bluebird bio (BLUE) 10-Q 5 Nov 21 2021 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
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		70,107,263
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001293971
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 402,461	$ 317,705
Marketable securities	375,140	833,546
Prepaid expenses	30,712	37,472
Receivables and other current assets	23,246	16,116
Inventory	766	10,698
Total current assets	832,325	1,215,537
Marketable securities	193,129	122,891
Property, plant and equipment, net	45,745	162,831
Intangible assets, net	11,009	10,041
Goodwill	12,056	13,128
Operating lease right-of-use assets	174,435	184,019
Restricted cash and other non-current assets	70,945	72,805
Total assets	1,339,644	1,781,252
Current liabilities:
Accounts payable	21,668	21,602
Accrued expenses and other current liabilities	203,790	145,406
Operating lease liability, current portion	29,441	25,024
Deferred revenue, current portion	2,530	2,320
Collaboration research advancement, current portion	9,130	9,236
Total current liabilities	266,559	203,588
Deferred revenue, net of current portion	25,761	25,762
Collaboration research advancement, net of current portion	16,767	21,581
Operating lease liability, net of current portion	152,126	167,997
Other non-current liabilities	7,904	7,268
Total liabilities	469,117	426,196
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at September 30, 2021 and December 31, 2020	0	0
Common stock, $0.01 par value, 125,000 shares authorized; 70,097 and 66,432 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively	701	665
Additional paid-in capital	4,440,605	4,260,443
Accumulated other comprehensive loss	(5,906)	(5,505)
Accumulated deficit	(3,564,873)	(2,900,547)
Total stockholders’ equity	870,527	1,355,056
Total liabilities and stockholders’ equity	$ 1,339,644	$ 1,781,252

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Stockholders’ equity:
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
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)	70,097,000	66,432,000
Common stock, shares, outstanding (in shares)	70,097,000	66,432,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue:
Collaborative arrangement revenue	$ 14,831	$ 2,422	$ 18,020	$ 114,398
Total revenues	22,677	19,273	42,943	240,026
Operating expenses:
Research and development	131,427	140,431	429,614	450,862
Selling, general and administrative	68,277	68,046	229,708	209,922
Share of collaboration loss	0	0	10,071	0
Cost of royalty and other revenue	19,704	1,318	37,286	3,897
Restructuring expense	20,175	0	24,800	0
Change in fair value of contingent consideration	48	(828)	464	(5,591)
Total operating expenses	239,631	208,967	731,943	659,090
Loss from operations	(216,954)	(189,694)	(689,000)	(419,064)
Interest income, net	319	1,964	1,468	10,258
Other income (expense), net	(294)	(6,686)	23,375	(9,582)
Loss before income taxes	(216,929)	(194,416)	(664,157)	(418,388)
Income tax benefit (expense)	113	(329)	(169)	(433)
Net loss	$ (216,816)	$ (194,745)	$ (664,326)	$ (418,821)
Net loss per share - basic: (in dollars per share)	$ (3.16)	$ (2.94)	$ (9.81)	$ (6.89)
Net loss per share - diluted: (in dollars per share)	$ (3.16)	$ (2.94)	$ (9.81)	$ (6.89)
Weighted-average number of common shares used in computing net loss per share - basic: (in shares)	68,621	66,251	67,701	60,762
Weighted-average number of common shares used in computing net loss per share - diluted: (in shares)	68,621	66,251	67,701	60,762
Other comprehensive loss:
Other comprehensive loss, net of tax benefit (expense) of $0.0 million and $0.1 million for the three months ended September 30, 2021 and 2020, respectively, and $0.0 million for the nine months ended September 30, 2021 and 2020.	$ (129)	$ (1,823)	$ (401)	$ (2,330)
Comprehensive loss	(216,945)	(196,568)	(664,727)	(421,151)
Service revenue
Revenue:
Revenue	6,312	13,352	17,544	108,542
Royalty and other revenue
Revenue:
Revenue	$ 1,534	$ 3,499	$ 7,379	$ 17,086

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Other comprehensive income (loss), tax	$ 0	$ 0.1	$ 0	$ 0

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, 2019		55,368
Beginning balance at Dec. 31, 2019	$ 1,284,993	$ 554	$ 3,568,184	$ (1,893)	$ (2,281,852)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		204
Vesting of restricted stock units	0	$ 2	(2)
Exercise of stock options, shares (in shares)		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 income (loss)	(906)			(906)
Net loss	(202,611)				(202,611)
Ending balance, shares (in shares) at Mar. 31, 2020		55,620
Ending balance at Mar. 31, 2020	1,120,433	$ 556	3,607,139	(2,799)	(2,484,463)
Beginning balance (in shares) at Dec. 31, 2019		55,368
Beginning balance at Dec. 31, 2019	1,284,993	$ 554	3,568,184	(1,893)	(2,281,852)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss)	(2,330)
Net loss	(418,821)
Ending balance, shares (in shares) at Sep. 30, 2020		66,339
Ending balance at Sep. 30, 2020	1,523,021	$ 663	4,227,254	(4,223)	(2,700,673)
Beginning balance (in shares) at Mar. 31, 2020		55,620
Beginning balance at Mar. 31, 2020	1,120,433	$ 556	3,607,139	(2,799)	(2,484,463)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)		10,455
Issuance of common stock	541,536	$ 105	541,431
Vesting of restricted stock units (in shares)		114
Vesting of restricted stock units	0	$ 1	(1)
Exercise of stock options, shares (in shares)		7
Exercise of stock options	347		347
Stock-based compensation	40,781		40,781
Other comprehensive income (loss)	399			399
Net loss	(21,465)				(21,465)
Ending balance, shares (in shares) at Jun. 30, 2020		66,196
Ending balance at Jun. 30, 2020	1,682,031	$ 662	4,189,697	(2,400)	(2,505,928)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		62
Vesting of restricted stock units	0	$ 1	(1)
Exercise of stock options, shares (in shares)		28
Exercise of stock options	249		249
Purchase of common stock under ESPP (in shares)		53
Purchase of common stock under ESPP	1,902		1,902
Stock-based compensation	35,407		35,407
Other comprehensive income (loss)	(1,823)			(1,823)
Net loss	(194,745)				(194,745)
Ending balance, shares (in shares) at Sep. 30, 2020		66,339
Ending balance at Sep. 30, 2020	$ 1,523,021	$ 663	4,227,254	(4,223)	(2,700,673)
Beginning balance (in shares) at Dec. 31, 2020	66,432	66,432
Beginning balance at Dec. 31, 2020	$ 1,355,056	$ 665	4,260,443	(5,505)	(2,900,547)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		294
Vesting of restricted stock units	0	$ 3	(3)
Exercise of stock options, shares (in shares)		207
Exercise of stock options	1,219	$ 2	1,217
Purchase of common stock under ESPP (in shares)		67
Purchase of common stock under ESPP	1,707	$ 1	1,706
Stock-based compensation	36,090		36,090
Issuance of stock to settle liability-classified restricted stock units (in shares)		422
Issuance of unrestricted stock awards to settle accrued employee compensation	12,013	$ 4	12,009
Other comprehensive income (loss)	56			56
Net loss	(205,808)				(205,808)
Ending balance, shares (in shares) at Mar. 31, 2021		67,422
Ending balance at Mar. 31, 2021	$ 1,200,333	$ 675	4,311,462	(5,449)	(3,106,355)
Beginning balance (in shares) at Dec. 31, 2020	66,432	66,432
Beginning balance at Dec. 31, 2020	$ 1,355,056	$ 665	4,260,443	(5,505)	(2,900,547)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares (in shares)	219
Other comprehensive income (loss)	$ (401)
Net loss	$ (664,326)
Ending balance, shares (in shares) at Sep. 30, 2021	70,097	70,097
Ending balance at Sep. 30, 2021	$ 870,527	$ 701	4,440,605	(5,906)	(3,564,873)
Beginning balance (in shares) at Mar. 31, 2021		67,422
Beginning balance at Mar. 31, 2021	1,200,333	$ 675	4,311,462	(5,449)	(3,106,355)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		127
Vesting of restricted stock units	0	$ 1	(1)
Exercise of stock options, shares (in shares)		2
Exercise of stock options	36		36
Stock-based compensation	26,222		26,222
Other comprehensive income (loss)	(328)			(328)
Net loss	(241,702)				(241,702)
Ending balance, shares (in shares) at Jun. 30, 2021		67,551
Ending balance at Jun. 30, 2021	984,561	$ 676	4,337,719	(5,777)	(3,348,057)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)		2,273
Issuance of common stock	37,499	$ 22	37,477
Vesting of restricted stock units (in shares)		80
Vesting of restricted stock units	0	$ 1	(1)
Exercise of stock options, shares (in shares)		10
Exercise of stock options	233		233
Purchase of common stock under ESPP (in shares)		53
Purchase of common stock under ESPP	875	$ 1	874
Issuance of pre-funded warrants	37,477		37,477
Stock-based compensation	24,352		24,352
Issuance of stock to settle liability-classified restricted stock units (in shares)		130
Issuance of unrestricted stock awards to settle accrued employee compensation	2,475	$ 1	2,474
Other comprehensive income (loss)	(129)			(129)
Net loss	$ (216,816)				(216,816)
Ending balance, shares (in shares) at Sep. 30, 2021	70,097	70,097
Ending balance at Sep. 30, 2021	$ 870,527	$ 701	$ 4,440,605	$ (5,906)	$ (3,564,873)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Jun. 30, 2020USD ($)
Statement of Stockholders' Equity [Abstract]
Costs from initial public offering	$ 33,465

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (664,326)	$ (418,821)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	464	(5,591)
Depreciation and amortization	17,335	14,378
Stock-based compensation expense	101,829	123,640
(Gain) loss on equity securities	(28,765)	9,068
Excess inventory reserve	29,712	0
Other non-cash items	13,358	(1,538)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	3,287	(2,534)
Inventory	(19,007)	(9,382)
Operating lease right-of-use assets	22,630	16,345
Accounts payable	2,004	(15,420)
Accrued expenses and other liabilities	54,774	(13,056)
Operating lease liabilities	(24,499)	(14,603)
Deferred revenue	210	8,558
Collaboration research advancement	(4,920)	(6,202)
Net cash used in operating activities	(495,914)	(315,158)
Cash flows from investing activities:
Purchase of property, plant and equipment	(12,944)	(21,098)
Purchases of marketable securities	(421,416)	(964,428)
Proceeds from maturities of marketable securities	802,367	722,487
Proceeds from sales of marketable securities	31,318	29,878
Proceeds from divestiture of business	110,300	0
Purchase of intangible assets	(8,000)	0
Net cash provided by (used in) investing activities	501,625	(233,161)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs	0	541,536
Proceeds From Issuance Of Common Stock And Warrants	74,982	0
Proceeds from exercise of stock options and ESPP contributions	5,078	3,747
Net cash provided by financing activities	80,060	545,283
Increase in cash, cash equivalents and restricted cash	85,771	(3,036)
Cash, cash equivalents and restricted cash at beginning of period	373,728	381,709
Cash, cash equivalents and restricted cash at end of period	459,499	378,673
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	402,461	324,164
Restricted cash included in receivables and other current assets	2,530	0
Restricted cash included in restricted cash and other non-current assets	54,508	54,509
Total cash, cash equivalents and restricted cash	459,499	378,673
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	732	1,686
Right-of-use assets obtained in exchange for operating lease liabilities	22,049	18,909
Reduction of right of use asset and associated lease liability due to lease reassessment	(9,004)	0
Issuance of unrestricted stock awards to settle accrued employee compensation	$ 14,488	$ 0

Description of the business

Description of the business	9 Months Ended
	Sep. 30, 2021
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, following marketing approval, potentially transformative gene therapies for severe genetic disease. 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 activities in Europe as well as commercial-readiness activities in the United States. In November 2021, the Company completed the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies, bluebird bio, Inc. and 2seventy bio, Inc., a Delaware corporation and wholly-owned subsidiary of the Company prior to the separation. bluebird bio, Inc. intends to retain its severe genetic disease programs, with a focus on the U.S. market. The Company’s programs in severe genetic diseases include programs for transfusion-dependent β-thalassemia, or TDT, sickle cell disease, or SCD, and cerebral adrenoleukodystrophy, or CALD. The Company also expects to make focused investments in research and development efforts on optimizing our existing programs as well as on pipeline programs in severe genetic diseases. 2seventy bio, Inc. is expected to focus on the Company's former oncology programs, including the anti-BCMA CAR T programs for multiple myeloma under the Company’s collaboration arrangement with Bristol-Myers Squibb ("BMS"). Please refer to Note 10, Collaborative arrangements and strategic partnerships, for further discussion of the Company’s collaboration with BMS. The results for the period ended September 30, 2021 reflect the combined results of the Company and 2seventy bio prior to the effectiveness of the separation, and the forward-looking statements contained within pertain to the Company's severe genetic disease operations, unless otherwise noted. In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern , the Company evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The Company has incurred losses since inception and to date has financed its operations primarily through the sale of equity securities and, to a lesser extent, through collaboration agreements and grants from governmental agencies and charitable foundations. As of September 30, 2021, the Company had an accumulated deficit of $3.56 billion. During the nine months ended September 30, 2021, the Company incurred a loss of $664.3 million and used $495.9 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 beti-cel, eli-cel, and LentiGlobin for SCD, and the achievement of a level of revenues adequate to support its cost structure. As of September 30, 2021, the Company had cash, cash equivalents and marketable securities of $970.7 million. Upon separation, the Company funded 2seventy bio with approximately $441.5 million, which has reduced the amount of cash available to the Company as of the date of separation. The Company expects its cash, cash equivalents and marketable securities, subsequent to the amount funded to 2seventy bio, will be sufficient to fund current planned operations for at least the next twelve months from the date of issuance of these financial statements. The Company anticipates reduced 2022 spending, including projected savings through the move of the Company's headquarters to Assembly Row in Somerville, Massachusetts, and the orderly wind down of European operations. This, together with other anticipated cash inflows, which include both the potential sale of priority review vouchers that would be issued with anticipated U.S. regulatory approvals of BLAs for beti-cel and eli-cel, and the pursuit of additional cash resources through public or private equity or debt financings, are expected to further strengthen the Company's financial condition. Management's expectations with respect to its ability to fund current planned operations are 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 cash resources through 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 cash resources 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 commercialize products following marketing approval.

Basis of presentation, principl

Basis of presentation, principles of consolidation and significant accounting policies	9 Months Ended
	Sep. 30, 2021
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 Accounting Standards Codification ("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 September 30, 2021 and 2020. 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, 2020, 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 23, 2021. Inventory in the prior year’s condensed consolidated financial statements has been reclassified to conform to the current presentation on the condensed consolidated balance sheets and condensed consolidated statements of cash flows. 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, including 2seventy bio, which on November 4, 2021 became an independent, publicly-traded company. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. Significant accounting policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and nine months ended September 30, 2021 are consistent with those discussed in Note 2 to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K, except as noted immediately below and as noted within the "Recent accounting pronouncements - Recently adopted" section. Collaborative arrangement revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements ("ASC 808"), which includes determining whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and therefore within the scope of ASC 606, Revenue from Contracts with Customers ("Topic 606" or "ASC 606"). For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the condensed consolidated statements of operations based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of losses in connection with commercialization of products Share of collaboration loss Net reimbursement of the Company's research and development expenses Collaborative arrangement revenue Net reimbursement of the collaborator's research and development expenses Research and development expense Where the collaborator is the principal in the product sales, the Company recognizes its share of any profits or losses, representing net product sales less cost of goods sold and shared commercial and other expenses, in the period in which such underlying sales occur and costs are incurred by the collaborator. The Company also recognizes its share of costs arising from research and development activities performed by collaborators in the period its collaborators incur such expenses. Royalty and other revenue During the nine months ended September 30, 2021, the Company recognized an immaterial amount of product revenue related to the sale of beti-cel (marketed as ZYNTEGLO) in the European Union and the related cost of goods sold, which is included within royalty and other revenue and cost of royalty and other revenue, respectively. Inventory Inventories are stated at the lower of cost or net realizable value under the first-expired, first-out (FEFO) methodology. Given human gene therapy products are a new and novel category of therapeutics and future economic benefit is not probable until regulatory approval for the product has been obtained, the Company has only considered inventory for capitalization upon regulatory approval. Manufacturing costs incurred prior to regulatory approval for pre-launch inventory that did not qualify for capitalization and clinical manufacturing costs are charged to research and development expense in the Company’s condensed consolidated statements of operations and comprehensive loss as costs are incurred. Additionally, inventory that initially qualifies for capitalization but that may ultimately be used for the production of clinical drug product is expensed as research and development expense when it has been designated for the manufacture of clinical drug product. Inventory consists of cell banks, plasmids, lentiviral vectors, other materials and compounds sourced from third party suppliers and utilized in the manufacturing process, and drug product, which has been produced for the treatment of specific patients, that are owned by the Company. Management periodically reviews inventories for excess or obsolescence, considering factors such as sales forecasts compared to quantities on hand and firm purchase commitments as well as remaining shelf life of on hand inventories. The Company writes-down its inventory that is obsolete or otherwise unmarketable to its estimated net realizable value in the period in which the impairment is first identified. Any such adjustments are included as a component of cost of goods sold within cost of royalty and other revenue on the Company’s condensed consolidated statements of operations. Common Stock Warrants The Company's common stock warrants are evaluated pursuant to ASC 480, Distinguishing Liabilities from Equity ("ASC 480"), and ASC 815, Derivatives and Hedging ("ASC 815"). Management classifies its freestanding warrants as (i) liabilities, if the warrant terms allow settlement of the warrant exercise in cash, or (ii) equity, if the warrant terms only allow settlement in shares of common stock. 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 recognition, income taxes, inventory capitalization, excess inventory analyses, 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. 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 was effective beginning January 1, 2021. The adoption of ASU 2019-12 did not have a material impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU 2020-06”). ASU 2020-06 simplifies the complexity associated with applying U.S. GAAP for certain financial instruments with characteristics of liabilities and equity. More specifically, the amendments focus on the guidance for convertible instruments and derivative scope exception for contracts in an entity's own equity. The Company early adopted the new standard, effective January 1, 2021. The adoption of ASU 2020-06 did not have an impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs In October 2020, the FASB issued ASU 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs (“ASU 2020-08”) to provide further clarification and update the previously issued guidance in ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20: Premium Amortization on Purchased Callable Debt Securities) (“ASU 2017-08”). ASU 2017-08 shortened the amortization period for certain callable debt securities purchased at a premium by requiring that the premium be amortized to the earliest call date. ASU 2020-08 requires that at each reporting period, to the extent that the amortized cost of an individual callable debt security exceeds the amount repayable by the issuer at the next call date, the excess premium shall be amortized to the next call date. The new standard was effective beginning January 1, 2021. The adoption of ASU 2020-08 did not have a material impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-10, Codification Improvements In October 2020, the FASB issued ASU 2020-10, Codification Improvements ("ASU 2020-10"). The amendments in this ASU represent changes to clarify the ASC, correct unintended application of the guidance, or make minor improvements to the ASC that are not expected to have a significant effect on current accounting practice or create a significant administrative cost to most entities. This new standard was effective beginning January 1, 2021. The adoption of ASU 2020-10 did not have a material impact on the Company's financial position or results of operations upon adoption.

Marketable securities

Marketable securities	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the marketable securities held at September 30, 2021 and December 31, 2020 (in thousands): Description Amortized cost / Cost Unrealized gains Unrealized losses Fair value September 30, 2021 U.S. government agency securities and treasuries $ 312,483 $ 67 $ (58) $ 312,492 Corporate bonds 111,616 7 (35) 111,588 Commercial paper 141,089 — — 141,089 Equity securities 4,305 — (1,205) 3,100 Total $ 569,493 $ 74 $ (1,298) $ 568,269 December 31, 2020 U.S. government agency securities and treasuries $ 675,043 $ 302 $ (74) $ 675,271 Corporate bonds 197,171 432 (40) 197,563 Commercial paper 77,949 1 — 77,950 Equity securities 20,017 — (14,364) 5,653 Total $ 970,180 $ 735 $ (14,478) $ 956,437 No available-for-sale debt securities held as of September 30, 2021 or December 31, 2020 had remaining maturities greater than five years.

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2021
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 September 30, 2021 and December 31, 2020 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) September 30, 2021 Assets: Cash and cash equivalents $ 402,461 $ 377,462 $ 24,999 $ — Marketable securities: U.S. government agency securities and treasuries 312,492 — 312,492 — Corporate bonds 111,588 — 111,588 — Commercial paper 141,089 — 141,089 — Equity securities 3,100 3,100 — — Total $ 970,730 $ 380,562 $ 590,168 $ — Liabilities: Contingent consideration $ 1,973 $ — $ — $ 1,973 Total $ 1,973 $ — $ — $ 1,973 December 31, 2020 Assets: Cash and cash equivalents $ 317,705 $ 317,705 $ — $ — Marketable securities: U.S. government agency securities and treasuries 675,271 — 675,271 — Corporate bonds 197,563 — 197,563 — Commercial paper 77,950 — 77,950 — Equity securities 5,653 5,653 — — Total $ 1,274,142 $ 323,358 $ 950,784 $ — Liabilities: Contingent consideration $ 1,509 $ — $ — $ 1,509 Total $ 1,509 $ — $ — $ 1,509 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 September 30, 2021, cash and cash equivalents comprise funds in cash, money market accounts, U.S. government agency securities and treasuries, and commercial paper. As of December 31, 2020, cash and cash equivalents comprise funds in cash and money market accounts. Marketable securities Marketable securities classified as Level 2 within the valuation hierarchy generally consist of U.S. government agency securities and treasuries, corporate bonds, and commercial paper. The Company estimates the fair values of these marketable securities by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for the same or similar securities, issuer credit spreads, benchmark yields, and other observable inputs. The Company validates the prices provided by its third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the next call date for premiums or to maturity for discounts. At September 30, 2021 and December 31, 2020, 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 debt securities during the three and nine months ended September 30, 2021 or 2020. Accrued interest receivable on the Company's available-for-sale debt securities totaled $1.2 million and $3.1 million as of September 30, 2021 and December 31, 2020, respectively. No accrued interest receivable was written off during the three and nine months ended September 30, 2021 or 2020. 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 September 30, 2021 and December 31, 2020 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses September 30, 2021 U.S. government agency securities $ 124,322 $ (58) $ — $ — $ 124,322 $ (58) Corporate bonds 74,279 (31) 11,377 (4) 85,656 (35) Total $ 198,601 $ (89) $ 11,377 $ (4) $ 209,978 $ (93) December 31, 2020 U.S. government agency securities $ 211,384 $ (74) $ — $ — $ 211,384 $ (74) Corporate bonds 76,598 (40) 1,205 — 77,803 (40) Total $ 287,982 $ (114) $ 1,205 $ — $ 289,187 $ (114) The Company determined that there was no material change in the credit risk of the above investments during the nine months ended September 30, 2021. As such, an allowance for credit losses was not recognized. As of September 30, 2021, 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 held equity securities with an aggregate fair value of $3.1 million and $5.7 million as of September 30, 2021 and December 31, 2020, respectively, within short-term marketable securities on its condensed consolidated balance sheets. In January 2021, the Company sold a portion of its equity securities for proceeds of $31.3 million. During the three months ended September 30, 2021 and 2020, the Company recorded gains of $0.5 million and losses of $5.8 million, respectively, related to its equity securities. During the nine months ended September 30, 2021 and 2020, the Company recorded gains of $28.8 million and losses of $9.1 million, respectively, related to its equity securities. Gains and losses related to equity securities are included in other income (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. Contingent consideration is included in accrued expenses and other current liabilities and other non-current liabilities on the condensed consolidated balance sheets. Upon the completion of the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies in November 2021, all future obligations related to the contingent consideration described above were assumed by 2seventy bio. Please refer to Note 9, Commitments and contingencies, for further information.

Inventory

Inventory	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Inventory	Inventory Inventory consists of the following (in thousands): As of September 30, 2021 As of December 31, 2020 Raw materials $ — $ 8,967 Finished goods 766 1,731 Inventory $ 766 $ 10,698 During the three and nine months ended September 30, 2021, the Company recorded a reserve for excess inventories of $14.6 million and $29.7 million, respectively, which is included within cost of royalty and other revenue within the condensed consolidated statements of operations.

Property, plant and equipment,

Property, plant and equipment, net	9 Months Ended
	Sep. 30, 2021
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 September 30, 2021 As of December 31, 2020 Land $ — $ 1,210 Building — 15,745 Computer equipment and software 5,699 6,950 Office equipment 6,686 7,665 Laboratory equipment 59,673 55,521 Leasehold improvements 31,579 34,286 Construction-in-progress 875 92,514 Total property, plant and equipment 104,512 213,891 Less accumulated depreciation and amortization (58,767) (51,060) Property, plant and equipment, net $ 45,745 $ 162,831 North Carolina manufacturing facility In November 2017, the Company acquired a manufacturing facility in Durham, North Carolina for the future manufacture of lentiviral vector for the Company’s gene therapies. In July 2021, the Company and National Resilience, Inc. ("Resilience") announced a strategic manufacturing collaboration aimed to accelerate the early research, development, and delivery of cell therapies. Agreements related to the collaboration were executed in September 2021. As part of the agreement, Resilience acquired the Company's manufacturing facility in Durham and retained all staff currently employed at the site. As a result of the transaction, the Company disposed of $111.2 million of net assets, primarily consisting of the building and laboratory equipment. Please refer to Note 10, Collaborative arrangements and strategic partnerships

Accrued expenses and other curr

Accrued expenses and other current liabilities	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Accrued expenses and other current liabilities	Accrued expenses and other current liabilities Accrued expenses and other current liabilities consist of the following (in thousands): As of September 30, 2021 As of December 31, 2020 Employee compensation $ 95,505 $ 55,802 Manufacturing costs 21,809 22,571 Clinical and contract research organization costs 19,017 23,766 Collaboration costs 28,031 20,004 Property, plant and equipment 618 789 License and milestone fees 467 278 Professional fees 2,549 1,541 Other 35,794 20,655 Accrued expenses and other current liabilities $ 203,790 $ 145,406 Accrued employee compensation includes severance costs associated with the Company's orderly wind down of its European operations. As of September 30, 2021, the Company had accrued expenses of $19.7 million related to these restructuring costs. Please refer to Note 16, Reduction in Workforce,

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	Leases The Company leases certain office and laboratory space, primarily located in Cambridge, Massachusetts and Seattle, Washington. Additionally, the Company has embedded leases at various contract manufacturing organizations in both the United States and internationally. Except as described below, there have been no material changes in lease obligations from those disclosed in Note 8 to the consolidated financial statements included in the Company's 2020 Annual Report on Form 10-K. 60 Binney Street lease In October 2021, the Company entered into a consent to assignment and amendment to its lease agreement for its 60 Binney Street Lease. The agreement reassigns the Company's interest in the lease to 2seventy bio, Inc. and releases the Company from its obligation to maintain the $13.8 million collateralized letter of credit required under the original lease. Following November 4, 2021, the date on which the separation of the Company and 2seventy bio was completed, the Company will reassess the accounting for this lease under ASC 842, Leases . Seattle, Washington leases In October 2021, the Company entered into a consent to assignment and amendment to its lease agreement for office and laboratory space in Seattle, Washington and the related sublease that was executed in September 2020 for a portion of the space. The agreement reassigns the Company's interest in the lease and the sublease to 2seventy bio, Inc. As part of the assignment, the sublease agreement associated with the expanded space was also assigned to 2seventy bio, Inc. Upon separation, the Company removed the related right-of-use asset and liability from its condensed consolidated balance sheets. Embedded operating leases In July 2020, the Company entered into an agreement reserving manufacturing capacity with a contract manufacturing organization. The Company concluded that this agreement contains an embedded operating lease as a controlled environment room at the facility is designated for the Company's exclusive use during the term of the agreement, with the option to sublease the space if the Company provides notice that it will not utilize it for a specified duration of time. Under the terms of the agreement, the Company will be required to pay up to $5.4 million per year in maintenance fees in addition to the cost of any services provided and may terminate this agreement with eighteen months' notice. The term of the agreement is five years, with the option to extend. The Company recorded a right-of-use asset and lease liability for this operating lease upon lease commencement in March 2021 and is recognizing rent expense on a straight-line basis throughout the remaining term of the embedded lease.

Commitments and contingencies

Commitments and contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies Contingent consideration related to business combinations In June 2014, the Company acquired Pregenen. The Company may be required to make up to $99.9 million in remaining future contingent cash payments to the former equity holders of Pregenen upon the achievement of certain commercial milestones related to the Pregenen technology. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the condensed 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. Upon the completion of the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies in November 2021, 2seventy bio assumed all future obligations related to the contingent consideration described above. Other funding commitments 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. Please refer to Note 10, Collaborative arrangements and strategic partnerships, for further information on the Company's collaboration agreements and to Note 11, Royalty and other revenue , for further information on the Company's license agreements. 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. There have been no material changes in future minimum purchase commitments from those disclosed in Note 9 to the consolidated financial statements included in the Company's 2020 Annual Report on Form 10-K. 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 has not recognized any liabilities relating to these obligations.

Collaborative arrangements and

Collaborative arrangements and strategic partnerships	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative arrangements and strategic partnerships	Collaborative arrangements and strategic partnerships Bristol-Myers Squibb In March 2013, the Company entered into a collaboration agreement with BMS. The details of the collaboration agreements and the payments the Company has received, and is entitled to receive, are further described in Note 11, Collaborative arrangements, to the Company’s consolidated financial statements included in its Annual Report on Form 10-K for the year ended December 31, 2020. During the third quarter of 2021, there have been no changes to the terms of the Company’s collaboration agreement with BMS. Upon the completion of the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies in November 2021, 2seventy bio assumed the collaboration agreement with BMS. Ide-cel Under the Company’s collaboration agreement with BMS, the Company shares equally in the profit and loss related to the development and commercialization of ide-cel in the United States. The Company has no remaining financial rights with respect to the development or commercialization of ide-cel outside of the United States. The Company accounts for its collaborative arrangement efforts with BMS in the United States 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 calculation of collaborative activity to be recognized for joint ide-cel efforts in the United States is performed on a quarterly basis and is independent of previous quarterly activity. This 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. The Company recognizes revenue related to the combined unit of accounting for the ex-U.S. license and lentiviral vector manufacturing services under Topic 606. Ide-cel U.S. Share of Collaboration Profit or Loss In March 2021, BMS received marketing approval from the U.S. Food and Drug Administration for ide-cel as a treatment for adult patients with relapsed or refractory multiple myeloma after four or more prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. BMS is primarily responsible for the commercialization of ide-cel and they are the principal for commercial activity. On a quarterly basis, the Company determines its share of collaboration profit or loss for commercial activities. The Company’s share of any collaboration profit for commercial activities is recognized as collaborative arrangement revenue and its share of any collaboration loss for commercial activity is recognized as an operating expense and classified as share of collaboration loss on the Company's condensed consolidated statement of operations. The Company also is responsible for equally sharing in the ongoing ide-cel research and development activities being conducted by BMS in the United States. The net amount owed to BMS for research and development activities is classified as research and development expense on the condensed consolidated statement of operations. If BMS is obligated to reimburse the Company because the Company’s research and development costs exceeds BMS’ research and development costs, the net amount is recorded as collaborative arrangement revenue. During the three and nine months ended September 30, 2021, the Company recognized $13.0 million, included as a component of collaborative arrangement revenue, on the condensed consolidated statement of operations and comprehensive loss, related to its share of collaboration profit associated with ide-cel commercial activities. During the three and nine months ended September 30, 2021, the Company recognized $0.0 million and $10.1 million, included as a component of share of collaborative arrangement loss, on the condensed consolidated statement of operations and comprehensive loss, related to its share of collaboration loss associated with ide-cel commercial activities. These amounts include the Company’s share of BMS’ ide-cel product revenue, cost of goods sold, and selling costs, offset by any reimbursement of commercial costs incurred by the Company during the three and nine month periods. The following table summarizes the amounts associated with the research activities under the collaboration included in research and development expense or recognized as collaborative arrangement revenue for the three and nine months ended September 30, 2021, and 2020 (in thousands): For the three months ended September 30, For the nine months ended September 30, 2021 2020 2021 2020 ASC 808 ide-cel research and development revenue - U.S. (1)(2) $ — $ — $ — $ 108,196 ASC 808 ide-cel research and development expense - U.S. (1) $ (5,660) $ (16,084) $ (31,678) $ (21,164) (1) As noted above, the calculation of collaborative arrangement activity to be recognized for joint ide-cel efforts in the United States 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. (2) In the second quarter of 2020, the Company recognized $169.2 million as a cumulative catch-up adjustment to revenue recorded in connection with the May 2020 First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Amended Ide-cel CCPS”), a portion of which was recognized as ASC 808 research and development collaboration revenue. Refer to Note 11, Collaborative arrangements, of the Company’s Annual Report on Form 10-K for further discussion on the Amended Ide-cel CCPS. Ide-cel ex-U.S. Service Revenue The following table summarizes the revenue recognized related to ide-cel ex-U.S. activities for the three and nine months ended September 30, 2021, and 2020 (in thousands): For the three months ended September 30, For the nine months ended September 30, 2021 2020 2021 2020 ASC 606 ide-cel license and manufacturing revenue - ex-U.S. (1) $ 5,314 $ 6,913 $ 14,698 $ 94,733 (1) In the second quarter of 2020, the Company recognized $169.2 million as a cumulative catch-up adjustment to revenue recorded in connection with the May 2020 First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Amended Ide-cel CCPS”), a portion of which was recognized as ASC 606 license and manufacturing revenue. Refer to Note 11, Collaborative arrangements, of the Company’s Annual Report on Form 10-K for further discussion on the Amended Ide-cel CCPS. bb21217 In addition to the activities related to ide-cel, BMS previously exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the collaboration arrangement with BMS which is further described in Note 11, Collaborative arrangements, to the Company’s consolidated financial statements included in its Annual Report on Form 10-K for the year ended December 31, 2020. Under the collaboration arrangement with BMS, the Company has an option to co-develop and co-promote bb21217 within the United States. The Company currently expects it will exercise its option to co-develop and co-promote bb21217 within the United States. The Company’s election to co-develop and co-promote bb21217 within the United States must be made by the substantial completion of CRB-402, 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 United States and to have the right to participate in the development and promotion of bb21217 within the United States. Under this scenario, the U.S. milestones and royalties payable would be adjusted and the Company would be eligible to receive a $10.0 million development milestone payment related to the development of bb21217 within the United States. The Company would not be eligible for royalties on U.S. sales of bb21217 under this scenario. In the event the Company does not exercise its option to co-develop and co-promote bb21217, the Company will receive an additional fee in the amount of $10.0 million. Under this scenario, the Company is eligible to receive U.S. milestones of up to $85.0 million for the first indication to be addressed by bb21217 and royalties for U.S. sales of bb21217. All of the remaining development, regulatory, and commercial milestones related to U.S. development, regulatory and commercialization activities are fully constrained and are therefore excluded from the transaction price. As part of its evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestones is outside the control of the Company and contingent upon the future success of its clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to U.S. sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to BMS and therefore are recognized at the later of when the performance obligation is satisfied or the related sales occur. The transaction price associated with the collaboration arrangement consists of $31.0 million of upfront payments and option payments received from BMS and $1.8 million in variable consideration which represents reimbursement to be received from BMS for manufacturing vector and associated payloads through development. The Company has identified two performance obligations with respect to the arrangement with BMS. The initial performance obligation was for research and development services substantially completed in September 2019, associated with the initial phase 1 clinical trial. The Company allocated $5.4 million of consideration to the research and development services performance obligation and fully recognized the consideration through September 2019. The other performance obligation relates to a combined performance obligation for the bb21217 license and vector manufacturing services through development, and the remaining $27.3 million in consideration was allocated to this combined performance obligation. The Company will satisfy this combined performance obligation as the bb21217 manufacturing services are performed . As of September 30, 2021, the Company has not commenced manufacturing and the full amount of the allocated transaction price remains unsatisfied. The Company re-evaluates the transaction price, including the 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. 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 nine months ended September 30, 2021 (in thousands): Balance at December 31, Additions Deductions Balance at Receivables $ 400 $ 12,661 $ (400) $ 12,661 Contract liabilities: Deferred revenue $ 26,582 $ — $ (820) $ 25,762 The increase in the receivables balance for the nine months ended September 30, 2021 is driven by amounts owed to the Company from BMS in the period under the settlement terms of the collaboration agreement. The decrease in deferred revenue during the nine months ended September 30, 2021 is driven by the release of the remaining $0.8 million of deferred revenue associated with the combined performance obligation consisting of the ide-cel license and manufacturing services. Regeneron Regeneron Collaboration Agreement In August 2018, the Company entered into a Collaboration Agreement (the “Regeneron Collaboration Agreement”) with Regeneron pursuant to which the parties will apply their respective technology platforms to the discovery, development, and commercialization of novel immune cell therapies for cancer. In August 2018, following the completion of required regulatory reviews, the Regeneron Collaboration Agreement became effective. Under the terms of the agreement, the parties will leverage Regeneron’s proprietary platform technologies for the discovery and characterization of fully human antibodies, as well as T cell receptors directed against tumor-specific proteins and peptides and the Company will contribute its field-leading expertise in gene therapy. Upon the completion of the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies in November 2021, the collaboration agreement with Regeneron was assumed by 2seventy bio. In accordance with the Regeneron Collaboration Agreement, the parties jointly selected six initial targets and intend to equally share the costs of research up to the point of submitting an IND application for a potential gene therapy product directed to a particular target. Additional targets may be selected to add to or replace any of the initial targets during the five-year research collaboration term as agreed to by the parties. Regeneron will accrue a certain number of option rights exercisable against targets as the parties reach certain milestones under the terms of the agreement. Upon the acceptance of an IND for the first product candidate directed to a target, Regeneron will have the right to exercise an option for co-development/co-commercialization of product candidates directed to such target on a worldwide or applicable opt-in territory basis, with certain exceptions. Where Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization and will share equally in any profits or losses therefrom in applicable opt-in territories. Outside of the applicable opt-in territories, the target becomes a licensed target and Regeneron would be eligible to receive, with respect to any resulting product, milestone payments of up to $130.0 million per product and royalties on net sales outside of the applicable opt-in territories at a rate ranging from the mid-single digits to low-double digits. A target would also become a licensed target in the event Regeneron does not have an option to such target, or Regeneron does not exercise its option with respect to such target. Either party may terminate a given research program directed to a particular target for convenience, and the other party may elect to continue such research program at its expense, receiving applicable cross-licenses. The terminating party will receive licensed product royalties and milestone payments on the potential applicable gene therapy products. Where the Company terminates a given research program for convenience, and Regeneron elects to continue such research program, the parties will enter into a transitional services agreement. Under certain conditions, following its opt-in, Regeneron may terminate a given collaboration program and the Company may elect to continue the development and commercialization of the applicable potential gene therapy products as licensed products. Regeneron Share Purchase Agreement A Share Purchase Agreement (“SPA”) was entered into by the parties in August 2018. In August 2018, the closing date of the transaction, the Company issued Regeneron 0.4 million shares of the Company’s common stock, subject to certain restrictions, for $238.10 per share, or $100.0 million in the aggregate. The purchase price represents $63.0 million worth of common stock plus a $37.0 million premium, which represents a collaboration research advancement, or credit to be applied to Regeneron’s initial 50 percent funding obligation for collaboration research, after which the collaborators will continue to fund ongoing research equally. The collaboration research advancement only applies to pre-IND research activities and is not refundable or creditable against post-IND research activities for any programs where Regeneron exercises their opt-in rights. Accounting analysis – Regeneron At the commencement of the arrangement, two units of accounting were identified, which are the issuance of 0.4 million shares of the Company’s common stock and joint research activities during the five-year research collaboration term. The Company determined the total transaction price to be $100.0 million, which comprises $54.5 million attributed to the equity sold to Regeneron and $45.5 million attributed to the joint research activities. In determining the fair value of the common stock at closing, the Company considered the closing price of the common stock on the closing date of the transaction and included a lack of marketability discount because Regeneron received shares subject to certain restrictions. The Company analyzed the joint research activities to assess whether they fall within the scope of ASC 808, and will reassess this throughout the life of the arrangement based on changes in the roles and responsibilities of the parties. Based on the terms of the arrangement as outlined above, for the collaboration research performed prior to submission of an IND application for a potential gene therapy product, both parties are deemed to be active participants in the collaboration. Both parties are performing research and development activities and will share equally in these costs through IND. Additionally, Regeneron and the Company are exposed to significant risks and rewards dependent on the commercial success of any product candidates that may result from the collaboration. As such, the collaboration arrangement is deemed to be within the scope of ASC 808. The $45.5 million attributed to the joint research activities includes the $37.0 million creditable against amounts owed to the Company by Regeneron. The collaboration research advancement will be reduced over time for amounts due to the Company by Regeneron as a result of the parties agreeing to share in the costs of collaboration research equally. The remainder of the amount attributed to the joint research activities will be recognized over the five-year research collaboration term. Consistent with its collaboration accounting policy, the Company will recognize collaborative arrangement revenue or research and development expense related to the joint research activities in future periods depending on the amounts incurred by each party in a given reporting period. That is, if the Company’s research costs incurred exceed those research costs incurred by Regeneron in a given quarter, the Company will record collaborative arrangement revenue and reduce the original $37.0 million advance by the amount due from Regeneron until such advancement is fully utilized, after which the Company would record an amount due from Regeneron. If Regeneron’s research costs incurred exceed those research costs incurred by the Company in a given quarter, the Company will record research and development expense and record a liability for the amount due to Regeneron. As of September 30, 2021 and December 31, 2020, the Company has $25.9 million and $30.8 million, respectively, of the amount attributed to the joint research activities remaining to be recognized, which is classified as collaboration research advancement, current portion and collaboration research advancement, net of current portion on the condensed consolidated balance sheets. The Company recognized $1.7 million and $4.9 million of collaborative arrangement revenue from the Regeneron Collaboration Agreement during the three and nine months ended September 30, 2021, respectively. The Company recognized $2.4 million and $6.2 million of collaborative arrangement revenue from the Regeneron Collaboration Agreement during the three and nine months ended September 30, 2020, respectively. Resilience Background In July 2021, the Company and Resilience US, Inc. (formerly known as Resilience Boston, Inc.), an affiliate of National Resilience, Inc. ("Resilience"), signed an Asset Purchase Agreement (the “Agreement”). As part of the Agreement, and upon the closing of the transaction which occurred in September 2021, Resilience acquired the Company's lentiviral vector manufacturing facility located in Durham, North Carolina and retained staff currently employed at the site. In exchange, the Company received $110.3 million for the facility and related fixed assets. Upon closing, the Company entered into certain ancillary agreements, including two manufacturing agreements and a license agreement (the “License Agreement”), among others (together referred to as the “Ancillary Agreements”). One manufacturing agreement will support the future manufacturing of lentiviral vector for the Company’s commercial product in collaboration with BMS, ide-cel (the “Commercial Supply Agreement”), while the other will support ongoing manufacturing for lentiviral vector for the Company's development candidates (the “Development Manufacturing Agreement”). The Company also agreed to reimburse Resilience for an amount equal to 50% of the net operating losses of and relating to the manufacturing facility’s business incurred during the twelve-month period ending on the first anniversary of the closing of the transaction, as calculated in accordance with the Agreement, subject to a cap of $15.0 million. In exchange, under the terms of the Development Manufacturing Agreement, the Company will receive up to eight batches of lentiviral vector during the twelve-month period ending on the first anniversary of the closing of the transaction. The License Agreement grants Resilience a worldwide, co-exclusive license to intellectual property controlled by the Company to perform Resilience’s obligations and exercise Resilience’s rights under the supply agreements, and a worldwide, nonexclusive right to offer certain manufacturing services to third-party customers under certain of the Company's intellectual property. Under the terms of the License Agreement, the Company may receive a high single-digit to low double-digit percentage tiered royalty based on Resilience’s gross margins for transactions entered into with parties other than the Company and which the Company's proprietary intellectual property is utilized as part of such transaction. Under the Commercial Supply Agreement, the Company will pay fully burdened manufacturing cost plus a markup for production of vector. Under the Development Manufacturing Agreement, services, manufacture, and delivery of batches of lentiviral vector during the first twelve months from the execution of this agreement will be free of cost, as the costs of these services are represented by the net operating loss sharing arrangement outlined within the Agreement. As such, the Company has committed to a minimum purchase of at least the Company's 50% share of the net operating losses during the first twelve months from the execution of such agreement. After the first twelve months, the Company will pay Resilience the fully burdened manufacturing cost plus a markup for production of vector. Upon the completion of the separation of its severe genetic disease and oncology programs into two separate, independent publicly traded companies in November 2021, 2seventy bio was assigned the Agreement and the Ancillary Agreements described above. Accounting analysis - Resilience The Company determined that the sale of the manufacturing facility was a sale of a business, as defined by ASC 805, Business Combinations (“ASC 805”) . As such, the Company calculated the gain or loss associated with the sale of the business under ASC 810 Consolidations (“ASC 810”). As the sale meets the definition of a business, the Company calculated the gain or loss under ASC 810 as the consideration received less the carrying amount of the net assets and liabilities, including any allocated goodwill, acquired and assumed by Resilience as part of the sale. As part of the computation, the Company determined that approximately $1.1 million of the goodwill balance was attributable to the portion of the reporting unit related to the Durham, North Carolina facility. As such, this amount was disposed of and is reflected in the Company’s condensed, consolidated balance sheets as of September 30, 2021, as part of the sale of the facility. The Company measured the fair value of the consideration received as the $110.3 million payment received from Resilience, future royalties under the License Agreement and any off-market component of the Ancillary Agreements. This

Royalty and other revenue

Royalty and other revenue	9 Months Ended
	Sep. 30, 2021
License And Royalty Revenue [Abstract]
Royalty and other revenue	Royalty and other revenue The Company has out-licensed intellectual property to various third parties. Under the terms of these agreements, the Company may be entitled to royalties and milestone payments. In April 2017, the Company entered into a worldwide license agreement with Novartis, which is further described in Note 12, Royalty and other revenue , to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K. Beginning in the fourth quarter of 2017, the Company began recognizing royalty revenue from sales of tisagenlecleucel under the agreement. This license agreement was terminated effective March 2021, at which point in time Novartis was no longer required to pay the Company royalty or other payments on net sales of tisagenlecleucel or any future products. Royalty revenue recognized from sales of tisagenlecleucel is included within royalty and other revenue on the condensed consolidated statement of operations and comprehensive loss. In April 2017, the Company entered into a worldwide license agreement with GlaxoSmithKline Intellectual Property Development Limited ("GSK"), which was assigned by GSK to Orchard Therapeutics Limited ("Orchard"), effective April 2018. The terms of this license agreement are further described in Note 12, Royalty and other revenue , to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K. During the second quarter of 2021, the Company and Orchard amended this license agreement to remove the potential milestone payments related to marketing authorization of covered products. In addition, the Company and Orchard entered into a new license agreement, under which the Company licensed to Orchard certain lentiviral vector-based technologies. Financial terms of the agreement include a potential milestone payment upon the first commercial sale of a licensed product in a territory, as well as low single-digit royalties on net sales of covered products. In May 2020, the Company entered into a non-exclusive license agreement with Juno Therapeutics, Inc. (“Juno”), a wholly-owned subsidiary of BMS, related to lentiviral vector technology to develop and commercialize CD-19-directed CAR T cell therapies. Upon regulatory approval of lisocabtagene maraleucel during the first quarter of 2021, the Company received a $2.5 million milestone payment from Juno, which is included within royalty and other revenue. Royalty revenue recognized from sales of lisocabtagene maraleucel is also included within royalty and other revenue on the condensed consolidated statement of operations and comprehensive loss. The Company may also be obligated to pay third-party licensors as a result of revenue recognized under out-license agreements, which is included within cost of royalty and other revenue on the condensed consolidated statement of operations and comprehensive loss. During the nine months ended September 30, 2021, the Company recognized an immaterial amount of product revenue related to the sale of beti-cel in the European Union and the related cost of goods sold, which is included within royalty and other revenue and cost of royalty and other revenue, respectively.

Equity

Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Equity	EquityIn September 2021, the Company entered into an equity purchase agreement with certain investors, pursuant to which the Company agreed to sell and issue, in a private placement offering of securities, an aggregate of (i) 2.3 million shares of the Company’s common stock at a purchase price per share of $16.50 and (ii) pre-funded warrants to purchase up to 2.3 million shares of common stock (the “Pre-Funded Warrants”) at an effective price of $16.49 per share ($16.49 paid to the Company upon the closing of the offering and $0.01 to be paid upon exercise of such Pre-Funded Warrants). This resulted in aggregate gross proceeds to the Company of approximately $75.0 million, before deducting placement agent fees and other offering expenses payable by the Company. The Pre-Funded Warrants can be exercised at any time or times on or after September 7, 2021, until exercised in full. The warrants have been evaluated to determine the appropriate accounting and classification pursuant to ASC 480 and ASC 815. Based on the terms of the Pre-Funded Warrants, management concluded that they should be classified within stockholder's equity on its condensed consolidated balance sheets, with no subsequent remeasurement as long as the underlying warrant agreements are not modified or amended.

Stock-based compensation

Stock-based compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-based compensation	Stock-based compensation In January 2021 and 2020, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.7 million and 2.2 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of September 30, 2021, 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 $28.3 million and $101.9 million for the three and nine months ended September 30, 2021, respectively. The Company recognized stock-based compensation expense totaling $38.8 million and $123.6 million for the three and nine months ended September 30, 2020, 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 September 30, For the nine months ended September 30, 2021 2020 2021 2020 Stock options 13,688 22,723 51,671 73,236 Restricted stock units 10,335 11,935 33,601 37,931 Employee stock purchase plan and other 4,315 4,160 16,591 12,473 28,338 38,818 101,863 123,640 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 September 30, For the nine months ended September 30, 2021 2020 2021 2020 Research and development $ 13,688 $ 18,837 $ 49,324 $ 58,204 Selling, general and administrative 14,650 19,981 52,539 65,436 $ 28,338 $ 38,818 $ 101,863 $ 123,640 Stock-based compensation of $0.1 million and $0.8 million was capitalized into inventory for the three and nine months ended September 30, 2021, respectively. Stock-based compensation of $0.3 million and $0.4 million was capitalized into inventory for the three and nine months ended September 30, 2020, respectively. As of September 30, 2021, the Company had approximately $179.1 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted-average period of approximately 2.1 years. Unrestricted stock awards During the first quarter of 2021, the Company granted 0.4 million unrestricted stock awards to employees as part of its 2020 annual incentive program. In addition, the Company implemented a retention program designed to incentivize and retain employees through the separation of its severe genetic disease and oncology programs. Under the retention program, employees are entitled to a one-time bonus payment, consisting of both a cash payment and unrestricted stock awards, with the condition that the employee remains employed at the end of 2021. For the three and nine months ended September 30, 2021, respectively, the Company recognized $3.1 million and $27.1 million in expense related to this program, which includes $1.6 million and $13.6 million in stock compensation expense related to the anticipated grants of stock. During the third quarter of 2021, the Company granted 0.1 million unrestricted stock awards, related to the retention program, to those employees impacted by the orderly wind down of the Company's operations in Europe. 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, 2020 6,262 $ 105.02 Granted 1,188 $ 27.85 Exercised (219) $ 6.82 Canceled, forfeited, or expired (1,557) $ 104.89 Outstanding at September 30, 2021 5,674 $ 92.68 Exercisable at September 30, 2021 3,527 $ 111.09 Vested and expected to vest at September 30, 2021 5,373 $ 92.68 During the nine months ended September 30, 2021, 0.2 million stock options were exercised, resulting in total proceeds to the Company of $1.5 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, 2020 1,495 $ 102.34 Granted 3,268 $ 26.81 Vested (501) $ 114.58 Forfeited (724) $ 60.33 Unvested balance at September 30, 2021 3,538 $ 39.43 Employee stock purchase plan In June 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. In June 2021, the Company amended the 2013 ESPP to include an additional 1.4 million shares of the Company’s common stock available to participating employees. During the nine months ended September 30, 2021 and 2020, respectively, 0.1 million shares and less than 0.1 million shares of common stock were issued under the 2013 ESPP.

Income taxes

Income taxes	9 Months Ended
	Sep. 30, 2021
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 benefit and expense recognized during the three and nine months ended September 30, 2021 is due to income taxes associated with foreign earnings. In March 2020, the Coronavirus Aid, Relief and Economic Security Act (“CARES Act”) was enacted. This law temporarily suspends and adjusts certain law changes enacted in the Tax Cuts and Jobs Act in 2017. In December 2020, the Consolidated Appropriations Act was enacted. This law modified the employee retention credit under the CARES Act and created credit extenders for certain credits. In March 2021, the American Rescue Plan Act (“ARPA”) was enacted and contained extenders to the refundable employee retention credit and provided further limitations to executive compensation effective for tax years beginning after 2026. The Company has concluded that the provisions in the CARES Act, Consolidated Appropriations Act, and ARPA have an immaterial impact on the Company’s income tax expense due to its cumulative losses and full valuation allowance position.

Net loss per share

Net loss per share	9 Months Ended
	Sep. 30, 2021
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 and nine months ended September 30, 2021 2020 Outstanding stock options 5,674 6,363 Restricted stock units 3,538 1,519 ESPP shares and other 946 285 10,158 8,167

Reduction in Workforce

Reduction in Workforce	9 Months Ended
	Sep. 30, 2021
Restructuring and Related Activities [Abstract]
Reduction in Workforce	Reduction in Workforce In April 2021, the Company announced its decision to withdraw ZYNTEGLO from the German market because reimbursement negotiations in Germany did not result in a price for ZYNTEGLO that reflects the value of the one-time gene therapy with potential life-long benefit for people living with TDT. A total of approximately 50 employees were impacted by this reduction. During the three months ended June 30, 2021, the Company substantially completed the implementation of this reduction and, in accordance with ASC 420, Exit and Disposal Activities, and ASC 712, Nonretirement Postemployment Benefits , recorded approximately $4.6 million of costs including severance, the portion of the employees' 2021 retention bonuses to be paid in cash, and the pro rata portion of the employees' 2021 performance bonus. In July 2021, the Company made the decision to focus its efforts on the U.S. market for beti-cel, eli-cel, and LentiGlobin for SCD and is executing an orderly wind down of its European operations. A total of approximately 90 employees were impacted by the reduction in workforce associated with this decision. The Company recorded $20.2 million of expense, in accordance with the related accounting standards mentioned above, for the affected employees. This amount includes expense for severance, the pro rata portion of the employees' 2021 performance bonus, the portion of the European employees' 2021 retention bonuses to be paid in cash, and the portion of retention bonuses to be paid in unrestricted stock awards, which were granted on September 30, 2021. As described in Note 13, Stock-based compensation , the Company recorded $2.5 million of costs associated with the grant of unrestricted stock awards to affected employees as a one time payment. All costs associated with the April 2021 and July 2021 reductions are reflected within restructuring expenses in the Company's condensed consolidated statements of operations and comprehensive loss. The Company expects that substantially all accrued restructuring charges will be paid in cash by March 31, 2022. The following table summarizes the accrued liabilities activity recorded in connection with the reduction in workforce for the nine months ended September 30, 2021: Charges Amount paid Amounts accrued at September 30, 2021 April 2021 reduction $ 4,625 $ (4,602) $ 23 July 2021 reduction 20,175 (546) 19,629 Total $ 24,800 $ (5,148) $ 19,652 During the three and nine months ended September 30, 2021, the Company recorded approximately $20.2 million and approximately $24.8 million, respectively, in restructuring expenses. During the three months ended September 30, 2021, the Company recorded $2.5 million in research and development expenses and selling, general and administrative expenses related to the grant of unrestricted stock awards to affected employees.

Subsequent events

Subsequent events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent events	Subsequent events On November 4, 2021, the Company completed the separation of its oncology portfolio and programs from its severe genetic disease portfolio and programs into a separate publicly traded company, 2seventy bio. The separation was effected by means of a distribution of all of the outstanding shares of common stock of 2seventy bio on the basis of one share of 2seventy bio common stock for every three shares of bluebird bio common stock issued and outstanding on October 19, 2021, the record date for the distribution. The distribution was effected at 12:01 a.m. on November 4, 2021. On November 3, 2021, in connection with the separation, bluebird bio and 2seventy bio executed a separation agreement, a tax matters agreement, an employee matters agreement, an intellectual property license agreement, and transition services agreements, under which both companies will temporarily provide and receive certain services from each other. These agreements effectuated the separation and govern 2seventy bio's relationship with bluebird bio after the distribution. As a result of the distribution and the separation, 2seventy bio is an independent, publicly traded company, effective as of November 4, 2021. In November 2021, the Company entered into a lease agreement with Assembly Row 5B, LLC ("Landlord") for office space located at 455 Grand Union Boulevard in Somerville, Massachusetts to serve as the Company's future headquarters. Under the terms of the arrangement, the Company will lease approximately 61,180 square feet starting at an annual rate of $45 per square foot, subject to annual increases of 2.5%, plus operating expenses and taxes. In addition, the Company will be eligible for a tenant work allowance of $160 per rentable square foot of the premises. The lease will commence on the date on which the Landlord tenders possession of the premises to the Company with any tenant work required to be performed by the Landlord substantially completed, which is anticipated to occur in the first quarter of 2022.

Basis of presentation, princi_2

Basis of presentation, principles of consolidation and significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification ("ASC") and Accounting Standards 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 September 30, 2021 and 2020.
Consolidation	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including 2seventy bio, which on November 4, 2021 became an independent, publicly-traded company. 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.
Collaborative arrangement revenue	Collaborative arrangement revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements ("ASC 808"), which includes determining whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and therefore within the scope of ASC 606, Revenue from Contracts with Customers ("Topic 606" or "ASC 606"). For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the condensed consolidated statements of operations based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of losses in connection with commercialization of products Share of collaboration loss Net reimbursement of the Company's research and development expenses Collaborative arrangement revenue Net reimbursement of the collaborator's research and development expenses Research and development expense Where the collaborator is the principal in the product sales, the Company recognizes its share of any profits or losses, representing net product sales less cost of goods sold and shared commercial and other expenses, in the period in which such underlying sales occur and costs are incurred by the collaborator. The Company also recognizes its share of costs arising from research and development activities performed by collaborators in the period its collaborators incur such expenses.
Royalty and other revenue	Royalty and other revenue During the nine months ended September 30, 2021, the Company recognized an immaterial amount of product revenue related to the sale of beti-cel (marketed as ZYNTEGLO) in the European Union and the related cost of goods sold, which is included within royalty and other revenue and cost of royalty and other revenue, respectively.
Inventory	Inventory Inventories are stated at the lower of cost or net realizable value under the first-expired, first-out (FEFO) methodology. Given human gene therapy products are a new and novel category of therapeutics and future economic benefit is not probable until regulatory approval for the product has been obtained, the Company has only considered inventory for capitalization upon regulatory approval. Manufacturing costs incurred prior to regulatory approval for pre-launch inventory that did not qualify for capitalization and clinical manufacturing costs are charged to research and development expense in the Company’s condensed consolidated statements of operations and comprehensive loss as costs are incurred. Additionally, inventory that initially qualifies for capitalization but that may ultimately be used for the production of clinical drug product is expensed as research and development expense when it has been designated for the manufacture of clinical drug product. Inventory consists of cell banks, plasmids, lentiviral vectors, other materials and compounds sourced from third party suppliers and utilized in the manufacturing process, and drug product, which has been produced for the treatment of specific patients, that are owned by the Company.
Common Stock Warrants	Common Stock Warrants The Company's common stock warrants are evaluated pursuant to ASC 480, Distinguishing Liabilities from Equity ("ASC 480"), and ASC 815, Derivatives and Hedging
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
Recent accounting pronouncements	Recent accounting pronouncements Recently 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 was effective beginning January 1, 2021. The adoption of ASU 2019-12 did not have a material impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU 2020-06”). ASU 2020-06 simplifies the complexity associated with applying U.S. GAAP for certain financial instruments with characteristics of liabilities and equity. More specifically, the amendments focus on the guidance for convertible instruments and derivative scope exception for contracts in an entity's own equity. The Company early adopted the new standard, effective January 1, 2021. The adoption of ASU 2020-06 did not have an impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs In October 2020, the FASB issued ASU 2020-08, Codification Improvements to Subtopic 310-20, Receivables - Nonrefundable Fees and Other Costs (“ASU 2020-08”) to provide further clarification and update the previously issued guidance in ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20: Premium Amortization on Purchased Callable Debt Securities) (“ASU 2017-08”). ASU 2017-08 shortened the amortization period for certain callable debt securities purchased at a premium by requiring that the premium be amortized to the earliest call date. ASU 2020-08 requires that at each reporting period, to the extent that the amortized cost of an individual callable debt security exceeds the amount repayable by the issuer at the next call date, the excess premium shall be amortized to the next call date. The new standard was effective beginning January 1, 2021. The adoption of ASU 2020-08 did not have a material impact on the Company's financial position or results of operations upon adoption. ASU No. 2020-10, Codification Improvements In October 2020, the FASB issued ASU 2020-10, Codification Improvements ("ASU 2020-10"). The amendments in this ASU represent changes to clarify the ASC, correct unintended application of the guidance, or make minor improvements to the ASC that are not expected to have a significant effect on current accounting practice or create a significant administrative cost to most entities. This new standard was effective beginning January 1, 2021. The adoption of ASU 2020-10 did not have a material impact on the Company's financial position or results of operations upon adoption.

Basis of presentation, princi_3

Basis of presentation, principles of consolidation and significant accounting policies (Tables)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Collaborative Arrangements, Nature Of Payment And Presentation	In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the condensed consolidated statements of operations based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of losses in connection with commercialization of products Share of collaboration loss Net reimbursement of the Company's research and development expenses Collaborative arrangement revenue Net reimbursement of the collaborator's research and development expenses Research and development expense

Marketable securities (Tables)

Marketable securities (Tables)	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities Held	The following table summarizes the marketable securities held at September 30, 2021 and December 31, 2020 (in thousands): Description Amortized cost / Cost Unrealized gains Unrealized losses Fair value September 30, 2021 U.S. government agency securities and treasuries $ 312,483 $ 67 $ (58) $ 312,492 Corporate bonds 111,616 7 (35) 111,588 Commercial paper 141,089 — — 141,089 Equity securities 4,305 — (1,205) 3,100 Total $ 569,493 $ 74 $ (1,298) $ 568,269 December 31, 2020 U.S. government agency securities and treasuries $ 675,043 $ 302 $ (74) $ 675,271 Corporate bonds 197,171 432 (40) 197,563 Commercial paper 77,949 1 — 77,950 Equity securities 20,017 — (14,364) 5,653 Total $ 970,180 $ 735 $ (14,478) $ 956,437

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of September 30, 2021 and December 31, 2020 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) September 30, 2021 Assets: Cash and cash equivalents $ 402,461 $ 377,462 $ 24,999 $ — Marketable securities: U.S. government agency securities and treasuries 312,492 — 312,492 — Corporate bonds 111,588 — 111,588 — Commercial paper 141,089 — 141,089 — Equity securities 3,100 3,100 — — Total $ 970,730 $ 380,562 $ 590,168 $ — Liabilities: Contingent consideration $ 1,973 $ — $ — $ 1,973 Total $ 1,973 $ — $ — $ 1,973 December 31, 2020 Assets: Cash and cash equivalents $ 317,705 $ 317,705 $ — $ — Marketable securities: U.S. government agency securities and treasuries 675,271 — 675,271 — Corporate bonds 197,563 — 197,563 — Commercial paper 77,950 — 77,950 — Equity securities 5,653 5,653 — — Total $ 1,274,142 $ 323,358 $ 950,784 $ — Liabilities: Contingent consideration $ 1,509 $ — $ — $ 1,509 Total $ 1,509 $ — $ — $ 1,509
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 September 30, 2021 and December 31, 2020 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses September 30, 2021 U.S. government agency securities $ 124,322 $ (58) $ — $ — $ 124,322 $ (58) Corporate bonds 74,279 (31) 11,377 (4) 85,656 (35) Total $ 198,601 $ (89) $ 11,377 $ (4) $ 209,978 $ (93) December 31, 2020 U.S. government agency securities $ 211,384 $ (74) $ — $ — $ 211,384 $ (74) Corporate bonds 76,598 (40) 1,205 — 77,803 (40) Total $ 287,982 $ (114) $ 1,205 $ — $ 289,187 $ (114)

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2021
Inventory Disclosure [Abstract]
Schedule of Inventory, Current	Inventory consists of the following (in thousands): As of September 30, 2021 As of December 31, 2020 Raw materials $ — $ 8,967 Finished goods 766 1,731 Inventory $ 766 $ 10,698

Property, plant and equipment_2

Property, plant and equipment, net (Tables)	9 Months Ended
	Sep. 30, 2021
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 September 30, 2021 As of December 31, 2020 Land $ — $ 1,210 Building — 15,745 Computer equipment and software 5,699 6,950 Office equipment 6,686 7,665 Laboratory equipment 59,673 55,521 Leasehold improvements 31,579 34,286 Construction-in-progress 875 92,514 Total property, plant and equipment 104,512 213,891 Less accumulated depreciation and amortization (58,767) (51,060) Property, plant and equipment, net $ 45,745 $ 162,831

Accrued expenses and other cu_2

Accrued expenses and other current liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
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 September 30, 2021 As of December 31, 2020 Employee compensation $ 95,505 $ 55,802 Manufacturing costs 21,809 22,571 Clinical and contract research organization costs 19,017 23,766 Collaboration costs 28,031 20,004 Property, plant and equipment 618 789 License and milestone fees 467 278 Professional fees 2,549 1,541 Other 35,794 20,655 Accrued expenses and other current liabilities $ 203,790 $ 145,406

Collaborative arrangements an_2

Collaborative arrangements and strategic partnerships (Tables)	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Agreement Transaction Price	The following table summarizes the amounts associated with the research activities under the collaboration included in research and development expense or recognized as collaborative arrangement revenue for the three and nine months ended September 30, 2021, and 2020 (in thousands): For the three months ended September 30, For the nine months ended September 30, 2021 2020 2021 2020 ASC 808 ide-cel research and development revenue - U.S. (1)(2) $ — $ — $ — $ 108,196 ASC 808 ide-cel research and development expense - U.S. (1) $ (5,660) $ (16,084) $ (31,678) $ (21,164) (1) As noted above, the calculation of collaborative arrangement activity to be recognized for joint ide-cel efforts in the United States 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. (2) In the second quarter of 2020, the Company recognized $169.2 million as a cumulative catch-up adjustment to revenue recorded in connection with the May 2020 First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Amended Ide-cel CCPS”), a portion of which was recognized as ASC 808 research and development collaboration revenue. Refer to Note 11, Collaborative arrangements, of the Company’s Annual Report on Form 10-K for further discussion on the Amended Ide-cel CCPS. The following table summarizes the revenue recognized related to ide-cel ex-U.S. activities for the three and nine months ended September 30, 2021, and 2020 (in thousands): For the three months ended September 30, For the nine months ended September 30, 2021 2020 2021 2020 ASC 606 ide-cel license and manufacturing revenue - ex-U.S. (1) $ 5,314 $ 6,913 $ 14,698 $ 94,733 (1) In the second quarter of 2020, the Company recognized $169.2 million as a cumulative catch-up adjustment to revenue recorded in connection with the May 2020 First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Amended Ide-cel CCPS”), a portion of which was recognized as ASC 606 license and manufacturing revenue. Refer to Note 11, Collaborative arrangements, of the Company’s Annual Report on Form 10-K for further discussion on the Amended Ide-cel CCPS.
Summary of Contract Assets and Liabilities	The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the nine months ended September 30, 2021 (in thousands): Balance at December 31, Additions Deductions Balance at Receivables $ 400 $ 12,661 $ (400) $ 12,661 Contract liabilities: Deferred revenue $ 26,582 $ — $ (820) $ 25,762

Stock-based compensation (Table

Stock-based compensation (Tables)	9 Months Ended
	Sep. 30, 2021
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 September 30, For the nine months ended September 30, 2021 2020 2021 2020 Stock options 13,688 22,723 51,671 73,236 Restricted stock units 10,335 11,935 33,601 37,931 Employee stock purchase plan and other 4,315 4,160 16,591 12,473 28,338 38,818 101,863 123,640
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 September 30, For the nine months ended September 30, 2021 2020 2021 2020 Research and development $ 13,688 $ 18,837 $ 49,324 $ 58,204 Selling, general and administrative 14,650 19,981 52,539 65,436 $ 28,338 $ 38,818 $ 101,863 $ 123,640
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, 2020 6,262 $ 105.02 Granted 1,188 $ 27.85 Exercised (219) $ 6.82 Canceled, forfeited, or expired (1,557) $ 104.89 Outstanding at September 30, 2021 5,674 $ 92.68 Exercisable at September 30, 2021 3,527 $ 111.09 Vested and expected to vest at September 30, 2021 5,373 $ 92.68
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, 2020 1,495 $ 102.34 Granted 3,268 $ 26.81 Vested (501) $ 114.58 Forfeited (724) $ 60.33 Unvested balance at September 30, 2021 3,538 $ 39.43

Net loss per share (Tables)

Net loss per share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): For the three and nine months ended September 30, 2021 2020 Outstanding stock options 5,674 6,363 Restricted stock units 3,538 1,519 ESPP shares and other 946 285 10,158 8,167

Reduction in Workforce (Tables)

Reduction in Workforce (Tables)	9 Months Ended
	Sep. 30, 2021
Restructuring and Related Activities [Abstract]
Restructuring and Related Costs	The following table summarizes the accrued liabilities activity recorded in connection with the reduction in workforce for the nine months ended September 30, 2021: Charges Amount paid Amounts accrued at September 30, 2021 April 2021 reduction $ 4,625 $ (4,602) $ 23 July 2021 reduction 20,175 (546) 19,629 Total $ 24,800 $ (5,148) $ 19,652

Description of the business - A

Description of the business - Additional Information (Detail) $ in Thousands	Nov. 04, 2021USD ($)company	Sep. 30, 2021USD ($)	Sep. 30, 2021USD ($)	Jun. 30, 2021USD ($)	Mar. 31, 2021USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Subsequent Event [Line Items]
Accumulated deficit		$ (3,564,873)	$ (3,564,873)						$ (3,564,873)		$ (2,900,547)
Net loss			(216,816)	$ (241,702)	$ (205,808)	$ (194,745)	$ (21,465)	$ (202,611)	(664,326)	$ (418,821)
Net cash used in operating activities									(495,914)	(315,158)
Cash, cash equivalents and marketable securities		970,700	970,700						970,700
Stock-based compensation expense			$ 28,338			$ 38,818			$ 101,863	$ 123,640
Unrestricted Stock Awards | July 2021 reduction
Subsequent Event [Line Items]
Stock-based compensation expense		$ 2,500
Subsequent Event
Subsequent Event [Line Items]
Number of independent publicly traded companies | company	2
Payments for advance to affiliate	$ 441,500

Basis of presentation, princi_4

Basis of presentation, principles of consolidation and significant accounting policies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2021segment
Accounting Policies [Abstract]
Number of operating segments	1

Marketable securities - Summary

Marketable securities - Summary of Marketable Securities Held (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Amortized cost / Cost
Equity securities	$ 4,305,000	$ 20,017,000
Total	569,493,000	970,180,000
Unrealized gains
Equity securities	0	0
Total	74,000	735,000
Unrealized losses
Equity securities	(1,205,000)	(14,364,000)
Total	(1,298,000)	(14,478,000)
Fair value
Equity securities	3,100,000	5,653,000
Total	568,269,000	956,437,000
Debt securities, available-for-sale, noncurrent	0	0
U.S. government agency securities and treasuries
Amortized cost / Cost
Debt securities	312,483,000	675,043,000
Unrealized gains
Debt securities	67,000	302,000
Unrealized losses
Debt securities	(58,000)	(74,000)
Fair value
Debt securities	312,492,000	675,271,000
Corporate bonds
Amortized cost / Cost
Debt securities	111,616,000	197,171,000
Unrealized gains
Debt securities	7,000	432,000
Unrealized losses
Debt securities	(35,000)	(40,000)
Fair value
Debt securities	111,588,000	197,563,000
Commercial paper
Amortized cost / Cost
Debt securities	141,089,000	77,949,000
Unrealized gains
Debt securities	0	1,000
Unrealized losses
Debt securities	0	0
Fair value
Debt securities	$ 141,089,000	$ 77,950,000

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Marketable securities:	$ 568,269	$ 956,437
Equity securities	3,100	5,653
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	402,461	317,705
Equity securities	3,100	5,653
Total	970,730	1,274,142
Liabilities:
Contingent consideration	1,973	1,509
Total	1,973	1,509
Fair Value, Measurements, Recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:	312,492	675,271
Fair Value, Measurements, Recurring | Corporate bonds
Assets:
Marketable securities:	111,588	197,563
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Marketable securities:	141,089	77,950
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	377,462	317,705
Equity securities	3,100	5,653
Total	380,562	323,358
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) | 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	24,999	0
Equity securities	0	0
Total	590,168	950,784
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:	312,492	675,271
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities:	111,588	197,563
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities:	141,089	77,950
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	1,973	1,509
Total	1,973	1,509
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) | 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 - Narra

Fair value measurements - Narrative (Detail)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2021USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Nov. 04, 2021company	Dec. 31, 2020USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) on available-for-sale securities		$ 0	$ 0	$ 0	$ 0
Interest receivable		1,200,000		1,200,000			$ 3,100,000
Interest receivable, write-offs		0	0	0	0
Investment at fair value		3,100,000		3,100,000			$ 5,653,000
Proceeds from sale of equity securities	$ 31,300,000
Subsequent Event
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of independent publicly traded companies | company						2
Other (Expense) Income, Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on equity securities		$ (500,000)	$ 5,800,000	$ (28,800,000)	$ 9,100,000

Fair value measurements - Sched

Fair value measurements - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair value
Less than 12 months	$ 198,601	$ 287,982
12 months or greater	11,377	1,205
Total	209,978	289,187
Unrealized losses
Less than 12 months	(89)	(114)
12 months or greater	(4)	0
Total	(93)	(114)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	124,322	211,384
12 months or greater	0	0
Total	124,322	211,384
Unrealized losses
Less than 12 months	(58)	(74)
12 months or greater	0	0
Total	(58)	(74)
Corporate bonds
Fair value
Less than 12 months	74,279	76,598
12 months or greater	11,377	1,205
Total	85,656	77,803
Unrealized losses
Less than 12 months	(31)	(40)
12 months or greater	(4)	0
Total	$ (35)	$ (40)

Inventory (Details)

Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
Raw materials	$ 0	$ 8,967
Finished goods	766	1,731
Inventory	$ 766	$ 10,698

Inventory - Narrative (Details)

Inventory - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Inventory Disclosure [Abstract]
Excess inventory reserve	$ 14,600	$ 29,712	$ 0

Property, plant and equipment_3

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 104,512	$ 213,891
Less accumulated depreciation and amortization	(58,767)	(51,060)
Property, plant and equipment, net	45,745	162,831
Land
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	0	1,210
Building
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	0	15,745
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	5,699	6,950
Office equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	6,686	7,665
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	59,673	55,521
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	31,579	34,286
Construction-in-progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 875	$ 92,514

Property, plant and equipment_4

Property, plant and equipment, net - Narrative (Detail) $ in Millions	Sep. 30, 2021USD ($)
Lentiviral Vector Manufacturing Facility
Property, Plant and Equipment [Line Items]
Net assets disposed of	$ 111.2

Accrued expenses and other cu_3

Accrued expenses and other current liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Employee compensation	$ 95,505	$ 55,802
Manufacturing costs	21,809	22,571
Clinical and contract research organization costs	19,017	23,766
Collaboration costs	28,031	20,004
Property, plant and equipment	618	789
License and milestone fees	467	278
Professional fees	2,549	1,541
Other	35,794	20,655
Accrued expenses and other current liabilities	$ 203,790	$ 145,406

Accrued expenses and other cu_4

Accrued expenses and other current liabilities - Narrative (Details) $ in Thousands	Sep. 30, 2021USD ($)
Statement of Financial Position [Abstract]
Accrued restructuring charges	$ 19,652

Leases - 60 Binney Street Narra

Leases - 60 Binney Street Narrative (Details) $ in Millions	1 Months Ended
	Oct. 31, 2021USD ($)
60 Binney Street Lease | Subsequent Event
Lessee, Lease, Description [Line Items]
Collateralized letter of credit	$ 13.8

Leases - Embedded Operating Lea

Leases - Embedded Operating Leases Narrative (Details) - Manufacturing Facility $ in Millions	1 Months Ended
	Jul. 31, 2020USD ($)
Lessee, Lease, Description [Line Items]
Annual maintenance fees	$ 5.4
Lease notice period	18 months
Lease period	5 years

Commitments and contingencies -

Commitments and contingencies - Narrative (Detail) $ in Millions	Nov. 04, 2021company	Jun. 30, 2014USD ($)
Subsequent Event
Commitments And Contingencies Disclosure [Line Items]
Number of independent publicly traded companies | company	2
Pregenen
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments | $		$ 99.9

Collaborative arrangements an_3

Collaborative arrangements and strategic partnerships - Narrative (Detail) $ / shares in Units, $ in Thousands, shares in Millions	Aug. 24, 2018USD ($)$ / sharesshares	Jul. 31, 2021USD ($)agreementbatch	Aug. 31, 2018target	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Nov. 04, 2021company	Dec. 31, 2020USD ($)	Sep. 30, 2019USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue				$ 14,831	$ 2,422	$ 18,020	$ 114,398
Share of collaboration loss				0	0	(10,071)	0
Deferred revenue						210	8,558
Investment in common stock						0	541,536
Proceeds from divestiture of business						110,300	0
Goodwill				12,056		12,056			$ 13,128
Lentiviral Vector Manufacturing Facility
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of manufacturing agreements | agreement		2
Proceeds from divestiture of business		$ 110,300
Net operating losses, percentage of reimbursement		50.00%
Net operating losses, reimbursement cap		$ 15,000
Goodwill		$ 1,100
Gain (loss) on disposal				2,000		2,000
Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of independent publicly traded companies | company								2
Maximum | Lentiviral Vector Manufacturing Facility
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of lentiviral vector batches | batch		8
Bristol-Myers Squibb
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price				31,000		31,000
Estimated variable consideration				1,800		1,800
Deferred revenue						800
Bristol-Myers Squibb | Phase I, Additional Obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration										$ 27,300
Bristol-Myers Squibb | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated variable consideration										$ 5,400
Bristol-Myers Squibb | Ide-cel License Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue				13,000		13,000
Share of collaboration loss				0		10,100
Bristol-Myers Squibb | bb21217 License Agreement | U.S.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional fee receivable if option to co-develop and co-promote is not exercised				10,000		10,000
Milestone payments receivable				85,000		85,000
Bristol-Myers Squibb | bb21217 Research And Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment achieved						10,000
Regeneron Collaboration Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of initial collaboration targets | target			6
Research collaboration term	5 years		5 years
Joint research activities remaining to be recognized				25,900		25,900			$ 30,800
Regeneron Collaboration Agreement | Collaboration
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Liability, revenue recognized				$ 1,700	$ 2,400	$ 4,900	$ 6,200
Regeneron Collaboration Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments receivable	$ 130,000
Regeneron Collaboration Agreement | Research and Development Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, transaction price	100,000
Purchase price premium	$ 37,000
Collaborative arrangement amortization period	5 years
Collaborative arrangement amount attributed to equity sold	$ 54,500
Collaborative arrangement amount attributed to joint research activities	$ 45,500
Regeneron Collaboration Agreement | Share Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock shares issued (in shares) | shares	0.4
Common stock price per share (in dollars per share) | $ / shares	$ 238.10
Investment in common stock	$ 100,000
Purchase price premium	$ 37,000
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	0.4
Investment in common stock	$ 63,000

Collaborative arrangements an_4

Collaborative arrangements and strategic partnerships - 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue	$ 14,831	$ 2,422	$ 18,020	$ 114,398
Research and development expense	(131,427)	(140,431)	(429,614)	(450,862)
Revenue catch-up adjustment	22,677	19,273	42,943	240,026
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	(5,660)	(16,084)	(31,678)	(21,164)
Bristol-Myers Squibb | U.S. | Ide Cel Revenue Services | License and Manufacturing Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement revenue	0	0	0	108,196
Revenue catch-up adjustment		169,200
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	$ 5,314	6,913	$ 14,698	$ 94,733
Revenue catch-up adjustment		$ 169,200

Collaborative arrangements an_5

Collaborative arrangements and strategic partnerships - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Bristol-Myers Squibb $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Receivables
Balance at beginning of period	$ 400
Additions	12,661
Deductions	(400)
Balance at end of period	12,661
Contract liabilities:
Balance at beginning of period	26,582
Additions	0
Deductions	(820)
Balance at end of period	$ 25,762

Royalty and other revenue - Add

Royalty and other revenue - Additional Information (Detail) - Royalty and other revenue - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
License And Royalty Revenue [Line Items]
Revenue	$ 1,534		$ 3,499	$ 7,379	$ 17,086
Juno Therapeutics
License And Royalty Revenue [Line Items]
Revenue		$ 2,500

Equity - Narrative (Detail)

Equity - Narrative (Detail) - Private Placement $ / shares in Units, shares in Thousands, $ in Millions	1 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares
Subsidiary, Sale of Stock [Line Items]
Shares sold | shares	2,300
Common stock price per share (in dollars per share)	$ 16.50
Proceeds from stock offering | $	$ 75
Pre-funded Warrants
Subsidiary, Sale of Stock [Line Items]
Number of securities called by warrants | shares	2,300
Warrant price per share (in dollars per share)	$ 16.49
Exercise price of warrants (in dollars per share)	$ 0.01

Stock-based compensation - Narr

Stock-based compensation - Narrative (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan (in shares)	2,700,000	2,200,000
Number of shares available for issuance (in shares)			2,500,000		2,500,000
Stock-based compensation expense			$ 28,338	$ 38,818	$ 101,863	$ 123,640
Amount capitalized			100		800
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan			179,100		$ 179,100
Expected weighted-average period related to unvested stock options, restricted stock units, performance-based restricted stock units and employee stock purchase plan					2 years 1 month 6 days
Granted (in shares)					1,188,000
Stock option share exercised (in shares)					219,000
Proceed from option share exercised					$ 1,500
Retention Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee benefits and share-based compensation expense			3,100		27,100
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount capitalized				300		400
ESPP shares and other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			4,315	4,160	$ 16,591	$ 12,473
Common shares reserved for future issuance (in shares)								200,000
Increase in shares authorized (in shares)							1,400,000
Shares of common stock issued under plan (in shares)					100,000
ESPP shares and other | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock issued under plan (in shares)						100,000
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			10,335	$ 11,935	$ 33,601	$ 37,931
Unrestricted Stock Awards | 2020 Annual Incentive Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	400,000
Unrestricted Stock Awards | Retention Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 1,600		$ 13,600
Granted (in shares)			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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 28,338	$ 38,818	$ 101,863	$ 123,640
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	13,688	22,723	51,671	73,236
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	10,335	11,935	33,601	37,931
ESPP shares and other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 4,315	$ 4,160	$ 16,591	$ 12,473

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 28,338	$ 38,818	$ 101,863	$ 123,640
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	13,688	18,837	49,324	58,204
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 14,650	$ 19,981	$ 52,539	$ 65,436

Stock-based compensation - Su_2

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) - $ / shares shares in Thousands	9 Months Ended
	Sep. 30, 2021
Shares (in thousands)
Outstanding at beginning of period (in shares)	6,262
Granted (in shares)	1,188
Exercised (in shares)	(219)
Canceled, forfeited, or expired (in shares)	(1,557)
Outstanding at end of period (in shares)	5,674
Exercisable at end of period (in shares)	3,527
Vested and expected to vest at end of period (in shares)	5,373
Weighted- average exercise price per share
Outstanding at beginning of period (in dollars per share)	$ 105.02
Granted (in dollars per share)	27.85
Exercised (in dollars per share)	6.82
Canceled, forfeited, or expired (in dollars per share)	104.89
Outstanding at end of period (in dollars per share)	92.68
Exercisable at end of period (in dollars per share)	111.09
Vested and expected to vest at end of period (in dollars per share)	$ 92.68

Stock-based compensation - Su_3

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted stock units shares in Thousands	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Shares (in thousands)
Unvested balance at beginning of period (in shares) | shares	1,495
Granted (in shares) | shares	3,268
Vested (in shares) | shares	(501)
Forfeited (in shares) | shares	(724)
Unvested balance at end of period (in shares) | shares	3,538
Weighted- average grant date fair value
Unvested balance at beginning of period (in dollars per share) | $ / shares	$ 102.34
Granted (in dollars per share) | $ / shares	26.81
Vested (in dollars per share) | $ / shares	114.58
Forfeited (in dollars per share) | $ / shares	60.33
Unvested balance at end of period (in dollars per share) | $ / shares	$ 39.43

Net loss per share - Common Sto

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
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)	10,158	8,167	10,158	8,167
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	5,674	6,363	5,674	6,363
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)	3,538	1,519	3,538	1,519
ESPP shares and other
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	946	285	946	285

Reduction in Workforce - Narrat

Reduction in Workforce - Narrative (Details) $ in Thousands	Sep. 30, 2021USD ($)	Jul. 31, 2021employee	Apr. 30, 2021USD ($)employee	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring expense				$ 20,175	$ 0	$ 24,800	$ 0
Stock-based compensation expense				28,338	$ 38,818	101,863	$ 123,640
April 2021 reduction
Restructuring Cost and Reserve [Line Items]
Number of employees impacted | employee			50
Restructuring expense			$ 4,600			4,625
July 2021 reduction
Restructuring Cost and Reserve [Line Items]
Number of employees impacted | employee		90
Restructuring expense				$ 20,200		$ 20,175
July 2021 reduction | Unrestricted Stock Awards
Restructuring Cost and Reserve [Line Items]
Stock-based compensation expense	$ 2,500

Reduction in Workforce - Summar

Reduction in Workforce - Summary of Total Restructuring Expenses (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Cost and Reserve [Line Items]
Charges		$ 20,175	$ 0	$ 24,800	$ 0
Amount paid				(5,148)
Amounts accrued at September 30, 2021		19,652		19,652
April 2021 reduction
Restructuring Cost and Reserve [Line Items]
Charges	$ 4,600			4,625
Amount paid				(4,602)
Amounts accrued at September 30, 2021		23		23
July 2021 reduction
Restructuring Cost and Reserve [Line Items]
Charges		20,200		20,175
Amount paid				(546)
Amounts accrued at September 30, 2021		$ 19,629		$ 19,629

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event	Nov. 04, 2021	Nov. 05, 2021ft²$ / ft²
Subsequent Event [Line Items]
Share distribution ratio	0.33
Grand Union Boulevard Lease
Subsequent Event [Line Items]
Leased building space (in sq ft) | ft²		61,180
Annual lease rent per square foot (in dollars per sq ft)		45
Lease arrangements annual increase percentage		2.50%
Tenant work allowance per square foot		160