including commercial-readiness activities. The Company’s programs in severe genetic diseases include ZYNTEGLO™ (autologous CD34+ cells encoding βA-T87Q-globin gene) gene therapy as a treatment for transfusion-dependent β-thalassemia, or TDT; its LentiGlobin ® product candidate as a treatment for sickle cell disease, or SCD; and its Lenti-D™ product candidate as a treatment for cerebral adrenoleukodystrophy, or CALD. The Company’s programs in oncology are focused on developing novel T cell-based immunotherapies, including chimeric antigen receptor (CAR) and T cell receptor (TCR) T cell therapies. Idecabtagene vicleucel, or ide-cel, and bb21217, which are product candidates in oncology under the Company’s collaboration arrangement with Bristol-Myers Squibb ("BMS"), formerly Celgene Corporation ("Celgene") prior to its acquisition by BMS in November 2019, are CAR T cell product candidates for the treatment of multiple myeloma. Please refer to Note 9, Collaborative arrangements, for further discussion of the Company’s collaboration with BMS. In June 2019, the Company received conditional marketing authorization from the European Commission for ZYNTEGLO (formerly referred to as LentiGlobin for TDT) for the treatment of patients 12 years and older with TDT who do not have a β0/β0 genotype, for whom hematopoietic stem cell (HSC) transplantation is appropriate but a human leukocyte-matched related HSC donor is not available. Since receiving conditional marketing authorization for ZYNTEGLO, the Company has continued to advance its commercialization readiness activities. Through March 31, 2020, the Company had not generated any revenue from product sales of ZYNTEGLO. In accordance with Accounting Standards Codification ("ASC") 205-40, Going Concern , the Company evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company has incurred losses since inception and to date has financed its operations primarily through the sale of shares of the Company's stock and, to a lesser extent, through collaboration agreements and grants from governmental agencies and charitable foundations. As of March 31, 2020, the Company had an accumulated deficit of $2.48 billion. During the three months ended March 31, 2020, the Company incurred a loss of $202.6 million and used $206.1 million of cash in operations. The Company expects to continue to generate operating losses and negative operating cash flows for the next few years and will need additional funding to support its planned operating activities through profitability. The transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. As of March 31, 2020, the Company had cash, cash equivalents and marketable securities of $1.02 billion. The Company expects its cash, cash equivalents and marketable securities will be sufficient to fund current planned operations for at least twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings, strategic collaborations or other sources. Management's expectations with respect to its ability to fund current planned operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management's estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any of these strategic or financing opportunities will be executed or executed on favorable terms, and some could be dilutive to existing stockholders. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates.

Basis of presentation, principl

Basis of presentation, principles of consolidation and significant accounting policies	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of presentation, principles of consolidation and significant accounting policies	Basis of presentation, principles of consolidation and significant accounting policies Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the ASC and Accounting Standards Updates (“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 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 March 31, 2020 and 2019. 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 condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2019, 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 18, 2020. Certain items in the prior year’s condensed consolidated financial statements have been reclassified to conform to the current presentation. However, no subtotals in the prior year condensed consolidated financial statements were impacted as a result. Amounts reported are computed based on thousands. As a result, certain totals may not sum due to rounding. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. Significant accounting policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2020 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s 2019 Annual Report on Form 10-K, except as noted immediately below and as noted within the "Recent accounting pronouncements - Recently adopted" section. Marketable securities Effective January 1, 2020, the Company adopted ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements ("ASU 2016-13" or "ASC 326"), using the effective date method. As the Company had never recorded any other-than-temporary-impairment adjustments to its available-for-sale debt securities prior to the effective date, no transition provisions are applicable to the Company. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASC 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the condensed consolidated statements of operations and comprehensive loss as credit loss expense within other expense, net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities. Accrued interest receivable related to the Company's available-for-sale debt securities is presented within receivables and other current assets on the Company's condensed consolidated balance sheets. The Company has elected the practical expedient available to exclude accrued interest receivable from both the fair value and the amortized cost basis of available-for-sale debt securities for the purposes of identifying and measuring any impairment. The Company writes off accrued interest receivable once it has determined that the asset is not realizable. Any write offs of accrued interest receivable are recorded by reversing interest income, recognizing credit loss expense, or a combination of both. To date, the Company has not written off any accrued interest receivables associated with its marketable securities. Stock-based compensation The Company estimates the fair value of its option awards using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Effective January 1, 2020, the Company eliminated the use of a representative peer group and uses only its own historical volatility data in its estimate of expected volatility given that there is now a sufficient amount of historical information regarding the volatility of its own stock price. Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: future undiscounted cash flows and subsequent fair value estimates used to assess potential and measure any impairment of long-lived assets, including goodwill and intangible assets, and the measurement of right-of-use assets and lease liabilities, contingent consideration, stock-based compensation expense, accrued expenses, revenue, income taxes, and the assessment of the Company's ability to fund its operations for at least the next twelve months from the date of issuance of these financial statements. Recent accounting pronouncements Recently adopted ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements, ASU No. 2019-5 Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief, ASU No. 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements . The new standard, as amended, requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The targeted transition relief standard allows filers an option to irrevocably elect the fair value option of ASC 825-10, Financial Instruments-Overall , applied on an instrument-by-instrument basis for eligible instruments. The Company adopted this standard on January 1, 2020 on a prospective basis and the adoption did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. The new standard removes certain disclosures, modifies certain disclosures, and adds additional disclosures related to fair value measurement. The Company adopted this standard on January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-15, Intangibles-Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract . The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The Company adopted this standard on a prospective basis as of January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , (“ASU 2018-18”). The amendments in this update clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 , Revenue from Contracts with Customers (“Topic 606” or "ASC 606") when the counter party is a customer in the context of a unit of account. ASU 2018-18 also precludes companies from presenting transactions with collaborative partners that are outside the scope of Topic 606 together with revenue within the scope of Topic 606. The Company adopted this standard on a retrospective basis on January 1, 2020. As a result, revenue for prior periods are presented in accordance with the new standard. Prior to the adoption of ASU 2018-18, the Company presented all revenue recognized under its collaborative arrangements as collaboration revenue on its condensed consolidated statement of operations and comprehensive loss. However, as the Company recognizes revenue under its collaborative arrangements both within and outside the scope of Topic 606, the Company has revised its presentation of revenue on its condensed consolidated statement of operations and comprehensive loss as follows: service revenue includes revenue from collaborative partners recognized within the scope of Topic 606 and collaborative arrangement revenue includes revenue from collaborative partners recognized outside the scope of Topic 606. The disaggregation of revenue recognized under Topic 606 and outside of Topic 606 had previously otherwise been disclosed in the Notes to Condensed Consolidated Financial Statements. ASU No. 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments In April 2019, the FASB issued ASU 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. This update provides clarifications for three topics related to financial instruments accounting, some of which apply to the Company. The Company adopted this standard on January 1, 2020 on a prospective basis, and it did not have a material impact on its financial position and results of operations upon adoption. Not yet adopted ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. The new standard will be effective beginning January 1, 2021. The Company is currently evaluating the potential impact ASU 2019-12 may have on its financial position and results of operations upon adoption.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the marketable securities held at March 31, 2020 and December 31, 2019 (in thousands): Description Amortized cost / Cost Unrealized gains Unrealized losses Fair value March 31, 2020 U.S. government agency securities and treasuries $ 377,551 $ 2,741 $ (15) $ 380,277 Certificates of deposit 240 — — 240 Corporate bonds 207,708 613 (1,110) 207,211 Commercial paper 75,650 — — 75,650 Equity securities 20,017 — (11,667) 8,350 Total $ 681,166 $ 3,354 $ (12,792) $ 671,728 December 31, 2019 U.S. government agency securities and treasuries $ 633,970 $ 2,014 $ (48) $ 635,936 Certificates of deposit 960 — — 960 Corporate bonds 185,827 824 (43) 186,608 Commercial paper 74,378 — — 74,378 Equity securities 20,017 — (7,147) 12,870 Total $ 915,152 $ 2,838 $ (7,238) $ 910,752 No available-for-sale debt securities held as of March 31, 2020 or December 31, 2019 had remaining maturities greater than three years.

Fair value measurements

Fair value measurements	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair value measurements	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 March 31, 2020 and December 31, 2019 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) March 31, 2020 Assets: Cash and cash equivalents $ 346,629 $ 336,630 $ 9,999 $ — Marketable securities: U.S. government agency securities and treasuries 380,277 — 380,277 — Certificates of deposit 240 — 240 — Corporate bonds 207,211 — 207,211 — Commercial paper 75,650 — 75,650 — Equity securities 8,350 8,350 — — Total $ 1,018,357 $ 344,980 $ 673,377 $ — Liabilities: Contingent consideration $ 4,869 $ — $ — $ 4,869 Total $ 4,869 $ — $ — $ 4,869 December 31, 2019 Assets: Cash and cash equivalents $ 327,214 $ 311,245 $ 15,969 $ — Marketable securities: U.S. government agency securities and treasuries 635,936 — 635,936 — Certificates of deposit 960 — 960 — Corporate bonds 186,608 — 186,608 — Commercial paper 74,378 — 74,378 — Equity securities 12,870 12,870 — — Total $ 1,237,966 $ 324,115 $ 913,851 $ — Liabilities: Contingent consideration $ 7,977 $ — $ — $ 7,977 Total $ 7,977 $ — $ — $ 7,977 Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of 90 days or less from the date of purchase to be cash equivalents. As of March 31, 2020, cash and cash equivalents comprise funds in cash, money market accounts, and U.S. Treasury securities. As of December 31, 2019, cash and cash equivalents comprise funds in cash, money market accounts, and commercial paper. Marketable securities Marketable securities classified as Level 2 within the valuation hierarchy generally consist of certificates of deposit, U.S. treasury securities and government agency securities, corporate bonds, and commercial paper. The Company estimates the fair values of these marketable securities by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for the same or similar securities, issuer credit spreads, benchmark yields, and other observable inputs. The Company validates the prices provided by its third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. At March 31, 2020 and December 31, 2019, the balance in the Company’s accumulated other comprehensive loss includes activity related to the Company’s available-for-sale debt securities. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three months ended March 31, 2020 or 2019, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same periods. Accrued interest receivable on the Company's available-for-sale debt securities totaled $2.9 million and $3.6 million as of March 31, 2020 and December 31, 2019, respectively. No accrued interest receivable was written off during the three months ended March 31, 2020 or 2019. The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at March 31, 2020 and December 31, 2019 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses March 31, 2020 U.S. government agency securities $ 5,000 $ — $ 40,614 $ (15) $ 45,614 $ (15) Corporate bonds 102,072 (1,110) — — 102,072 (1,110) Total $ 107,072 $ (1,110) $ 40,614 $ (15) $ 147,686 $ (1,125) December 31, 2019 U.S. government agency securities $ 13,234 $ (3) $ 79,618 $ (45) $ 92,852 $ (48) Corporate bonds 53,983 (43) — — 53,983 (43) Total $ 67,217 $ (46) $ 79,618 $ (45) $ 146,835 $ (91) The Company determined that there was no material change in the credit risk of the above investments during the three months ended March 31, 2020. As such, an allowance for credit losses would not be necessary. As of March 31, 2020, the Company does not intend to sell such securities and it is not more likely than not that the Company will be required to sell the securities before recovery of their amortized cost bases. The Company holds equity securities with an aggregate fair value of $8.4 million and $12.9 million as of March 31, 2020 and December 31, 2019, respectively, within short-term marketable securities on its condensed consolidated balance sheets. The Company has recorded unrealized losses of $4.5 million and $3.1 million during the three months ended March 31, 2020 and 2019, respectively, related to its equity securities, which is included in other expense, net on the condensed consolidated statements of operations and comprehensive loss. Contingent consideration In connection with its prior acquisition of Precision Genome Engineering, Inc. (“Pregenen”), the Company may be required to pay future consideration that is contingent upon the achievement of specified development, regulatory approvals or sales-based milestone events. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the condensed consolidated statements of operations and comprehensive loss. In the absence of new information, changes in fair value will reflect changing discount rates and the passage of time. The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from 2021 to 2028, and discount rates ranging from 11.8% to 13.5%. Significant increases or decreases in any of the probabilities of success would result in a significantly higher or lower fair value measurement, respectively. Significant increases or decreases in the other inputs would result in a significantly lower or higher fair value measurement, respectively. The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands): For the three months ended March 31, 2020 Beginning balance $ 7,977 Additions — Changes in fair value (3,108) Payments — Ending balance $ 4,869 Please refer to Note 8, Commitments and contingencies, for further information.

Property, plant and equipment,

Property, plant and equipment, net	3 Months Ended
	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Property, plant and equipment, net	. Property, plant and equipment, net Property, plant and equipment, net, consists of the following (in thousands): As of March 31, 2020 As of December 31, 2019 Land $ 1,210 $ 1,210 Building 15,745 15,664 Computer equipment and software 6,837 6,947 Office equipment 7,607 7,599 Laboratory equipment 46,832 44,560 Leasehold improvements 33,953 33,788 Construction-in-progress 82,048 77,981 Total property, plant and equipment 194,232 187,749 Less accumulated depreciation and amortization (40,312) (36,573) Property, plant and equipment, net $ 153,920 $ 151,176 North Carolina manufacturing facility In November 2017, the Company acquired a manufacturing facility, which is in the process of construction, in Durham, North Carolina for the future manufacture of lentiviral vector for the Company’s gene therapies. As of March 31, 2020, a portion of the facility has been placed into service, and the remainder of the facility is still in process of construction. Construction-in-progress as of March 31, 2020 and December 31, 2019 includes $78.4 million and $74.2 million, respectively, related to the North Carolina manufacturing facility.

Accrued expenses and other curr

Accrued expenses and other current liabilities	3 Months Ended
	Mar. 31, 2020
Payables and Accruals [Abstract]
Accrued expenses and other current liabilities	Accrued expenses and other current liabilitiesAccrued expenses and other current liabilities consist of the following (in thousands): As of March 31, 2020 As of December 31, 2019 Employee compensation $ 24,018 $ 44,679 Manufacturing costs 25,957 23,126 Clinical and contract research organization costs 21,266 16,799 Collaboration research costs 19,732 27,142 Property, plant, and equipment 730 2,354 License and milestone fees 768 300 Professional fees 2,604 1,827 Other 23,016 25,329 Total accrued expenses and other current liabilities $ 118,091 $ 141,556

Leases

Leases	3 Months Ended
	Mar. 31, 2020
Leases [Abstract]
Leases	Leases The Company leases certain office and laboratory space. Additionally, the Company has embedded leases at contract manufacturing organizations. Effective January 1, 2019, the Company adopted ASU 2016-02, Leases (Topic 842), (“ASC 842”), using the required modified retrospective approach and utilizing the effective date as the date of initial application. 60 Binney Street Lease On September 21, 2015, the Company entered into a lease agreement for office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”), which is now the Company’s corporate headquarters. Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. The Company currently maintains a $13.8 million collateralized letter of credit and, subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease to $9.2 million over time. Pursuant to a work letter entered into in connection with the 60 Binney Street Lease, the landlord contributed an aggregate of $42.4 million toward the cost of construction and tenant improvements for the Building. The Company occupied the Building beginning on March 27, 2017 and the 60 Binney Street Lease will continue until March 31, 2027. The Company has the option to extend the 60 Binney Street Lease for two successive five s. I n applying the ASC 842 transition guidance, the Company classified this lease as an operating lease and recorded a right-of-use asset and lease liability on the effective date. The Company is recognizing rent expense on a straight-line basis throughout the remaining term of the lease. 50 Binney Street Sublease In April 2019, the Company entered into a sublease agreement for office space located at 50 Binney Street in Cambridge, Massachusetts (the “50 Binney Street Sublease”) to supplement the Company’s corporate headquarters located at 60 Binney Street in Cambridge, Massachusetts. Under the terms of the 50 Binney Street Sublease, the Company will lease 267,278 square feet of office space for $99.95 per square foot, or $26.7 million per year in base rent subject to certain operating expenses, taxes and annual rent increases of approximately 3%. The lease will commence when the space is available for use by the Company, which is anticipated to be in the second half of 2021, and end on December 31, 2030, unless the Company earlier occupies the premises or other conditions specified in the 50 Binney Street Sublease occur. The sublessor has the right to postpone the commencement date until January 1, 2022 by providing not less than nine recorded within restricted cash and other non-current assets on the Company’s condensed consolidated balance sheets as of March 31, 2020. The Company will assess the lease classification of the 50 Binney Street Sublease and commence recognition of the associated rent expense upon lease commencement. Seattle, Washington leases In July 2018, the Company entered into a lease agreement for office and laboratory space located in a portion of a building in Seattle, Washington. The lease was amended in October 2018 to increase the total rentable space to approximately 36,126 square feet at $54.00 per square foot in base rent per year, which is subject to scheduled annual rent increases of 2.5% plus certain operating expenses and taxes. The lease commenced on January 1, 2019, and the lease term will continue through January 31, 2027. The Company moved into the facility in June 2019. The Company determined the classification of this lease to be an operating lease and recorded a right-of-use asset and lease liability at lease commencement. In September 2019, the Company entered into a second amendment to the lease (the “Second Amendment”). The Second Amendment added approximately 22,188 square feet to the existing space and extended the lease term of the entire premises by 16 months, or until April 2028. Fixed monthly rent for the expanded space will be incurred at a rate of $62.80 per square foot per year beginning in January 2021, subject to annual increases of 2.5%. The Second Amendment includes a five Upon the execution of the Second Amendment, which was deemed to be a lease modification, the Company re-evaluated the assumptions made at the original lease commencement date. The Company determined the Second Amendment consists of two separate contracts under ASC 842. One contract is related to a new right-of-use for the expanded 22,188 square feet of space, which is to be accounted for as a new lease, and the other is related to the modification of term for the original 36,126 square feet of space. The Company recorded an additional right-of-use asset and lease liability upon lease commencement of the expanded space. The Company is recognizing rent expense on a straight-line basis through the remaining extended term of the respective leases. Embedded operating leases On June 3, 2016, the Company entered into a manufacturing agreement for the future commercial production of the Company’s ZYNTEGLO, LentiGlobin, and Lenti-D drug products with a contract manufacturing organization. Under this 12-year agreement, the contract manufacturing organization will complete the design, construction, validation, and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company paid $12.0 million upon the achievement of certain contractual milestones and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. Construction was completed in March 2018 and beginning in April 2018, the Company pays $5.1 million per year in fixed suite fees, as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services, and may pay additional suite fees if it elects its option to reserve or lease additional suites. The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company determined that this agreement contains an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company recorded a right-of-use asset and lease liability for this operating lease on the effective date of ASC 842 and is recognizing rent expense on a straight-line basis throughout the remaining term of the embedded lease. On November 18, 2016, the Company entered into an agreement for clinical and commercial production of the Company’s ZYNTEGLO, LentiGlobin for SCD and Lenti-D drug products with a contract manufacturing organization at an existing facility. The Company concluded that this agreement contains an embedded operating lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement. The term of the agreement is five years with subsequent three estimated

Commitments and contingencies

Commitments and contingencies	3 Months Ended
	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies Contingent consideration related to business combinations On June 30, 2014, the Company acquired Pregenen. 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 clinical and commercial milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved, and discount rates. The use of different assumptions could result in materially different estimates of fair value. Please refer to Note 4, Fair value measurements, for additional information. Other funding commitments The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones that may be met in subsequent periods or royalties on future sales of specified products, which includes the collaboration agreement entered into with Regeneron Pharmaceuticals, Inc. (“Regeneron”) in August 2018. Please refer to Note 9, Collaborative arrangements, for further information on the collaboration agreement with Regeneron. Additionally, the Company is party to various contracts with contract research organizations and contract manufacturers that generally provide for termination on notice, with the exact amounts in the event of termination to be based on the timing of the termination and the terms of the agreement. As compared to the contractual obligations and commitments as disclosed in the Company’s Annual Report on Form 10-K filed with the SEC on February 18, 2020, the Company’s future minimum purchase commitments for the period ended March 31, 2020 increased by $57.6 million under agreements with certain contract manufacturing organizations. The Company may be obligated to make future development, regulatory, and commercial milestone payments, and royalty payments on future sales of specified products associated with its collaboration and license agreements. Payments under these agreements generally become due and payable upon achievement of such milestones or sales. When the achievement of these milestones or sales have occurred, the corresponding amounts are recognized in the Company’s financial statements. Refer to Note 13, Subsequent events , for discussion of the May 2020 amendments to the BMS arrangement. While there are no material legal proceedings the Company is aware of, the Company may become party to various claims and complaints arising in the ordinary course of business. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. Management does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, management cannot give any assurance regarding the ultimate outcome of any claims, and their resolution could be material to operating results for any particular period. The Company also indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company's request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and by-laws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director or officer in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company's exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, it is has not recognized any liabilities relating to these obligations.

Collaborative arrangements

Collaborative arrangements	3 Months Ended
	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative arrangements	Collaborative arrangementsTo date, the Company’s revenue has been primarily generated from its collaboration arrangements with BMS, formerly Celgene prior to its acquisition by BMS in November 2019, and Regeneron Pharmaceuticals, Inc. ("Regeneron"), each as further described below. Bristol-Myers Squibb BMS Original Collaboration Agreement On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “BMS Collaboration Agreement”) with Celgene (now BMS following its acquisition of Celgene in November 2019) to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with BMS pursuant to which the Company obtained a sublicense to certain intellectual property from BMS, originating under BMS’s license from Baylor College of Medicine, for use in the collaboration. Under the terms of the BMS Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. The Company was responsible for conducting discovery, research and development activities through completion of phase 1 clinical studies, if any, during the initial term of the BMS 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 BMS. The JSC, among other activities, reviews the collaboration program, reviews and evaluates product candidates and approves regulatory plans. In addition to the JSC, the BMS Collaboration Agreement provides that the Company and BMS each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration. BMS Amended Collaboration Agreement On June 3, 2015, the Company and BMS amended and restated the BMS Collaboration Agreement (the “Amended BMS Collaboration Agreement”). Under the Amended BMS Collaboration Agreement, the parties narrowed the focus of the collaboration to exclusively work on anti- B-cell maturation antigen (“BCMA”) product candidates for a new three On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial phase 1 clinical study for such product candidate (the “Option Period”), the Company had granted BMS an option to obtain an exclusive worldwide license to develop and commercialize such product. Following BMS’s license of each product candidate, the Company is entitled to elect to co-develop and co-promote each product candidate in the U.S. BMS Ide-cel License Agreement On February 10, 2016, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize ide-cel, the first product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“Ide-cel License Agreement”) entered into by the parties on February 16, 2016 and paid to the Company the associated $10.0 million option fee. Pursuant to the Ide-cel License Agreement, BMS was responsible for development and related funding of ide-cel after the substantial completion of the phase 1 clinical trial. The Company was responsible for the manufacture of vector and associated payload throughout development and upon BMS’s request, throughout commercialization, the costs of which were reimbursable by BMS in accordance with the terms of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement, as further described below. BMS was responsible for the manufacture of drug product throughout development and commercialization. BMS Ide-cel Co-Development, Co-Promote and Profit Share Agreement On March 28, 2018, the Company elected to co-develop and co-promote ide-cel within the U.S. pursuant to the execution of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Ide-cel CCPS”), which replaced the Ide-cel License Agreement. The responsibilities of the parties remain unchanged from those under the Ide-cel License Agreement, however, the Company will share equally in all profits and losses relating to developing, commercializing and manufacturing ide-cel within the U.S. and has the right to participate in the development and promotion of ide-cel in the U.S. BMS is responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. Under the Ide-cel CCPS, the Company may receive up to a total of $70.0 million in development milestone payments for the first indication to be addressed by ide-cel, with the ability to obtain additional milestone payments for a second indication and modified licensed products. In the second quarter of 2019, a $10.0 million development milestone was achieved such that as of March 31, 2020, the total remaining potential development milestones on the first indication of ide-cel is $60.0 million. In addition, to the extent ide-cel is commercialized, the Company is entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions. Refer to Note 13, Subsequent events , for discussion of the May 2020 amendments to the BMS arrangement. BMS bb21217 License Agreement On September 22, 2017, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“bb21217 License Agreement”) entered into by the parties on September 28, 2017 and paid the Company an option fee of $15.0 million. Pursuant to the bb21217 License Agreement, BMS is responsible for development and related funding of bb21217 after the substantial completion of the ongoing phase 1 clinical trial. In 2019, the parties amended the protocol for the ongoing phase 1 clinical trial to enroll additional patients for which the Company will be reimbursed based upon an agreed-upon amount per patient. The Company is responsible for the manufacture of vector and associated payload throughout development and upon BMS’s request, throughout commercialization. Expenses incurred by the Company associated with these activities are fully reimbursable by BMS at cost plus a mark-up. Throughout both development and commercialization, BMS is responsible for the manufacture of drug product. The Company currently expects it will exercise its option to co-develop and co-promote bb21217 within the U.S. The Company’s election to co-develop and co-promote bb21217 must be made by the substantial completion of the on-going phase 1 clinical trial of bb21217. If elected, the Company expects the responsibilities of the parties to remain largely unchanged, however, the Company expects it will share equally in all profits and losses relating to developing, commercializing and manufacturing bb21217 within the U.S. and to have the right to participate in the development and promotion of bb21217 in the U.S. BMS would be responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. Under this scenario, the Company expects to receive, per product, up to $70.0 million in development milestone payments for the first indication to be addressed by the bb21217 product candidate, with the ability to obtain additional milestone payments for a second indication and modified licensed products. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions. In the event the Company does not exercise its option to co-develop and co-promote bb21217, the Company will receive an additional fee in the amount of $10.0 million. Under this scenario, the Company may be eligible to receive up to $10.0 million in clinical milestone payments, up to $117.0 million in regulatory milestone payments, and up to $78.0 million in commercial milestone payments. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales, subject to certain reductions. Refer to Note 13, Subsequent events , for discussion of the May 2020 amendments to the BMS arrangement. Accounting Analysis – Ide-cel The transition provisions of Topic 606 allowed entities to elect the practical expedient to reflect the aggregate effect of all contract modifications when identifying the satisfied and unsatisfied performance obligations for contracts that were modified prior to Topic 606 adoption (the Company adopted Topic 606 on January 1, 2018). BMS’s option to in-license the first product candidate, ide-cel, under the arrangement was considered a material right at the time the Amended BMS Collaboration Agreement was executed in June 2015 given the product candidate had been formally nominated by the JSC and that substantially all investigational new drug application, or IND, enabling activities had been completed by that time. In making this determination, the Company also considered the option price relative to the value of the underlying license. BMS’s exercise of this material right in February 2016 was determined to represent a contract modification and represents the last contract modification prior to the adoption of Topic 606. As a result, the BMS Collaboration Agreement, Amended BMS Collaboration Agreement, and Ide-cel CCPS are combined for accounting purposes and treated as a single arrangement. As of February 2016, BMS’s option to license an additional product candidate under the collaboration did not represent a material right due primarily to the significant uncertainty regarding whether any additional product candidates would be identified under the Amended BMS Collaboration Agreement. Therefore, the license to the Company’s second product candidate, bb21217, which was executed in September 2017, is accounted for as a separate contract. Refer below for discussion of the bb21217 accounting analysis. As of the February 2016 contract modification date, the Company concluded the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to ide-cel, and (iii) manufacture of vectors and associated payload for incorporation into ide-cel through development. The Company determined that the manufacture of commercial vector represented an option to acquire additional goods and services that is not representative of a material right. In addition, as of the February 2016 contract modification date, BMS had not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector was not considered to be a performance obligation at that time. The Company concluded that the research and development services are distinct from the other promised goods and services under the arrangement given that BMS can benefit from the research and development services on their own and such services are distinct within the context of the contract. Thus, such services are considered to be a separate performance obligation. The Company concluded that the license to ide-cel is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation. Ide-cel transaction price The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of March 31, 2020 (in thousands): Ide-cel transaction price as of March 31, Up-front non-refundable payment - BMS Collaboration Agreement $ 75,000 Up-front non-refundable payment - Amended BMS Collaboration Agreement 25,000 Ide-cel license fee - Ide-cel License Agreement 10,000 Ide-cel development milestone 10,000 Estimated variable consideration 87,189 $ 207,189 Allocation of Transaction price unsatisfied as of March 31, 2020 Ide-cel research and development services $ 40,912 $ — Ide-cel license and manufacturing services 166,277 13,362 $ 207,189 $ 13,362 The estimated variable consideration of $87.2 million relates to the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development. The total transaction price has been allocated to the performance obligations identified based on a relative standalone selling price (“SSP”) basis. The Company estimated the SSP of the license after considering potential future cash flows under the license. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the research and development services and manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services. All of the clinical and regulatory milestones which have not been met as of period end are fully constrained and are excluded from the transaction price. As part of its evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestones is outside the control of the Company and contingent upon the future success of clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to BMS and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, at each reporting period and as uncertain events are resolved or other changes in circumstances occur. Ide-cel research and development services The Company allocated $40.9 million of the transaction price to the research and development services. The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was three years through projected initial phase 1 clinical study substantial completion, or through May 2018. The Company recognized revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. Although the Company fully satisfied this performance obligation during the second quarter of 2018, any changes to the total transaction price following the completion of this performance obligation in May 2018 will be allocated to the performance obligations under the arrangement based on a relative SSP basis and therefore the allocation of any changes to the total transaction price may impact the revenue recognized for this performance obligation in the period of change. The Company recognized a revenue adjustment related to ide-cel research and development services of $0.0 million for the three months ended March 31, 2020 and $0.2 million for the three months ended March 31, 2019. Ide-cel license and manufacturing services The Company allocated $166.3 million of the transaction price to the combined unit of accounting which consists of the license and manufacture of vectors and associated payload for incorporation into ide-cel. The Company accounts for its vector manufacturing services for development in the U.S. and BMS’s U.S. development efforts within the scope of ASC 808 given that both parties are active participants in the activities and both parties are exposed to significant risks and rewards dependent on the commercial success of the activities. The Company recognizes revenue for its U.S. manufacturing services by analogy to Topic 606. The portion of BMS’s U.S. development costs that the Company is responsible for are recognized as a reduction to its collaborative arrangement revenues, or, if in excess of such revenues in a given quarter, the excess is recorded as research and development expense. Revenue recognition for the combined unit of accounting commenced during the first quarter of 2017. The Company recognizes revenue associated with the combined unit of accounting using the proportional performance method, as the Company will satisfy this performance obligation as the manufacturing services are performed through development. In using this method, the Company estimated its development plan for ide-cel, including expected demand from BMS, and the costs associated with the manufacture of vectors and associated payload for incorporation into ide-cel. On a quarterly basis, the Company determines the proportion of effort incurred as a percentage of total effort it expects to expend. This ratio is applied to the transaction price, which includes variable consideration, allocated to the combined performance obligation consisting of the ide-cel license and manufacturing services. Management has applied significant judgment in the process of developing its budget estimates and any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. The following table summarizes the net collaborative arrangement expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808 related to the combined performance obligation for the license and vector manufacturing of ide-cel in the U.S. for the three months ended March 31, 2020, and 2019 (in thousands): For the three months ended March 31, 2020 2019 ASC 808 ide-cel research and development expense - U.S. (1) $ (5,080) $ (3,244) (1) As noted above, the calculation of collaborative arrangement revenue or research and development expense to be recognized for joint ide-cel development efforts in the U.S. is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. Revenue related to the combined unit of accounting for the non-US license and vector manufacturing services is accounted for in accordance with Topic 606. The following table summarizes the revenue recognized related to the combined unit of accounting for the ide-cel non-US license and vector manufacturing services for the three months ended March 31, 2020, and 2019 (in thousands): For the three months ended March 31, 2020 2019 ASC 606 ide-cel license and manufacturing revenue - outside of U.S. $ 13,970 $ 9,064 As of March 31, 2020, the aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the ide-cel license and manufacturing services, that is unsatisfied, or partially unsatisfied, is $13.4 million, which the Company expects to recognize as revenue as manufacturing services are provided through the remaining development period which is estimated to be through 2020. As of March 31, 2020 and December 31, 2019, the Company had $6.4 million and $8.5 million, respectively, of deferred revenue associated with the combined performance obligation consisting of the ide-cel license and manufacturing services. Accounting Analysis – bb21217 On September 22, 2017, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second optioned product candidate, pursuant to the bb21217 License Agreement entered into by the parties on September 28, 2017. The bb21217 License Agreement is considered a separate contract for accounting purposes as the option to obtain an exclusive worldwide license to develop and commercialize bb21217, or any other product candidate, was not considered a material right to BMS at the time the practical expedient was applied. The Company made this evaluation after considering the significant uncertainty at that time regarding whether any additional product candidates would be identified under the Amended BMS Collaboration Agreement. In particular, the Company considered that bb21217 had not been formally nominated as a product candidate under the collaboration at that time, primarily due to a lack of preclinical data as well as uncertainty surrounding the ability to successfully complete various IND-enabling activities. At contract inception, the Company concluded that the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to the second product candidate, bb21217, and (iii) manufacture of vectors and associated payload for incorporation into bb21217 through development. The Company determined that the manufacture of commercial vector represents an option to acquire additional goods and services that is not representative of a material right. In addition, at this time BMS has not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector is not considered to be a performance obligation at this time. The Company concluded that the research and development services for the phase 1 clinical trial as originally contemplated by the parties are distinct from the other promised goods and services under the arrangement given that BMS can benefit from the research and development services on their own and such services are distinct within the context of the contract. Thus, such services are considered to be a separate performance obligation. Similar to ide-cel, the Company concluded that the license to bb21217 is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation. The agreement to expand the bb21217 phase 1 trial is treated as a contract modification for accounting purposes, because the trial expansion is the addition of a promised good or service that is distinct and the associated consideration reflects the standalone selling price of the additional promised good or service. It will be accounted for prospectively as a separate contract from the bb21217 License Agreement. The transaction price associated with these additional patients consists of variable consideration and is based upon an agreed-upon amount per patient which will be recognized as revenue as the patients are treated. The Company began fulfilling the performance obligation in the fourth quarter of 2019 and it remained partially unsatisfied as of March 31, 2020. bb21217 License Agreement transaction price The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of March 31, 2020 (in thousands): bb21217 transaction price as of March 31, 2020 bb21217 license fee - bb21217 License Agreement $ 15,000 Estimated variable consideration 26,687 $ 41,687 Allocation of transaction Transaction price unsatisfied as of March 31, 2020 bb21217 research and development services $ 5,444 $ — bb21217 license and manufacturing services 36,243 36,243 $ 41,687 $ 36,243 The estimated variable consideration of $26.7 million relates to reimbursement from BMS for the manufacturing services during development. The total transaction price has been allocated to the performance obligations identified based on a relative SSP basis. The Company estimated the SSP of the license after considering potential future cash flows under the license. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the research and development services and manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services. All of the clinical and regulatory milestones are fully constrained and are excluded from the transaction price. As part of its evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestones is outside the control of the Company and contingent upon the future success of its clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to BMS and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, each reporting period and as uncertain events are resolved or other changes in circumstances occur. bb21217 research and development services The Company allocated $5.4 million of the transaction price to the research and development services under the phase 1 trial as originally contemplated. The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was two years through projected initial phase 1 clinical study substantial completion for these patients, or through September 2019. The Company recognized revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. Although the Company fully satisfied its initial performance obligation as originally contemplated during the third quarter of 2019, any changes to the total transaction price following the completion of this performance obligation in September 2019 will be allocated to the performance obligations under the arrangement based on a relative SSP basis and therefore the allocation of any changes to the total transaction price may impact the revenue recognized for this performance obligation in the period of change. As part of performing its initial obligation to complete a phase 1 trial as originally contemplated, the Company recognized revenue of $0.0 million and $0.7 million for the three months ended March 31, 2020 and 2019, respectively. In connection with treating additional patients in the phase 1 trial, the Company recognized revenue of $2.4 million for the three months ended March 31, 2020. bb21217 license and manufacturing services The Company will satisfy its performance obligation related to the manufacture of vectors and associated payload for incorporation into bb21217 through development as the bb21217 manufacturing services are performed. As of March 31, 2020, the manufacturing services for bb21217 had not yet commenced. Therefore, no amounts have been recognized for the combined performance obligation in the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2020, and 2019. The aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the bb21217 license and manufacturing services, is $36.2 million. The Company does not expect that recognition will begin in the next twelve months and has therefore classified deferred revenue associated with the combined performance obligation as deferred revenue, net of current portion on its consolidated balance sheets. The Company had $9.8 million of remaining deferred revenue as of March 31, 2020 and December 31, 2019 associated with the combined performance obligation consisting of the bb21217 license and manufacturing services. Contract assets and liabilities – ide-cel and bb21217 The Company receives payments from its collaborative partners based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until such time as the Company satisfies its performance obligations under these arrangements. A contract asset is a conditional right to consideration in exchange for goods or services that the Company has transferred to a customer. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the three months ended March 31, 2020 (in thousands): Balance at December 31, Additions Deductions Balance at March 31, Receivables $ 400 $ 2,400 $ (400) $ 2,400 Contract liabilities: Deferred revenue $ 18,265 $ — $ (2,118) $ 16,147 The change in the receivables balance for the three months ended March 31, 2020 is primarily driven by amounts owed to the Company for bb21217 research and development services provided in the first quarter of 2020, offset by amounts collected from BMS in the period. The decrease in deferred revenue during the three months ended March 31, 2020 is driven by amounts recognized for the combined performance obligation consisting of the ide-cel license and manufacturing services. During the three months ended March 31, 2020, $2.1 million of the deferred revenue balance at the beginning of the period was released from deferred revenue. Regeneron Regeneron Collaboration Agreement On August 3, 2018, the Company entered into a Collaboration Agreement (the “Regeneron Collaboration Agreement”) with Regeneron pursuant to which the parties will apply their respective technology platforms to the discovery, development, and commercialization of novel immune cell therapies for cancer. On August 24, 2018, following the completion of required regulatory reviews, the Regeneron Collaboration Agreement became effective. Under the terms of the agreement, the parties will leverage Regeneron’s proprietary platform technologies for the discovery and characterization of fully human antibodies, as well as T cell receptors directed against tumor-specific proteins and peptides and the Company will contribute its field-leading expertise in gene therapy. In accordance with the Regeneron Collaboration Agreement, the parties jointly selected six initial targets and intend to equally share the costs of research up to the point of submitting an IND application for a potential gene therapy product directed to a particular target. Additional targets may be selected to add to or replace any of the initial targets during the five Regeneron will accrue a certain number of option rights exercisable against targets as the parties reach certain milestones under the terms of the agreement. Upon the acceptance of an IND for the first product candidate directed to a target, Regeneron will have the right to exercise an option for co-development/co-commercialization of product candidates directed to such target on a worldwide or applicable opt-in territory basis, with certain exceptions. Where Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories. Outside of the applicable opt-in territories, the target becomes a licensed target and Regeneron would be eligible to receive, with respect to any resulting product, milestone payments of up to $130.0 million per product and royalties on net sales outside of the applicable opt-in territories at a rate ranging from the mid-single digits to low-double digits. A target would also become a licensed target in the event Regeneron does not have an option to such target, or Regeneron does not exercise its option with respect to such target. Either party may terminate a given research program directed to a particular target for convenience,

Stock-based compensation

Stock-based compensation	3 Months Ended
	Mar. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-based compensation	Stock-based compensationIn January 2020 and 2019, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.2 million and 2.2 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2020, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.5 million. Stock-based compensation expense The Company recognized stock-based compensation expense totaling $36.3 million and $32.3 million for the three months ended March 31, 2020 and 2019, respectively. 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 three months ended March 31, 2020 2019 Stock options 24,440 23,183 Restricted stock units 11,853 8,881 Employee stock purchase plan — 277 36,293 32,341 Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): For the three months ended March 31 2020 2019 Research and development 16,269 15,516 Selling, general and administrative 20,024 16,825 36,293 32,341 As of March 31, 2020, the Company had $361.8 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units, and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.7 years. Stock option activity The following table summarizes the stock option activity under the Company’s equity award plans: Shares (in thousands) Weighted- average exercise price per share Outstanding at December 31, 2019 5,483 $ 116.30 Granted 1,147 $ 75.65 Exercised (20) $ 37.71 Canceled, forfeited, or expired (132) $ 148.55 Outstanding at March 31, 2020 6,478 $ 108.68 Exercisable at March 31, 2020 3,248 $ 99.92 Vested and expected to vest at March 31, 2020 6,478 $ 108.68 During the three months ended March 31, 2020, less than 0.1 million shares of common stock were exercised, resulting in total proceeds to the Company of $0.8 million. Restricted stock unit activity The following table summarizes the restricted stock unit activity under the Company’s equity award plans: Shares Weighted- Unvested balance at December 31, 2019 1,127 $ 146.10 Granted 709 $ 77.69 Vested (204) $ 132.15 Forfeited (47) $ 150.73 Unvested balance at March 31, 2020 1,585 $ 117.15 Employee stock purchase plan On June 3, 2013, the Company adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized the initial issuance of up to a total of 0.2 million shares of the Company’s common stock to participating employees. During each of the three months ended March 31, 2020 and 2019, less than 0.1 million shares of common stock were issued under the 2013 ESPP.

Net loss per share

Net loss per share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net loss per share	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 months ended March 31, 2020 2019 Outstanding stock options 6,478 5,384 Restricted stock units 1,585 1,128 ESPP shares — 15 8,063 6,527

Basis of presentation, princi_2

Basis of presentation, principles of consolidation and significant accounting policies (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the ASC and Accounting Standards Updates (“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 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 March 31, 2020 and 2019.
Consolidation	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment.
Marketable securities	Marketable securities Effective January 1, 2020, the Company adopted ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements ("ASU 2016-13" or "ASC 326"), using the effective date method. As the Company had never recorded any other-than-temporary-impairment adjustments to its available-for-sale debt securities prior to the effective date, no transition provisions are applicable to the Company. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASC 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the condensed consolidated statements of operations and comprehensive loss as credit loss expense within other expense, net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities. Accrued interest receivable related to the Company's available-for-sale debt securities is presented within receivables and other current assets on the Company's condensed consolidated balance sheets. The Company has elected the practical expedient available to exclude accrued interest receivable from both the fair value and the amortized cost basis of available-for-sale debt securities for the purposes of identifying and measuring any impairment. The Company writes off accrued interest receivable once it has determined that the asset is not realizable. Any write offs of accrued interest receivable are recorded by reversing interest income, recognizing credit loss expense, or a combination of both. To date, the Company has not written off any accrued interest receivables associated with its marketable securities.
Stock-based compensation	Stock-based compensation The Company estimates the fair value of its option awards using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Effective January 1, 2020, the Company eliminated the use of a representative peer group and uses only its own historical volatility data in its estimate of expected volatility given that there is now a sufficient amount of historical information regarding the volatility of its own stock price.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: future undiscounted cash flows and subsequent fair value estimates used to assess potential and measure any impairment of long-lived assets, including goodwill and intangible assets, and the measurement of right-of-use assets and lease liabilities, contingent consideration, stock-based compensation expense, accrued expenses, revenue, income taxes, and the assessment of the Company's ability to fund its operations for at least the next twelve months from the date of issuance of these financial statements.
Recent accounting pronouncements	Recent accounting pronouncements Recently adopted ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements, ASU No. 2019-5 Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief, ASU No. 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements . The new standard, as amended, requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The targeted transition relief standard allows filers an option to irrevocably elect the fair value option of ASC 825-10, Financial Instruments-Overall , applied on an instrument-by-instrument basis for eligible instruments. The Company adopted this standard on January 1, 2020 on a prospective basis and the adoption did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. The new standard removes certain disclosures, modifies certain disclosures, and adds additional disclosures related to fair value measurement. The Company adopted this standard on January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-15, Intangibles-Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract . The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The Company adopted this standard on a prospective basis as of January 1, 2020, and it did not have a material impact on its financial position and results of operations upon adoption. ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , (“ASU 2018-18”). The amendments in this update clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 , Revenue from Contracts with Customers (“Topic 606” or "ASC 606") when the counter party is a customer in the context of a unit of account. ASU 2018-18 also precludes companies from presenting transactions with collaborative partners that are outside the scope of Topic 606 together with revenue within the scope of Topic 606. The Company adopted this standard on a retrospective basis on January 1, 2020. As a result, revenue for prior periods are presented in accordance with the new standard. Prior to the adoption of ASU 2018-18, the Company presented all revenue recognized under its collaborative arrangements as collaboration revenue on its condensed consolidated statement of operations and comprehensive loss. However, as the Company recognizes revenue under its collaborative arrangements both within and outside the scope of Topic 606, the Company has revised its presentation of revenue on its condensed consolidated statement of operations and comprehensive loss as follows: service revenue includes revenue from collaborative partners recognized within the scope of Topic 606 and collaborative arrangement revenue includes revenue from collaborative partners recognized outside the scope of Topic 606. The disaggregation of revenue recognized under Topic 606 and outside of Topic 606 had previously otherwise been disclosed in the Notes to Condensed Consolidated Financial Statements. ASU No. 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments In April 2019, the FASB issued ASU 2019-4, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. This update provides clarifications for three topics related to financial instruments accounting, some of which apply to the Company. The Company adopted this standard on January 1, 2020 on a prospective basis, and it did not have a material impact on its financial position and results of operations upon adoption. Not yet adopted ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. The new standard will be effective beginning January 1, 2021. The Company is currently evaluating the potential impact ASU 2019-12 may have on its financial position and results of operations upon adoption.

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities Held	The following table summarizes the marketable securities held at March 31, 2020 and December 31, 2019 (in thousands): Description Amortized cost / Cost Unrealized gains Unrealized losses Fair value March 31, 2020 U.S. government agency securities and treasuries $ 377,551 $ 2,741 $ (15) $ 380,277 Certificates of deposit 240 — — 240 Corporate bonds 207,708 613 (1,110) 207,211 Commercial paper 75,650 — — 75,650 Equity securities 20,017 — (11,667) 8,350 Total $ 681,166 $ 3,354 $ (12,792) $ 671,728 December 31, 2019 U.S. government agency securities and treasuries $ 633,970 $ 2,014 $ (48) $ 635,936 Certificates of deposit 960 — — 960 Corporate bonds 185,827 824 (43) 186,608 Commercial paper 74,378 — — 74,378 Equity securities 20,017 — (7,147) 12,870 Total $ 915,152 $ 2,838 $ (7,238) $ 910,752

Fair value measurements (Tables

Fair value measurements (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2020 and December 31, 2019 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) March 31, 2020 Assets: Cash and cash equivalents $ 346,629 $ 336,630 $ 9,999 $ — Marketable securities: U.S. government agency securities and treasuries 380,277 — 380,277 — Certificates of deposit 240 — 240 — Corporate bonds 207,211 — 207,211 — Commercial paper 75,650 — 75,650 — Equity securities 8,350 8,350 — — Total $ 1,018,357 $ 344,980 $ 673,377 $ — Liabilities: Contingent consideration $ 4,869 $ — $ — $ 4,869 Total $ 4,869 $ — $ — $ 4,869 December 31, 2019 Assets: Cash and cash equivalents $ 327,214 $ 311,245 $ 15,969 $ — Marketable securities: U.S. government agency securities and treasuries 635,936 — 635,936 — Certificates of deposit 960 — 960 — Corporate bonds 186,608 — 186,608 — Commercial paper 74,378 — 74,378 — Equity securities 12,870 12,870 — — Total $ 1,237,966 $ 324,115 $ 913,851 $ — Liabilities: Contingent consideration $ 7,977 $ — $ — $ 7,977 Total $ 7,977 $ — $ — $ 7,977
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value	The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at March 31, 2020 and December 31, 2019 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses March 31, 2020 U.S. government agency securities $ 5,000 $ — $ 40,614 $ (15) $ 45,614 $ (15) Corporate bonds 102,072 (1,110) — — 102,072 (1,110) Total $ 107,072 $ (1,110) $ 40,614 $ (15) $ 147,686 $ (1,125) December 31, 2019 U.S. government agency securities $ 13,234 $ (3) $ 79,618 $ (45) $ 92,852 $ (48) Corporate bonds 53,983 (43) — — 53,983 (43) Total $ 67,217 $ (46) $ 79,618 $ (45) $ 146,835 $ (91)
Roll-Forward of Fair Value of 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 three months ended March 31, 2020 Beginning balance $ 7,977 Additions — Changes in fair value (3,108) Payments — Ending balance $ 4,869

Property, plant and equipment_2

Property, plant and equipment, net (Tables)	3 Months Ended
	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment Net	Property, plant and equipment, net, consists of the following (in thousands): As of March 31, 2020 As of December 31, 2019 Land $ 1,210 $ 1,210 Building 15,745 15,664 Computer equipment and software 6,837 6,947 Office equipment 7,607 7,599 Laboratory equipment 46,832 44,560 Leasehold improvements 33,953 33,788 Construction-in-progress 82,048 77,981 Total property, plant and equipment 194,232 187,749 Less accumulated depreciation and amortization (40,312) (36,573) Property, plant and equipment, net $ 153,920 $ 151,176

Accrued expenses and other cu_2

Accrued expenses and other current liabilities (Tables)	3 Months Ended
	Mar. 31, 2020
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): As of March 31, 2020 As of December 31, 2019 Employee compensation $ 24,018 $ 44,679 Manufacturing costs 25,957 23,126 Clinical and contract research organization costs 21,266 16,799 Collaboration research costs 19,732 27,142 Property, plant, and equipment 730 2,354 License and milestone fees 768 300 Professional fees 2,604 1,827 Other 23,016 25,329 Total accrued expenses and other current liabilities $ 118,091 $ 141,556

Collaborative arrangements (Tab

Collaborative arrangements (Tables)	3 Months Ended
	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied	The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of March 31, 2020 (in thousands): Ide-cel transaction price as of March 31, Up-front non-refundable payment - BMS Collaboration Agreement $ 75,000 Up-front non-refundable payment - Amended BMS Collaboration Agreement 25,000 Ide-cel license fee - Ide-cel License Agreement 10,000 Ide-cel development milestone 10,000 Estimated variable consideration 87,189 $ 207,189 Allocation of Transaction price unsatisfied as of March 31, 2020 Ide-cel research and development services $ 40,912 $ — Ide-cel license and manufacturing services 166,277 13,362 $ 207,189 $ 13,362 The following table summarizes the net collaborative arrangement expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808 related to the combined performance obligation for the license and vector manufacturing of ide-cel in the U.S. for the three months ended March 31, 2020, and 2019 (in thousands): For the three months ended March 31, 2020 2019 ASC 808 ide-cel research and development expense - U.S. (1) $ (5,080) $ (3,244) (1) As noted above, the calculation of collaborative arrangement revenue or research and development expense to be recognized for joint ide-cel development efforts in the U.S. is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. Revenue related to the combined unit of accounting for the non-US license and vector manufacturing services is accounted for in accordance with Topic 606. The following table summarizes the revenue recognized related to the combined unit of accounting for the ide-cel non-US license and vector manufacturing services for the three months ended March 31, 2020, and 2019 (in thousands): For the three months ended March 31, 2020 2019 ASC 606 ide-cel license and manufacturing revenue - outside of U.S. $ 13,970 $ 9,064 The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of March 31, 2020 (in thousands): bb21217 transaction price as of March 31, 2020 bb21217 license fee - bb21217 License Agreement $ 15,000 Estimated variable consideration 26,687 $ 41,687 Allocation of transaction Transaction price unsatisfied as of March 31, 2020 bb21217 research and development services $ 5,444 $ — bb21217 license and manufacturing services 36,243 36,243 $ 41,687 $ 36,243
Changes in Balances of Company's Receivables and Contract Liabilities	The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the three months ended March 31, 2020 (in thousands): Balance at December 31, Additions Deductions Balance at March 31, Receivables $ 400 $ 2,400 $ (400) $ 2,400 Contract liabilities: Deferred revenue $ 18,265 $ — $ (2,118) $ 16,147

Stock-based compensation (Table

Stock-based compensation (Tables)	3 Months Ended
	Mar. 31, 2020
Share-based Payment Arrangement [Abstract]
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 three months ended March 31, 2020 2019 Stock options 24,440 23,183 Restricted stock units 11,853 8,881 Employee stock purchase plan — 277 36,293 32,341
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 three months ended March 31 2020 2019 Research and development 16,269 15,516 Selling, general and administrative 20,024 16,825 36,293 32,341
Summary of Stock Option Activity Under Plan	The following table summarizes the stock option activity under the Company’s equity award plans: Shares (in thousands) Weighted- average exercise price per share Outstanding at December 31, 2019 5,483 $ 116.30 Granted 1,147 $ 75.65 Exercised (20) $ 37.71 Canceled, forfeited, or expired (132) $ 148.55 Outstanding at March 31, 2020 6,478 $ 108.68 Exercisable at March 31, 2020 3,248 $ 99.92 Vested and expected to vest at March 31, 2020 6,478 $ 108.68
Summary of Restricted Common Stock Awards	The following table summarizes the restricted stock unit activity under the Company’s equity award plans: Shares Weighted- Unvested balance at December 31, 2019 1,127 $ 146.10 Granted 709 $ 77.69 Vested (204) $ 132.15 Forfeited (47) $ 150.73 Unvested balance at March 31, 2020 1,585 $ 117.15

Net loss per share (Tables)

Net loss per share (Tables)	3 Months Ended
	Mar. 31, 2020
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 months ended March 31, 2020 2019 Outstanding stock options 6,478 5,384 Restricted stock units 1,585 1,128 ESPP shares — 15 8,063 6,527

Subsequent Events (Details)

Subsequent Events (Details) $ in Millions	8 Months Ended
	Dec. 31, 2020USD ($)
Forecast | Bristol-Myers Squibb | Ide-cel CCPS and BB21217 License Agreements | Subsequent Event
Subsequent Event [Line Items]
Upfront payment	$ 200

Description of the business - A

Description of the business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 2,484,463		$ 2,281,852
Net loss	202,611	$ 164,446
Net cash used in operating activities	206,121	$ 154,154
Cash, cash equivalents and marketable securities	$ 1,020,000

Basis of presentation, princi_3

Basis of presentation, principles of consolidation and significant accounting policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020segment
Accounting Policies [Abstract]
Number of operating segments	1

Marketable Securities - Summary

Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Equity Securities, Amortized cost / Cost	$ 20,017,000	$ 20,017,000
Equity Securities, Unrealized gains	0	0
Equity Securities, Unrealized losses	(11,667,000)	(7,147,000)
Equity securities	8,350,000	12,870,000
Marketable Securities, Amortized cost / Cost	681,166,000	915,152,000
Marketable Securities, Unrealized gains	3,354,000	2,838,000
Marketable Securities, Unrealized losses	(12,792,000)	(7,238,000)
Marketable securities:	671,728,000	910,752,000
Debt securities, available-for-sale, noncurrent	0		$ 0
U.S. government agency securities and treasuries
Debt Securities, Available-for-sale [Line Items]
Debt Securities. Amortized cost / Cost	377,551,000	633,970,000
Debt Securities, Unrealized gains	2,741,000	2,014,000
Debt Securities, Unrealized losses	(15,000)	(48,000)
Debt Securities, Fair value	380,277,000	635,936,000
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Debt Securities. Amortized cost / Cost	240,000	960,000
Debt Securities, Unrealized gains	0	0
Debt Securities, Unrealized losses	0	0
Debt Securities, Fair value	240,000	960,000
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Debt Securities. Amortized cost / Cost	207,708,000	185,827,000
Debt Securities, Unrealized gains	613,000	824,000
Debt Securities, Unrealized losses	(1,110,000)	(43,000)
Debt Securities, Fair value	207,211,000	186,608,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Debt Securities. Amortized cost / Cost	75,650,000	74,378,000
Debt Securities, Unrealized gains	0	0
Debt Securities, Unrealized losses	0	0
Debt Securities, Fair value	$ 75,650,000	$ 74,378,000

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Marketable securities:	$ 671,728	$ 910,752
Equity securities	8,350	12,870
Liabilities:
Contingent consideration	4,869	7,977
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	346,629	327,214
Equity securities	8,350	12,870
Total	1,018,357	1,237,966
Liabilities:
Contingent consideration	4,869	7,977
Total	4,869	7,977
Fair Value, Measurements, Recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:	380,277	635,936
Fair Value, Measurements, Recurring | Certificates of deposit
Assets:
Marketable securities:	240	960
Fair Value, Measurements, Recurring | Corporate bonds
Assets:
Marketable securities:	207,211	186,608
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Marketable securities:	75,650	74,378
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	336,630	311,245
Equity securities	8,350	12,870
Total	344,980	324,115
Liabilities:
Contingent consideration	0	0
Total	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Certificates of deposit
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Corporate bonds
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Commercial paper
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	9,999	15,969
Equity securities	0	0
Total	673,377	913,851
Liabilities:
Contingent consideration	0	0
Total	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	380,277	635,936
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Certificates of deposit
Assets:
Marketable securities:	240	960
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities:	207,211	186,608
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities:	75,650	74,378
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Equity securities	0	0
Total	0	0
Liabilities:
Contingent consideration	4,869	7,977
Total	4,869	7,977
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Certificates of deposit
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Corporate bonds
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Commercial paper
Assets:
Marketable securities:	$ 0	$ 0

Fair value measurements - Addit

Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) on available-for-sale securities	$ 0	$ 0
Fair value, less than twelve months	107,072,000		$ 67,217,000
Unrealized loss on securities, more than twelve months	40,614,000		79,618,000
Unrealized losses, less than twelve months	1,110,000		46,000
Unrealized loss on securities for more than twelve months	15,000		45,000
Interest receivable	2,900,000	3,600,000
Interest receivable, write-offs	0	0
Investment at fair value	$ 8,350,000		$ 12,870,000
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	11.80%
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	13.50%
Other (Expense) Income, Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized loss on equity securities	$ 4,500,000	$ 3,100,000

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair value
Fair value, less than 12 months	$ 107,072	$ 67,217
Fair value, 12 months or greater	40,614	79,618
Fair value	147,686	146,835
Unrealized losses
Unrealized losses, less than 12 months	(1,110)	(46)
Unrealized losses, 12 months or greater	(15)	(45)
Unrealized losses	(1,125)	(91)
U.S. government agency securities and treasuries
Fair value
Fair value, less than 12 months	5,000	13,234
Fair value, 12 months or greater	40,614	79,618
Fair value	45,614	92,852
Unrealized losses
Unrealized losses, less than 12 months	0	(3)
Unrealized losses, 12 months or greater	(15)	(45)
Unrealized losses	(15)	(48)
Corporate bonds
Fair value
Fair value, less than 12 months	102,072	53,983
Fair value, 12 months or greater	0	0
Fair value	102,072	53,983
Unrealized losses
Unrealized losses, less than 12 months	(1,110)	(43)
Unrealized losses, 12 months or greater	0	0
Unrealized losses	$ (1,110)	$ (43)

Fair value measurements - Roll-

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Additions	$ 0
Payments	0
Significant unobservable inputs (Level 3) | Contingent consideration obligations
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7,977
Changes in fair value	(3,108)
Ending balance	$ 4,869

Property, plant and equipment_3

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 194,232	$ 187,749
Less accumulated depreciation and amortization	(40,312)	(36,573)
Property, plant and equipment, net	153,920	151,176
Land
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	1,210	1,210
Building
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	15,745	15,664
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	6,837	6,947
Office equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	7,607	7,599
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	46,832	44,560
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	33,953	33,788
Construction-in-progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 82,048	$ 77,981

Property, plant and equipment_4

Property, plant and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 194,232	$ 187,749
Accumulated depreciation of de-recognized asset	(40,312)	(36,573)
Remaining Construction In Progress North Carolina Manufacturing Facility
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 78,400	$ 74,200

Accrued expenses and other cu_3

Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Employee compensation	$ 24,018	$ 44,679
Manufacturing costs	25,957	23,126
Clinical and contract research organization costs	21,266	16,799
Collaboration research costs	19,732	27,142
Property, plant, and equipment	730	2,354
License and milestone fees	768	300
Professional fees	2,604	1,827
Other	23,016	25,329
Total accrued expenses and other current liabilities	$ 118,091	$ 141,556

Leases - Additional Information

Leases - Additional Information (Detail) € in Millions	Sep. 21, 2015USD ($)ft²$ / ft²	Sep. 30, 2019ft²lease_term$ / ft²	Apr. 30, 2019USD ($)ft²$ / ft²	Jul. 31, 2018ft²$ / ft²	Mar. 31, 2020USD ($)lease_term	Mar. 31, 2018USD ($)	Mar. 06, 2020EUR (€)	Nov. 18, 2016	Jun. 03, 2016
Seattle, Washington
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) | ft²		22,188		36,126
Annual lease rent per square foot (in dollars per sq ft) | $ / ft²		62.80		54
Renewal term		5 years
Lessee, Operating Lease, Term Of Contract Extension		16 months
Lease Arrangements Annual Increase Percentage		2.50%		2.50%
Number of contracts | lease_term		2
60 Binney Street Lease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) | ft²	253,108
Annual lease rent per square foot (in dollars per sq ft) | $ / ft²	72.50
Lease payments base annual rent	$ 18,400,000
Lease rent, annual increase percentage	1.75%
Collateralized letter of credit	$ 13,800,000
Decrease in letter of credit under the terms of the lease	9,200,000
Landlord contribution for cost of construction and tenant improvements	$ 42,400,000
Number of terms | lease_term					2
Renewal term					5 years
50 Binney Street Sublease
Lessee, Lease, Description [Line Items]
Collateralized letter of credit			$ 40,100,000
Area of office space for sublease | ft²			267,278
Rate of area for sublease per square feet | $ / ft²			99.95
Annual expense			$ 26,700,000
Annual rent increase percentage			3.00%
Payment commencement, term			90 days
Upfront payment for purchase of furniture and equipment					$ 7,500,000
50 Binney Street Sublease | Minimum
Lessee, Lease, Description [Line Items]
Option to postpone commencement, term			9 months
Manufacturing Facility
Lessee, Lease, Description [Line Items]
Renewal term								3 years
Annual expense					$ 5,100,000
Lease period								5 years	12 years
Lease notice period					12 months
Milestone paid						$ 12,000,000
Lease not yet commenced, amount					$ 8,000,000
Termination fees, term					24 months
Annual maintenance and production fees | €							€ 16.5

Commitments and contingencies -

Commitments and contingencies - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Line Items]
Increase in expected future minimum purchase commitments	$ 57.6
Pregenen
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	120
Pregenen | Clinical Milestone Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	20.1
Pregenen | Commercial Milestones Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 99.9

Collaborative arrangements - Ad

Collaborative arrangements - Additional Information (Detail)	Aug. 24, 2018USD ($)target$ / sharesshares	Sep. 28, 2017USD ($)	Feb. 16, 2016USD ($)	Mar. 19, 2013USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Mar. 31, 2019USD ($)	Mar. 28, 2018USD ($)	Jun. 03, 2015USD ($)product
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability					$ 16,147,000	$ 18,265,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues					$ 2,100,000
Bristol-Myers Squibb | Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability				$ 75,000,000
Bristol-Myers Squibb | Amended Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability									$ 25,000,000
Term of collaboration agreement					3 years
Deferred revenue recognition period					3 years
Number of product candidates | product									2
Estimated variable consideration					$ 87,200,000
Bristol-Myers Squibb | Amended Collaborative Arrangement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					40,900,000
Bristol-Myers Squibb | Ide-cel License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, option fee received					10,000,000
Bristol-Myers Squibb | Ide-cel License Agreement | First Product Candidates
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, option fee received			$ 10,000,000
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Development milestone payments receivable					10,000,000
Development milestone payments achieved						10,000,000
Remaining potential development milestone payment receivable					$ 60,000,000
Bristol-Myers Squibb | Ide-cel Co-Development, Co-Promote and Profit Share Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Development milestone payments receivable								$ 70,000,000
Bristol-Myers Squibb | bb21217 License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue recognition period					2 years
Collaboration agreement, option fee received		$ 15,000,000			$ 15,000,000
Development milestone payments receivable					70,000,000
Additional fee receivable if option to co-develop and co-promote is not exercised		$ 10,000,000
Estimated variable consideration					26,700,000
Bristol-Myers Squibb | bb21217 License Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Clinical milestone payments receivable					10,000,000
Regulatory milestone payments receivable					117,000,000
Commercial milestone payments receivable					78,000,000
Bristol-Myers Squibb | bb21217 License Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					5,400,000
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					166,300,000
Bristol-Myers Squibb | Ide-cel License and Manufacturing Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability					6,400,000	8,500,000
Remaining performance obligation revenue					13,400,000
Bristol-Myers Squibb | Ide-cel Research And Development
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					0
Bristol-Myers Squibb | Ide-cel Research And Development | Restatement Adjustment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue							$ 200,000
Bristol-Myers Squibb | bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					5,444,000
Remaining performance obligation revenue					0
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Initial Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					0		700,000
Bristol-Myers Squibb | bb21217 Research and Development Services | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue					2,400,000
Bristol-Myers Squibb | bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price					36,243,000
Remaining performance obligation revenue					36,243,000
Bristol-Myers Squibb | bb21217 license and manufacturing services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer liability					9,800,000	9,800,000
Remaining performance obligation revenue					36,200,000
Deferred revenue balance recognized as gross revenues					0		0
Regeneron Collaboration Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of initial collaboration targets | target	6
Research collaboration term	5 years
Joint research activities remaining to be recognized					35,900,000	$ 38,200,000
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue balance recognized as gross revenues					$ 2,300,000		$ 2,000,000
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable	$ 130,000,000
Regeneron Collaboration Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price	100,000,000
Purchase price premium	$ 37,000,000
Collaborative arrangement amortization period	5 years
Collaborative arrangement amount attributed to equity sold	$ 54,500,000
Collaborative arrangement amount attributed to joint research activities	$ 45,500,000
Regeneron Collaboration Agreement | Share Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	400,000
Common stock price per share | $ / shares	$ 238.10
Investment in common stock	$ 100,000,000
Purchase price premium	$ 37,000,000
Collaborative arrangement research initial funding obligation, percentage	50.00%
Regeneron Collaboration Agreement | Share Purchase Agreement | Common Shares
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	400,000
Investment in common stock	$ 63,000,000

Collaborative arrangements - Su

Collaborative arrangements - Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied (Detail) - Bristol-Myers Squibb - USD ($) $ in Thousands	Sep. 28, 2017	Mar. 31, 2020
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Up-front non-refundable payment		$ 75,000
Amended BMS Collaboration Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Up-front non-refundable payment		25,000
Ide-cel License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, license fee		10,000
Ide-cel Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		166,277
Transaction price unsatisfied		13,362
Vectors and Associated Payload
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration		87,189
Ide-cel Transaction Price
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		207,189
Transaction price unsatisfied		13,362
Ide-cel Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		40,912
Transaction price unsatisfied		0
bb21217 License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, license fee	$ 15,000	15,000
Estimated variable consideration		26,700
Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration		26,687
bb21217 Transaction Price
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		41,687
Transaction price unsatisfied		36,243
bb21217 Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		5,444
Transaction price unsatisfied		0
bb21217 license and manufacturing services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price		36,243
Transaction price unsatisfied		$ 36,243

Collaborative arrangements - _2

Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred for Joint Ide-cel Development Efforts Related to Combined Unit of Accounting for its License and Vector Manufacturing of Ide-cel (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense	$ (154,123)	$ (122,640)
Bristol-Myers Squibb | U.S. | Ide-cel Research and Development Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense	(5,080)	(3,244)
Bristol-Myers Squibb | Outside of U.S. | Ide-cel License and Manufacturing Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	$ 13,970	$ 9,064

Collaborative arrangements - Ch

Collaborative arrangements - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Bristol-Myers Squibb $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Receivables
Balance at beginning of period	$ 400
Additions	2,400
Deductions	400
Balance at end of period	2,400
Contract liabilities:
Balance at beginning of period	18,265
Additions	0
Deductions	2,118
Balance at end of period	$ 16,147

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Jun. 03, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan (in shares)	2,200,000	2,200,000
Number of shares available for issuance (in shares)			2,500,000
Stock-based compensation expense			$ 36,293	$ 32,341
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			$ 361,800
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			2 years 8 months 12 days
Stock option share exercised (in shares)			20,000
Proceed from option share exercised			$ 800
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option share exercised (in shares)			100,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 24,440	23,183
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 0	$ 277
Common shares reserved for future issuance (in shares)					200,000
Shares of common stock issued under plan (in shares)			100,000	100,000

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 36,293	$ 32,341
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	24,440	23,183
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	11,853	8,881
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0	$ 277

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 36,293	$ 32,341
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	16,269	15,516
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 20,024	$ 16,825

Stock-based compensation - Su_2

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Shares (in thousands)
Outstanding at beginning of period (in shares) | shares	5,483
Granted (in shares) | shares	1,147
Exercised (in shares) | shares	(20)
Canceled, forfeited, or expired (in shares) | shares	(132)
Outstanding at end of period (in shares) | shares	6,478
Exercisable at end of period (in shares) | shares	3,248
Vested and expected to vest at end of period (in shares) | shares	6,478
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 116.30
Granted (in dollars per share) | $ / shares	75.65
Exercised (in dollars per share) | $ / shares	37.71
Canceled, forfeited, or expired (in dollars per share) | $ / shares	148.55
Outstanding at end of period (in dollars per share) | $ / shares	108.68
Exercisable at end of period (in dollars per share) | $ / shares	99.92
Vested and expected to vest at end of period (in dollars per share) | $ / shares	$ 108.68

Stock-based compensation - Su_3

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted stock units shares in Thousands	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Shares
Unvested balance at beginning of period (in shares) | shares	1,127
Granted (in shares) | shares	709
Vested (in shares) | shares	(204)
Forfeited (in shares) | shares	(47)
Unvested balance at end of period (in shares) | shares	1,585
Weighted-average grant date fair value
Unvested balance at beginning of period (in dollars per share) | $ / shares	$ 146.10
Granted (in dollars per share) | $ / shares	77.69
Vested (in dollars per share) | $ / shares	132.15
Forfeited (in dollars per share) | $ / shares	150.73
Unvested balance at end of period (in dollars per share) | $ / shares	$ 117.15

Uncategorized Items - blue-2020

Label	Element	Value
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 6,564,000
Accounting Standards Update 2016-02 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 6,564,000