bluebird bio (BLUE) 10-Q 2 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 27, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	1,293,971
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		54,157,916

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 333,949	$ 758,505
Marketable securities	743,010	531,604
Tenant improvement receivable	19	3,112
Prepaid expenses	24,053	21,171
Receivables and other current assets	4,713	8,377
Total current assets	1,105,744	1,322,769
Marketable securities	380,284	324,193
Property, plant and equipment, net	219,226	199,606
Intangible assets, net	15,050	16,931
Goodwill	13,128	13,128
Restricted cash and other non-current assets	28,079	23,940
Total assets	1,761,511	1,900,567
Current Liabilities:
Accounts payable	17,936	12,873
Accrued expenses and other current liabilities	71,003	57,065
Deferred revenue, current portion	27,323	25,674
Total current liabilities	116,262	95,612
Deferred revenue, net of current portion	27,900	21,763
Contingent consideration	3,027	2,231
Financing lease obligation, net of current portion	154,103	154,749
Other non-current liabilities	2,685	2,780
Total liabilities	303,977	277,135
Commitments and contingencies (Note 7)
Stockholders’ Equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at June 30, 2018 and December 31, 2017
Common stock, $0.01 par value, 125,000 shares authorized; 50,225 and 49,406 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively	502	494
Additional paid-in capital	2,666,729	2,540,951
Accumulated other comprehensive loss	(5,394)	(4,205)
Accumulated deficit	(1,204,303)	(913,808)
Total stockholders’ equity	1,457,534	1,623,432
Total liabilities and stockholders’ equity	$ 1,761,511	$ 1,900,567

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	50,225,000	49,406,000
Common stock, shares outstanding	50,225,000	49,406,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue:
Total revenues	$ 7,851	$ 16,716	$ 23,808	$ 23,548
Operating expenses:
Research and development	115,014	63,891	212,123	118,919
General and administrative	41,168	21,197	76,094	41,481
Cost of license and royalty revenue	$ 21	$ 420	$ 36	$ 420
Type of cost, good or service [Extensible List]	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember
Change in fair value of contingent consideration	$ 262	$ (970)	$ 796	$ 463
Total operating expenses	156,465	84,538	289,049	161,283
Loss from operations	(148,614)	(67,822)	(265,241)	(137,735)
Interest income (expense), net	2,436	(2,242)	3,824	(687)
Other income (expense), net	182	(834)	297	(1,189)
Loss before income taxes	(145,996)	(70,898)	(261,120)	(139,611)
Net loss	$ (145,996)	$ (70,898)	$ (261,120)	$ (139,611)
Net loss per share - basic and diluted:	$ (2.91)	$ (1.73)	$ (5.22)	$ (3.41)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	50,153	41,035	50,038	40,936
Other comprehensive loss:
Other comprehensive loss, net of tax expense of $0.0 and $0.0 million for the three and six months ended June 30, 2018 and 2017, respectively	$ (345)	$ (165)	$ (1,189)	$ (266)
Total other comprehensive loss	(345)	(165)	(1,189)	(266)
Comprehensive loss	(146,341)	(71,063)	(262,309)	(139,877)
Collaboration [Member]
Revenue:
Total revenues	7,437	6,146	23,045	12,978
License and Royalty [Member]
Revenue:
Total revenues	$ 414	$ 10,570	$ 763	$ 10,570

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Other comprehensive (loss) tax expense	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (261,120)	$ (139,611)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	796	463
Depreciation and amortization	8,199	6,281
Stock-based compensation expense	51,051	24,971
Other non-cash items	2,834	2,334
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,279)	(9,594)
Accounts payable	4,097	2,003
Accrued expenses and other liabilities	9,862	(8,509)
Deferred revenue	(21,589)	(7,682)
Deferred rent	(73)	507
Net cash used in operating activities	(209,222)	(128,837)
Cash flows from investing activities:
Purchase of property, plant and equipment, including assets under financing lease obligation	(20,689)	(39,509)
Purchases of marketable securities	(689,163)	(116,740)
Proceeds from maturities of marketable securities	417,640	210,810
Net cash (used in) provided by investing activities	(292,212)	54,561
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs	48,702	436,805
Reimbursement of assets under financing lease obligation	3,098	36,399
Payments on financing lease obligation	(446)	(120)
Proceeds from issuance of common stock	25,624	8,639
Net cash provided by financing activities	76,978	481,723
(Decrease) increase in cash, cash equivalents and restricted cash	(424,456)	407,447
Cash, cash equivalents and restricted cash at beginning of period	772,268	293,277
Cash, cash equivalents and restricted cash at end of period	347,812	700,724
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	7,815	1,194
Assets acquired under financing lease obligation		2,297
Tenant improvements under financing lease included in tenant improvements receivable	$ 14	$ 3,635

Description of the business

Description of the business	6 Months Ended
	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the business	1. Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company researches, develops, manufactures and plans to commercialize gene therapies for the treatment of severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide general and administrative support for these operations. The Company’s clinical programs in severe genetic diseases include its LentiGlobin ® TM Collaboration revenue As of June 30, 2018, the Company had cash, cash equivalents and marketable securities of $1.46 billion. Although the Company has incurred recurring losses and expects to continue to incur losses for the foreseeable future, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund current operations for at least the next twelve months.

Basis of presentation, principl

Basis of presentation, principles of consolidation and significant accounting policies	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of presentation, principles of consolidation and significant accounting policies	2. Basis of presentation, principles of consolidation and significant accounting policies Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 21, 2018. Certain items in the prior year’s condensed consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year condensed consolidated financial statements were impacted. Amounts reported are computed based on thousands. As a result, certain totals may not sum due to rounding. Principles of consolidation The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. As of June 30, 2018, there have been no changes to the Company’s subsidiary listing, which is included as an exhibit to its Annual Report on Form 10-K for the year ended December 31, 2017. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States. Significant accounting policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2018 are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s 2017 Annual Report on Form 10-K, except as noted below with respect to the Company’s revenue recognition, collaboration revenue and license and royalty revenue accounting policies and as noted within the “Recent accounting pronouncements – Recently adopted” Revenue recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right is accounted for as a contract modification for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method. Collaboration revenue To date, the Company’s collaboration revenue has been exclusively generated from its collaboration arrangement with Celgene, which was originally entered into in March 2013 and was subsequently amended in June 2015, as further described in Note 8, “ Collaboration revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements License and royalty revenue The Company enters into out-licensing agreements that are within the scope of Topic 606. The Company does not have any material license arrangements that contain more than one performance obligation. The terms of such out-license agreements include the license of functional intellectual property, given the functionality of the intellectual property is not expected to change substantially as a result of the licensor’s ongoing activities, and typically include payment of one or more of the following: non-refundable up-front license fees; development and regulatory milestone payments and milestone payments based on the level of sales; and royalties on net sales of licensed products. Nonrefundable up-front license fees are recognized as revenue at a point in time when the licensed intellectual property is made available for the customer’s use and benefit, which is generally at the inception of the arrangement. Milestone fees, which are a type of variable consideration, are recognized as revenue to the extent that it is probable that a significant reversal will not occur. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. For a complete discussion of accounting for collaboration and other revenue-generating arrangements, see Note 8, “Collaboration revenue” “License and royalty revenue” Recent accounting pronouncements - Recently adopted Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially Recent accounting pronouncements Recently adopted ASU No. 2014 -09, In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue Recognition Revenue from Contracts with Customers • For contracts that were modified prior to Topic 606 adoption, the Company has not retrospectively accounted for each contract modification in accordance with the contract modification guidance. Instead, the Company reflected the aggregate effect of all modifications occurring prior to Topic 606 adoption when identifying the sa tisfied and unsatisfied performance obligations, determining the transaction price and allocating the transaction price. As a result of adopting Topic 606, the Company recorded a $29.4 million adjustment to the opening balance of accumulated deficit in the first quarter of 2018 primarily as a result of the accounting for the up-front consideration received in March 2013 in connection with the collaboration arrangement with Celgene under ASC 605-25 versus Topic 606. Refer below for a summary of t he amount by which each financial statement line item was affected by Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of January 1, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 45,344 $ 19,670 $ 25,674 Deferred revenue, net of current portion $ 31,468 $ 9,705 $ 21,763 Accumulated deficit $ (943,183 ) $ (29,375 ) $ (913,808 ) The amount by which each financial statement line item is affected in the current reporting period by Topic 606 as compared with the guidance that was in effect prior to adoption is disclosed below. Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 27,323 $ 12,010 $ 15,313 Deferred revenue, net of current portion $ 27,900 $ 8,545 $ 19,355 Accumulated deficit $ (1,204,303 ) $ (20,555 ) $ (1,183,748 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Three Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 7,437 $ 887 $ 6,550 Research and development expense $ 115,014 $ (1,800 ) $ 116,814 Net loss $ (145,996 ) $ 2,687 $ (148,683 ) Net loss per share - basic and diluted: $ (2.91 ) $ 0.05 $ (2.96 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Six Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 23,045 $ 7,020 $ 16,025 Research and development expense $ 212,123 $ (1,800 ) $ 213,923 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Net loss per share - basic and diluted: $ (5.22 ) $ 0.17 $ (5.39 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Cash Flows for the Six Months Ended June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Changes in deferred revenue $ (21,589 ) $ (8,820 ) $ (12,769 ) The most significant change above relates to the Company’s collaboration revenue, which to date has been exclusively generated from its collaboration arrangement with Celgene. Under ASC 605, the Company accounted for contract modifications to the Celgene collaboration as they occurred and the accounting for those changes was prospective in nature. Through the application of the practical expedient discussed above in connection with the adoption of Topic 606, the Company reflected the aggregate effect of all modifications to the Celgene collaboration when identifying the satisfied and unsatisfied performance obligations, determining the transaction price, and allocating the transaction price. As a result, although the performance obligations identified under Topic 606 were generally consistent with the units of account identified under ASC 605, the timing of the allocation of the transaction price to the identified performance obligations under Topic 606 differed from the allocations of consideration under ASC 605. Accordingly, the transaction price ultimately allocated to each performance obligation under Topic 606 differed from the amounts allocated under ASC 605. As a result of adopting Topic 606, the Company established a deferred revenue deferred tax asset, and an offsetting valuation allowance, of $7.9 million through its accumulated deficit given it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there was no tax impact on the Company’s condensed consolidated financial statements as a result of adopting Topic 606. ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”) ASU 2016-18, Statement of Cash Flows: Restricted Cash In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”) (in thousands) As of June 30, 2018 As of June 30, 2017 Cash and cash equivalents $ 333,949 $ 686,334 Restricted cash included in receivables and other current assets 100 627 Restricted cash included in restricted cash and other non-current assets 13,763 13,763 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 347,812 $ 700,724 ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting The new standard was effective beginning January 1, 2018. The adoption of this standard did not have a material impact on the Company’s Not yet adopted ASU 2016-02, Leases In February 2016, the FASB issued ASU 2016-02, Leases, (“ASU 2016-02”) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019. The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position and results of operations. The Company’s assessment will include, but is not limited to, evaluating the impact that this standard has on the lease of its corporate headquarters at 60 Binney in Cambridge, Massachusetts, its laboratory space in Seattle, Washington, its office space in Zug, Switzerland, and any of its embedded leases. ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (“Subtopic 310-20”) Subtopic 310-20 calls for a modified retrospective application under which . The new standard will be effective beginning January 1, 2019 and early adoption is permitted for public entities. financial position or results of operations upon adoption. ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (“ASU 2018-02”) The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s financial position and results of operations ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”) simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The Company is currently evaluating the potential impact ASU 2018-07 may have on its results of operations

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Marketable securities	3. Marketable securities The following table summarizes the available-for-sale securities held at June 30, 2018 and December 31, 2017 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value June 30, 2018 U.S. government agency securities and treasuries $ 1,116,012 $ 36 $ (4,714 ) $ 1,111,334 Certificates of deposit 11,960 — — 11,960 Total $ 1,127,972 $ 36 $ (4,714 ) $ 1,123,294 December 31, 2017 U.S. government agency securities and treasuries $ 841,895 $ — $ (3,579 ) $ 838,316 Certificates of deposit 17,480 1 — 17,481 Total $ 859,375 $ 1 $ (3,579 ) $ 855,797 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At June 30, 2018 and December 31, 2017, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale marketable securities. There were no material realized gains or losses recognized The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2018 and December 31, 2017 was $807.2 million and $704.1 million, respectively. As of June 30, 2018 and December 31, 2017, there were $101.2 million and $134.4 million in securities held by the Company in an unrealized loss position for more than twelve months, respectively. No available-for-sale securities held as of June 30, 2018 or December 31, 2017 had remaining maturities greater than three years.

Fair value measurements

Fair value measurements	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair value measurements	4. Fair value measurements The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2018 and December 31, 2017 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) June 30, 2018 Assets: Cash and cash equivalents $ 333,949 $ 333,949 $ — $ — Marketable securities: U.S. government agency securities and treasuries 1,111,334 — 1,111,334 — Certificates of deposit 11,960 — 11,960 — Total assets $ 1,457,243 $ 333,949 $ 1,123,294 $ — Liabilities: Contingent consideration $ 3,027 $ — $ — $ 3,027 Total liabilities $ 3,027 $ — $ — $ 3,027 December 31, 2017 Assets: Cash and cash equivalents $ 758,505 $ 758,505 $ — $ — Marketable securities: U.S. government agency securities and treasuries 838,316 — 838,316 — Certificates of deposit 17,481 — 17,481 — Total assets $ 1,614,302 $ 758,505 $ 855,797 $ — Liabilities: Contingent consideration $ 2,231 $ — $ — $ 2,231 Total liabilities $ 2,231 $ — $ — $ 2,231 Cash, cash equivalents and marketable securities The Company considers all highly liquid securities with original final maturities of 90 days or less from the date of purchase to be cash equivalents. As of June 30, 2018 and December 31, 2017, cash and cash equivalents comprises funds in cash and money market accounts. Marketable securities classified as Level 2 within the valuation hierarchy generally consist of U.S. treasury securities and government agency securities. 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. Contingent consideration In connection with its prior acquisition of Pregenen, the Company may be required to pay future consideration that is contingent upon the achievement of specified development, regulatory approvals or sales-based milestone events. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the condensed consolidated statements of operations and comprehensive loss. In the absence of new information, changes in fair value will reflect changing discount rates and the passage of time. The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands): For the six months ended June 30, 2018 Beginning balance $ 2,231 Additions — Changes in fair value 796 Payments — Ending balance $ 3,027 Please refer to Note 7, “Commitments and contingencies”

Property, plant and equipment,

Property, plant and equipment, net	6 Months Ended
	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Property, plant and equipment, net	5. Property, plant and equipment, net Property, plant and equipment, net, consists of the following (in thousands): As of As of June 30, 2018 December 31, 2017 Land $ 1,210 $ 1,210 Building 167,155 164,414 Computer equipment and software 5,551 5,134 Office equipment 5,036 4,478 Laboratory equipment 30,293 24,914 Leasehold improvements 272 116 Construction-in-progress 31,876 15,189 Total property, plant and equipment 241,393 215,455 Less accumulated depreciation and amortization (22,167 ) (15,849 ) Property, plant and equipment, net $ 219,226 $ 199,606 North Carolina manufacturing facility In November 2017, the Company acquired a manufacturing facility, which is in the process of construction, in Durham, North Carolina for the future manufacture of lentiviral vector for the Company’s gene and cell therapies. Construction-in-progress as of June 30, 2018 and December 31, 2017 includes $28.1 million and $12.9 million, respectively, related to the North Carolina manufacturing facility. 60 Binney Street lease As of June 30, 2018, total property, plant and equipment, gross, includes $167.2 million related to the Company’s headquarters at 60 Binney Street in Cambridge, Massachusetts, of which $156.0 million was incurred by the landlord. As of December 31, 2017, total property, plant and equipment, gross, includes $164.4 million related to the Company’s headquarters at 60 Binney Street in Cambridge, Massachusetts, of which $156.0 million was incurred by the landlord. Please refer to Note 7, "Commitments and contingencies"

Accrued expenses and other curr

Accrued expenses and other current liabilities	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Accrued expenses and other current liabilities	6. Accrued expenses and other current liabilities Accrued expenses and other current liabilities consist of the following (in thousands): As of As of June 30, 2018 December 31, 2017 Accrued goods and services $ 52,683 $ 29,533 Employee compensation 13,383 19,657 Accrued professional fees 1,748 1,402 Financing lease obligation, current portion 1,214 1,051 Accrued license and milestone fees 914 4,584 Other 1,061 838 Total accrued expenses and other current liabilities $ 71,003 $ 57,065

Commitments and contingencies

Commitments and contingencies	6 Months Ended
	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	7. Commitments and contingencies Operating lease commitments On June 3, 2016, the Company entered into a strategic manufacturing agreement for the future commercial production of the Company’s Lenti-D and LentiGlobin product candidates with a contract manufacturing organization. Under this 12 year agreement, the contract manufacturing organization will complete the design, construction, validation and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company was required to pay $12.5 million upon the achievement of certain contractual milestones, and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. The Company paid $5.0 million for the achievement of the first and second contractual milestones during 2016 and paid $5.5 million for the third and fourth contractual milestones achieved during 2017. In March 2018, $1.5 million of the possible $2.0 million related to the fifth contractual milestone was achieved and was paid in the second quarter of 2018. Given that construction was completed in March 2018, beginning in April 2018 the Company will pay $5.1 million per year in fixed suite fees as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services, and may pay additional suite fees if it elects its option to reserve or lease additional suites. The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company concluded that this agreement contains an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company concluded that it is not the deemed owner during construction nor is it a capital lease under ASC 840-10, Leases – Overall On November 18, 2016, the Company entered into an agreement for future clinical and commercial production of the Company’s LentiGlobin and Lenti-D gene therapy drug products with a contract manufacturing organization at an existing facility. The term of the agreement is five years with a three year renewal at the mutual option of each party. Under the agreement, the Company is required to pay an up-front fee of €3.0 million, €2.0 million of which was paid in the fourth quarter of 2016 and €1.0 million of which is expected to be paid in the third quarter of 2018, and annual maintenance and production fees of up to €9.8 million, depending on its production needs. The Company may terminate this agreement with twelve months’ notice and a one-time termination fee. The Company concluded that this agreement contains an embedded lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement, and determined that it is not a capital lease under ASC 840-10, Leases – Overall non-cancellable 60 Binney Street Lease commitments On September 21, 2015, the Company entered into a lease agreement for office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”) to become its new corporate headquarters with a term through March 31, 2027. Although the Company does not legally own the premises, it is deemed to be the owner of the building for accounting purposes because the Company was involved in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building during the construction period. Accordingly, construction costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance have been recorded as an asset in “Property, plant and equipment, net” with a related financing obligation in “Accrued expenses and other current liabilities” and “ ” on the Company’s condensed consolidated balance sheets The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the building over a useful life of 40 years and incurred interest expense related to the financing obligation of $3.9 million and $7.8 million for the three and six months ended June 30, 2018, respectively. The Company incurred interest expense related to the financing obligation of $3.7 million for the three and six months ended June 30, 2017. The Company currently maintains a $13.8 million letter of credit as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease to $9.2 million over time. Contingent consideration related to business combinations On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to $120.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain clinical and commercial milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value. Please refer to Note 4, “Fair value measurements” Other funding commitments The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2018 and December 31, 2017 or royalties on future sales of specified products. 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 due in the event of termination to be based on the timing of the termination and the terms of the agreement. In each of 2018 and 2019, the Company expects to make payments of approximately $12.0 million under an agreement with a contract manufacturer.

Collaboration revenue

Collaboration revenue	6 Months Ended
	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration revenue	8. Collaboration revenue To date, the Company’s collaboration revenue has been exclusively generated from its collaboration arrangement with Celgene, which was originally entered into in March 2013 and was subsequently amended in June 2015, as further described below. Original Collaboration Agreement On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Collaboration Agreement”) with Celgene to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine, for use in the collaboration. Under the terms of the Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. The Company is responsible for conducting discovery, research and development activities through completion of Phase I clinical studies during the initial term of the Collaboration Agreement, or three years. The collaboration is governed by a joint steering committee (“JSC”) formed by an equal number of representatives from the Company and Celgene. The JSC, among other activities, reviews the collaboration program, reviews and evaluates product candidates and approves regulatory plans. In addition to the JSC, the Collaboration Agreement provides that the Company and Celgene each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration. Amended Collaboration Agreement On June 3, 2015, the Company and Celgene amended and restated the Collaboration Agreement (the “Amended Collaboration Agreement”). Under the Amended Collaboration Agreement, the parties narrowed the focus of the collaboration exclusively to anti- B-cell maturation antigen (“BCMA”) product candidates for a new three-year term that ended in June 2018. In connection with the Amended Collaboration Agreement, the Company received an upfront, one-time, non-refundable, non-creditable payment of $25.0 million. The collaboration will continue to be governed by the JSC. Under the terms of the Amended Collaboration Agreement, for up to two product candidates selected for development under the collaboration, the Company is responsible for conducting and funding all research and development activities performed up through completion of the initial Phase I clinical study of such product candidate. On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial Phase I clinical study for such product candidate (the “Option Period”), the Company granted Celgene an option to obtain an exclusive worldwide license to develop and commercialize such product. Following Celgene’s license of each product candidate, the Company is entitled to elect to co-develop and co-promote each product candidate in the U.S. bb2121 License Agreement On February 10, 2016, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb2121, the first product candidate under the Amended Collaboration Agreement, pursuant to an executed license agreement (“bb2121 License Agreement”) entered into by the parties on February 16, 2016 and paid the Company an option fee of $10.0 million. Pursuant to the bb2121 License Agreement, Celgene is responsible for development and related funding of bb2121 after the substantial completion of the on-going Phase I clinical trial. The Company is responsible for the manufacture of vector and associated payload throughout development and, upon Celgene’s request, commercialization, which is fully reimbursed by Celgene, and Celgene is responsible for the manufacture of drug product throughout development and commercialization. bb2121 Co-Development, Co-Promote and Profit Share Agreement On March 28, 2018, the Company elected to co-develop and co-promote bb2121 within the U.S. pursuant to the execution of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“bb2121 CCPS”). The responsibilities of the parties remain unchanged from those under the bb2121 License Agreement, however, the Company will share equally in all profits and losses relating to developing, commercializing and manufacturing bb2121 within the U.S. and has the right to participate in the development and promotion of bb2121 in the U.S. Under the bb2121 CCPS, the Company may receive up to $70.0 million in development milestone payments for the first indication to be addressed by the bb2121 product candidate, with the ability to obtain additional milestone payments for a second indication and modified licensed products. In addition, to the extent bb2121 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions. bb21217 License Agreement On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second product candidate under the Amended Collaboration Agreement, pursuant to an executed license agreement (“bb21217 License Agreement”) entered into by the parties on September 28, 2017 and paid the Company an option fee of $15.0 million. Pursuant to the bb21217 License Agreement, Celgene is responsible for development and related funding of bb21217 after the substantial completion of the on-going Phase I clinical trial. The Company is responsible for the manufacture of vector and associated payload throughout development and, upon Celgene’s request, commercialization, which is fully reimbursed by Celgene, and Celgene is responsible for the manufacture of drug product throughout development and commercialization. The Company currently expects it will exercise its option to co-develop and co-promote bb21217 within the U.S. The Company’s election to co-develop and co-promote bb21217 must be made by the substantial completion of the ongoing Phase I trial of bb21217. If elected, the Company expects the responsibilities of the parties to remain largely unchanged, however, the Company expects it will share equally in all profits and losses relating to developing, commercializing and manufacturing bb21217 within the U.S. and to have the right to participate in the development and promotion of bb21217 in the U.S. In the event the Company does not exercise its option to co-develop and co-promote bb21217, the Company will receive an additional fee in the amount of $10.0 million. Under this scenario, the Company may be eligible to receive up to $10.0 million in clinical milestone payments, up to $117.0 million in regulatory milestone payments, and up to $78.0 million in commercial milestone payments. In addition, to the extent bb21217 is commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales, subject to certain reductions. Accounting analysis – bb2121 The Company has elected to use a practical expedient within Topic 606 that allow entities to reflect the aggregate effect of all contract modifications when identifying the satisfied and unsatisfied performance obligations for contracts that were modified prior to Topic 606 adoption. Celgene’s option to in-license the first product candidate, bb2121, under the arrangement was considered a material right at the time the Amended Collaboration Agreement was executed in June 2015 given the product candidate had been formally nominated by the JSC and that substantially all investigational new drug application, or IND, enabling activities had been completed by that time. Therefore, Celgene’s February 2016 exercise of its option that was considered a material right to obtain an exclusive worldwide license to develop and commercialize the first product candidate, bb2121, under the collaboration represented a contract modification. As a result, the Collaboration Agreement, Amended Collaboration Agreement, and bb2121 License Agreement are combined for accounting purposes and treated as a single arrangement. As of February 2016, Celgene’s option to license an additional product candidate under the collaboration did not represent a material right. Therefore, the license to the Company’s second product candidate, bb21217, which was executed in September 2017, is accounted for as a separate contract. Refer below for discussion of the bb21217 accounting analysis. As of the February 2016 contract modification date, the Company concluded the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to the first product candidate, bb2121, and (iii) manufacture of vectors and associated payload for incorporation into bb2121 through development. The Company determined that the manufacture of commercial vector represents an option to acquire additional goods and services that is not representative of a material right. In addition, at this time Celgene has not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector is not considered to be a performance obligation at this time. The Company concluded that the research and development services are distinct from the other promised goods and services under the arrangement and thus such services are considered to be a separate performance obligation. The Company concluded that the license to bb2121 is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation. As of June 30, 2018, the total transaction price of $182.5 million comprises the up-front non-refundable fees of $100.0 million, the option fee of $10.0 million, and the estimated variable consideration of $72.5 million related to the estimated reimbursement from Celgene for the manufacture of vectors and associated payload through development. The total transaction price has been allocated to the performance obligations identified based on a relative SSP basis. The Company estimated the SSP of the license after considering potential future cash flows under the license. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the research and development services and manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services. None of the clinical or regulatory milestones have been included in the transaction price, as all milestone amounts are fully constrained. 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, and the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Celgene and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company will re-evaluate the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, in each reporting period and as uncertain events are resolved or other changes in circumstances occur. bb2121 research and development services The Company allocated $36.0 million of the transaction price to the research and development services. The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was three years through projected initial Phase I clinical study substantial completion, or through May 2018. The Company recognized revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. Although the Company has fully satisfied this performance obligation during the second quarter of 2018, any future changes to the total transaction price allocated to the performance obligations under the arrangement may impact the revenue recognized for this performance obligation in the period of change. The Company recognized revenue related to bb2121 research and development services of $1.0 million and $3.1 million for the three and six months ended June 30, 2018, respectively. The Company recognized revenue related to bb2121 research and development services of $1.6 million and $3.1 million for the three and six months ended June 30, 2017, respectively. bb2121 license and manufacturing services The Company allocated $146.4 million of the transaction price to the combined unit of accounting which consists of the license and manufacture of vectors and associated payload for incorporation into bb2121. The Company accounts for its vector manufacturing services for development in the U.S. and Celgene’s U.S. development efforts within the scope of ASC 808 given that both parties are active participants in the activities and both parties are exposed to significant risks and rewards dependent on the commercial success of the activities. The Company recognizes revenue for its U.S. manufacturing services by analogy to Topic 606. The portion of Celgene’s U.S. development costs that bluebird is responsible for are recognized as a reduction to its collaboration revenues, or, if in excess of such revenues in a given quarter, the excess is recorded as research and development expense. Revenue recognition for the combined unit of accounting commenced during the first quarter of 2017. The Company recognizes revenue associated with the combined unit of accounting using the proportional performance method, as the Company will satisfy this performance obligation as the manufacturing services are performed through development. In using this method the Company estimated its development plan for bb2121, including expected demand from Celgene, and the costs associated with the manufacture of vectors and associated payload for incorporation into bb2121. On a quarterly basis, the Company determines the proportion of effort incurred as a percentage of total effort it expects to expend. This ratio is applied to the transaction price, which includes variable consideration, allocated to the combined performance obligation consisting of the bb2121 license and manufacturing services. Management has applied significant judgment in the process of developing its budget estimates and any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. In developing the SSP for the combined performance obligation, management assumed that the Company would exercise its option to co-develop and co-promote bb2121, and therefore will recognize revenue related to 67.5% of worldwide development costs incurred, which represents the percentage the Company is contractually entitled to bill Celgene under the cost share provisions of the co-development and co-promotion agreement, upon its execution. The Company exercised its option to co-develop and co-promote bb2121 in March 2018. The period of performance and pattern of revenue recognition remained unchanged upon its execution and will be revisited as the development plan changes or if other events occur. For the three months ended June 30, 2018, the portion of Celgene’s U.S. development costs that bluebird is responsible for are in excess of the Company’s corresponding U.S. development costs, and, as such, the Company recorded research and development expense of $3.3 million (which is representative of gross revenue of $8.5 million offset by approximately $11.8 million of cost reimbursement to Celgene) related to the combined unit of accounting. For the three months ended March 31, 2018, the Company recognized revenue of $3.9 million (representative of gross revenue of $11.8 million offset by approximately $7.9 million of cost reimbursement to Celgene) related to the combined unit of accounting for its license and vector manufacturing of bb2121 in the U.S. Revenue related to the combined unit of accounting for its rest of world license and vector manufacturing services is accounted for in accordance with Topic 606 and is recognized as collaboration revenue. The Company recognized $5.8 million and $14.7 million of revenue related to the combined unit of accounting for its rest of world license and vector manufacturing services for the three and six months ended June 30, 2018, respectively. The Company recognized $6.0 million and $10.9 million of revenue related to the combined unit of accounting of accounting for its license and vector manufacturing services for the three and six months ended June 30, 2017, respectively, in accordance with ASC 605. As of June 30, 2018, the aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the bb2121 license and manufacturing services, that is unsatisfied, or partially unsatisfied, is $80.8 million, which the Company expects to recognize as revenue as manufacturing services are provided through the remaining development period which is estimated to be through 2020. The Company had $41.8 million remaining deferred revenue as of June 30, 2018 associated with the combined performance obligation consisting of the bb2121 license and manufacturing services. Accounting analysis – bb21217 On September 22, 2017, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, the second optioned product candidate, pursuant to the bb21217 License Agreement entered into by the parties on September 28, 2017. The bb21217 License Agreement is considered a separate contract for accounting purposes as the option to obtain an exclusive worldwide license to develop and commercialize bb21217, or any other product candidate, was not considered a material right to Celgene at the time the practical expedient was applied. The Company made this evaluation after considering the significant uncertainty at that time regarding whether any additional product candidates would be identified under the Amended Collaboration Agreement. In particular, the Company considered that bb21217 had not been formally nominated as a product candidate under the collaboration at that time, primarily due to a lack of pre-clinical data as well as uncertainty surrounding the ability to successfully complete various IND-enabling activities. At contract inception, the Company concluded that the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to the second product candidate, bb21217, and (iii) manufacture of vectors and associated payload for incorporation into bb21217 through development. The Company determined that the manufacture of commercial vector represents an option to acquire additional goods and services that is not representative of a material right. In addition, at this time Celgene has not exercised its option to purchase any commercial vector. Accordingly, the manufacture of commercial vector is not considered to be a performance obligation at this time. The Company concluded that the research and development services are distinct from the other promised goods and services under the arrangement and thus is considered to be a performance obligation. Similar to bb2121, the Company concluded that the license to bb21217 is not distinct from the vector manufacturing services because the manufacturing is essential to the use of the license. Accordingly, these two promised goods and services are considered a single combined performance obligation. As of June 30, 2018, the total transaction price of $41.7 million comprises the option fee of $15.0 million and the estimated variable consideration of $26.7 million related to reimbursement from Celgene for the manufacturing services during development. The total transaction price has been allocated to the performance obligations identified based on a relative SSP basis. The Company estimated the SSP of the license after considering potential future cash flows under the license. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the SSP of each of the research and development services and manufacturing services to be provided based on the Company’s estimated cost of providing the services plus an applicable profit margin commensurate with observable market data for similar services. None of the clinical or regulatory milestones have been included in the transaction price, as all milestone amounts are fully constrained. 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, and the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Celgene and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company will re-evaluate the transaction price, including is estimated variable consideration included in the transaction price and all constrained amounts, in each reporting period and as uncertain events are resolved or other changes in circumstances occur. bb21217 research and development services The Company allocated $5.4 million of the transaction price to the research and development services. The Company will satisfy this performance obligation as the research and development services are performed. The Company determined that the period of performance of the research and development services was two years through projected initial Phase I clinical study substantial completion, or through September 2019. The Company recognizes revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. The Company recognized revenue related to bb21217 research and development services for Celgene of $0.7 million and $1.4 million for the three and six months ended June 30, 2018, respectively. During the three and six months ended June 30, 2017, the Company did not recognize any revenue under the bb21217 License Agreement. As of June 30, 2018, the aggregate amount of the transaction price allocated to the bb21217 research and development services performance obligation that are unsatisfied, or partially unsatisfied, and deferred is $3.6 million, which the Company expects to recognize through September 2019 as research and development services are performed. bb21217 license and manufacturing services The Company will satisfy its performance obligation related to the manufacture of vectors and associated payload for incorporation into bb21217 through development as the bb21217 manufacturing services are performed. As of June 30, 2018, the manufacturing services for bb21217 had not yet commenced. Therefore, no revenue has been recognized for the combined unit of accounting for the three and six months ended June 30, 2018 and 2017. The aggregate amount of the transaction price allocated to the combined performance obligation, which consists of the bb21217 license and manufacturing services, is $36.2 million. The Company does not expect that recognition will begin in the next twelve months and has therefore classified deferred revenue associated with the combined performance obligation as deferred revenue, net of current portion on its consolidated balance sheet. The Company had $9.8 million remaining deferred revenue as of June 30, 2018 associated with the combined performance obligation consisting of the bb21217 license and manufacturing services. Contract assets and liabilities The Company receives payments from Celgene based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until such time as the Company satisfies its performance obligations under these arrangements. A contract asset is a conditional right to consideration in exchange for goods or services that the Company has transferred to a customer. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The following table presents changes in the balances of the Company’s Celgene receivables and contract liabilities during the six months ended June 30, 2018: For the six months ended June 30, 2018 (in thousands) Balance at beginning of period under Topic 606 Additions Deductions Balance at end of period under Topic 606 Receivables $ 4,635 $ 1,641 (6,276 ) $ — Contract liabilities: Payable included in accrued expenses $ — $ 2,824 $ — $ 2,824 Deferred revenue $ 76,812 — (21,589 ) $ 55,223 The change in the receivables balance for the six months ended June 30, 2018 is primarily driven by cash collected from Celgene for the amounts owed to the Company for the satisfaction of vector manufacturing services performed under the collaboration to date. As of June 30, 2018, the Company does not have a receivable given that any amounts owed to the Company through June 30, 2018 were collected in the period and Celgene’s U.S. development costs incurred in the second quarter of 2018 for which bluebird is responsible are in excess of the Company’s U.S. development costs that Celgene is responsible for. The decrease in deferred revenue during the six months ended June 30, 2018 is primarily driven by revenue recognized for the combined performance obligation consisting of the bb2121 license and manufacturing services. To date, there have been no impairment losses recognized on any receivables arising from the Company’s contracts with customers. During the six months ended June 30, 2018, $21.6 million of the deferred revenue balance at the beginning of the period was recognized as gross revenues.

License and royalty revenue

License and royalty revenue	6 Months Ended
	Jun. 30, 2018
License And Royalty Revenue [Abstract]
License and royalty revenue	9. License and royalty revenue Novartis Pharma AG On April 26, 2017, the Company entered into a worldwide license agreement with Novartis Pharma AG, or Novartis. Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including Kymriah (formerly known as CTL019), Novartis’s anti-CD19 CAR T therapy. At contract inception, financial terms of the agreement included a $7.5 million payment upon execution, $7.5 million of potential future milestone payments associated with regulatory approvals, and $1.1 million of payments for each subsequently licensed product, as well as low single digit royalty payments on net sales of covered products. In August 2017, Novartis received FDA approval for Kymriah and paid the Company $2.5 million as a result of the achievement of a related milestone. Given this arrangement is within the scope of Topic 606, the Company assessed this arrangement in accordance with Topic 606 and concluded that at the date of contract inception, only one performance obligation, consisting of the license which was satisfied at contract inception, was identified. Accordingly, the nonrefundable license fee of $7.5 million was recognized as revenue upon contract execution in the second quarter of 2017 and the $2.5 million regulatory milestone was recognized as revenue upon milestone achievement, also in the second quarter of 2017, given there were no other unsatisfied performance obligations in the arrangement. This accounting conclusion was unchanged from its historical treatment under ASC 605. Because the single performance obligation was previously satisfied, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved. Regulatory approvals are not within the Company’s control or the licensee’s control and are generally not considered probable of being achieved until those approvals are received. As such, these milestones are constrained and excluded from the transaction price until such time as regulatory approvals are received. The Company began recognizing royalty revenue from sales of Kymriah in the fourth quarter of 2017. As the license was deemed to be the predominant item to which the royalties relate, the Company recognizes royalties from the sales of Kymriah when the related sales occur. For the three and six months ended June 30, 2018, the Company recognized royalty revenue of $0.4 million and $0.7 million, respectively. The Company did not recognize royalty revenue in the comparative periods in the prior year given that Kymriah was approved for commercial sale in the second half of 2017. The associated cost of license and royalty revenue for the three and six months ended June 30, 2018 was less than $0.1 million in both periods. For the three and six months ended June 30, 2017, the cost of license and royalty revenue was $0.3 million. Orchard Therapeutics Limited (formerly GlaxoSmithKline Intellectual Property Development Limited) On April 28, 2017, the Company entered into a worldwide license agreement with GlaxoSmithKline Intellectual Property Development Limited, or GSK. Under the terms of the agreement, GSK non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize gene therapies for Wiscott-Aldrich syndrome and metachromatic leukodystrophy, two rare genetic diseases. Financial terms of the agreement include a nonrefundable upfront payment of $3.0 million as well as $1.3 million of potential milestone payments for each marketing authorization for each indication in any country as well as low single digit royalties on net sales of covered products. This license agreement was assigned by GSK to Orchard Therapeutics Limited, effective as of April 11, 2018. Given this arrangement is within the scope of Topic 606, the Company assessed this arrangement in accordance with Topic 606 and concluded that at the date of contract inception, only one performance obligation, consisting of the license which was satisfied at contract inception, was identified. Accordingly, the entire nonrefundable license fee of $3.0 million was recognized as revenue upon contract execution in the second quarter of 2017 given there were no other unsatisfied performance obligations in the arrangement. This accounting conclusion was unchanged from its historical treatment under ASC 605. Because the single performance obligation was previously satisfied, all regulatory milestones will be recognized as revenue in full in the period in which the associated milestone is achieved. Regulatory approvals are not within the Company’s control or the licensee’s control and are generally not considered probable of being achieved until those approvals are received. As such, these milestones are constrained and excluded from the transaction price until such time as regulatory approvals are received. During the three and six months ended June 30, 2017, the Company recognized revenue of $3.0 million upon delivery of the license, as there were no other undelivered elements in the arrangements. There was no revenue recognized under this arrangement in the three and six months ended June 30, 2018. For the three and six months ended June 30, 2017 the cost of license and royalty revenue was $0.1 million. Given there was no revenue recognized under this arrangement in the three and six months ended June 30, 2018, there was no associated cost of license and royalty revenue.

Equity

Equity	6 Months Ended
	Jun. 30, 2018
Equity [Abstract]
Equity	10. Equity On December 15, 2017, the Company sold 3.2 million shares of common stock (excluding any shares sold pursuant to an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $185.00 per share for aggregate net proceeds of $569.8 million. In January 2018, the Company sold 0.3 million shares of common stock pursuant to the partial exercise of an overallotment option granted to the underwriters in connection with the December 2017 underwritten public offering at a price of $185.00 per share for aggregate net proceeds of $48.7 million. In July 2018, the Company sold 3.9 million shares of common stock (inclusive of shares sold pursuant to an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $162.50 per share for aggregate net proceeds of $600.6 million.

Stock-based compensation

Stock-based compensation	6 Months Ended
	Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation	11. Stock-based compensation In January 2018 and 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.0 million and 1.6 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2018, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 1.8 million. Stock-based compensation expense The Company recognized stock-based compensation expense totaling $28.1 million and $51.1 million for the three and six months ended June 30, 2018, respectively. The Company recognized stock-based compensation expense totaling $13.5 million and $25.0 million for the three and six months ended June 30, 2017, 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 For the three months ended June 30, six months ended June 30, 2018 2017 2018 2017 Stock options $ 20,932 $ 10,764 $ 38,227 $ 20,107 Restricted stock units 6,953 2,636 12,494 4,592 Employee stock purchase plan 171 89 330 272 $ 28,056 $ 13,489 $ 51,051 $ 24,971 Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): For the For the three months ended June 30, six months ended June 30, 2018 2017 2018 2017 Research and development $ 14,196 $ 6,797 $ 25,820 $ 12,451 General and administrative 13,860 6,692 25,231 12,520 $ 28,056 $ 13,489 $ 51,051 $ 24,971 In February 2018, the Company issued restricted stock units with service and performance conditions to employees, approximately 0.2 million of which are outstanding as of June 30, 2018 and none of which vested during the three or six months ended June 30, 2018. Vesting of these awards is contingent on the occurrence of a certain regulatory milestone event and fulfillment of any remaining service condition. As a result, the related compensation cost will be first recognized as expense if and when achievement of the regulatory milestone is considered probable. These awards were modified in the second quarter of 2018 as a result of the adoption of a broad-based employee plan. The Company did not recognize any expense during the three or six months ended June 30, 2018 related to these awards and may recognize up to $40.4 million in stock-based compensation expense related to these awards upon achievement of the performance condition and subject to the service based condition. As of June 30, 2018, the Company had $298.5 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 3.1 years, exclusive of any potential future stock-based compensation expense that may be recognized on any of the Company’s outstanding performance-based awards for which the performance conditions were deemed not probable of achievement as of June 30, 2018. 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, 2017 3,755 $ 67.91 Granted 1,220 $ 198.41 Exercised (426 ) $ 59.45 Canceled or forfeited (44 ) $ 120.49 Outstanding at June 30, 2018 4,505 $ 103.55 Exercisable at June 30, 2018 1,954 $ 56.65 Vested and expected to vest at June 30, 2018 4,497 $ 103.61 During the six months ended June 30, 2018, 0.4 million shares of common stock were exercised, resulting in total proceeds to the Company of $25.3 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans. Restricted stock unit activity The following table summarizes the restricted stock unit activity under the Company’s equity award plans: Shares (in thousands) Weighted- average grant date fair value Unvested balance at December 31, 2017 477 $ 80.72 Granted 575 $ 201.53 Vested (107 ) $ 77.53 Forfeited (14 ) $ 142.86 Unvested balance at June 30, 2018 931 $ 154.73 Refer above for discussion of the performance-based restricted stock units granted in February 2018, which are included in the table above. Employee stock purchase plan On June 3, 2013, the Company adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized the initial issuance of up to a total of 238,000 shares 2017, 9,055 and shares

Income taxes

Income taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income taxes	12. 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. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allows the recording of provisional amounts during a measurement period not to extend beyond one year of the enactment date. In accordance with SAB 118, the Company determined a provisional amount for the impact on its prior year deferred tax assets and valuation allowance in its prior year financial statements. The Company has not updated the provisional amounts and expects to complete the final assessment of the impact within the measurement period.

Net loss per share

Net loss per share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Net loss per share	13. Net loss per share The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): For the three and six months ended June 30, 2018 2017 Outstanding stock options 4,505 4,191 Restricted stock units 931 460 ESPP shares 5 11 5,441 4,662

Subsequent events

Subsequent events	6 Months Ended
	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent events	14. Subsequent events As discussed in Note 10, “Equity,” in July 2018, the Company sold 3.9 million shares of common stock (inclusive of shares sold pursuant to an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $162.50 per share for aggregate net proceeds of approximately $600.6 million.

Basis of presentation, princi21

Basis of presentation, principles of consolidation and significant accounting policies (Policies)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 21, 2018. Certain items in the prior year’s condensed consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year condensed consolidated financial statements were impacted. Amounts reported are computed based on thousands. As a result, certain totals may not sum due to rounding.
Principles of consolidation	Principles of consolidation The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. As of June 30, 2018, there have been no changes to the Company’s subsidiary listing, which is included as an exhibit to its Annual Report on Form 10-K for the year ended December 31, 2017. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States.
Significant accounting policies	Significant accounting policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2018 are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s 2017 Annual Report on Form 10-K, except as noted below with respect to the Company’s revenue recognition, collaboration revenue and license and royalty revenue accounting policies and as noted within the “Recent accounting pronouncements – Recently adopted” Revenue recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right is accounted for as a contract modification for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method. Collaboration revenue To date, the Company’s collaboration revenue has been exclusively generated from its collaboration arrangement with Celgene, which was originally entered into in March 2013 and was subsequently amended in June 2015, as further described in Note 8, “ Collaboration revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements License and royalty revenue The Company enters into out-licensing agreements that are within the scope of Topic 606. The Company does not have any material license arrangements that contain more than one performance obligation. The terms of such out-license agreements include the license of functional intellectual property, given the functionality of the intellectual property is not expected to change substantially as a result of the licensor’s ongoing activities, and typically include payment of one or more of the following: non-refundable up-front license fees; development and regulatory milestone payments and milestone payments based on the level of sales; and royalties on net sales of licensed products. Nonrefundable up-front license fees are recognized as revenue at a point in time when the licensed intellectual property is made available for the customer’s use and benefit, which is generally at the inception of the arrangement. Milestone fees, which are a type of variable consideration, are recognized as revenue to the extent that it is probable that a significant reversal will not occur. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. For a complete discussion of accounting for collaboration and other revenue-generating arrangements, see Note 8, “Collaboration revenue” “License and royalty revenue” Recent accounting pronouncements - Recently adopted
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially
Recent accounting pronouncements	Recent accounting pronouncements Recently adopted ASU No. 2014 -09, In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers Revenue Recognition Revenue from Contracts with Customers • For contracts that were modified prior to Topic 606 adoption, the Company has not retrospectively accounted for each contract modification in accordance with the contract modification guidance. Instead, the Company reflected the aggregate effect of all modifications occurring prior to Topic 606 adoption when identifying the sa tisfied and unsatisfied performance obligations, determining the transaction price and allocating the transaction price. As a result of adopting Topic 606, the Company recorded a $29.4 million adjustment to the opening balance of accumulated deficit in the first quarter of 2018 primarily as a result of the accounting for the up-front consideration received in March 2013 in connection with the collaboration arrangement with Celgene under ASC 605-25 versus Topic 606. Refer below for a summary of t he amount by which each financial statement line item was affected by Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of January 1, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 45,344 $ 19,670 $ 25,674 Deferred revenue, net of current portion $ 31,468 $ 9,705 $ 21,763 Accumulated deficit $ (943,183 ) $ (29,375 ) $ (913,808 ) The amount by which each financial statement line item is affected in the current reporting period by Topic 606 as compared with the guidance that was in effect prior to adoption is disclosed below. Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 27,323 $ 12,010 $ 15,313 Deferred revenue, net of current portion $ 27,900 $ 8,545 $ 19,355 Accumulated deficit $ (1,204,303 ) $ (20,555 ) $ (1,183,748 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Three Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 7,437 $ 887 $ 6,550 Research and development expense $ 115,014 $ (1,800 ) $ 116,814 Net loss $ (145,996 ) $ 2,687 $ (148,683 ) Net loss per share - basic and diluted: $ (2.91 ) $ 0.05 $ (2.96 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Six Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 23,045 $ 7,020 $ 16,025 Research and development expense $ 212,123 $ (1,800 ) $ 213,923 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Net loss per share - basic and diluted: $ (5.22 ) $ 0.17 $ (5.39 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Cash Flows for the Six Months Ended June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Changes in deferred revenue $ (21,589 ) $ (8,820 ) $ (12,769 ) The most significant change above relates to the Company’s collaboration revenue, which to date has been exclusively generated from its collaboration arrangement with Celgene. Under ASC 605, the Company accounted for contract modifications to the Celgene collaboration as they occurred and the accounting for those changes was prospective in nature. Through the application of the practical expedient discussed above in connection with the adoption of Topic 606, the Company reflected the aggregate effect of all modifications to the Celgene collaboration when identifying the satisfied and unsatisfied performance obligations, determining the transaction price, and allocating the transaction price. As a result, although the performance obligations identified under Topic 606 were generally consistent with the units of account identified under ASC 605, the timing of the allocation of the transaction price to the identified performance obligations under Topic 606 differed from the allocations of consideration under ASC 605. Accordingly, the transaction price ultimately allocated to each performance obligation under Topic 606 differed from the amounts allocated under ASC 605. As a result of adopting Topic 606, the Company established a deferred revenue deferred tax asset, and an offsetting valuation allowance, of $7.9 million through its accumulated deficit given it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there was no tax impact on the Company’s condensed consolidated financial statements as a result of adopting Topic 606. ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”) ASU 2016-18, Statement of Cash Flows: Restricted Cash In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”) (in thousands) As of June 30, 2018 As of June 30, 2017 Cash and cash equivalents $ 333,949 $ 686,334 Restricted cash included in receivables and other current assets 100 627 Restricted cash included in restricted cash and other non-current assets 13,763 13,763 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 347,812 $ 700,724 ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting The new standard was effective beginning January 1, 2018. The adoption of this standard did not have a material impact on the Company’s Not yet adopted ASU 2016-02, Leases In February 2016, the FASB issued ASU 2016-02, Leases, (“ASU 2016-02”) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019. The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position and results of operations. The Company’s assessment will include, but is not limited to, evaluating the impact that this standard has on the lease of its corporate headquarters at 60 Binney in Cambridge, Massachusetts, its laboratory space in Seattle, Washington, its office space in Zug, Switzerland, and any of its embedded leases. ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (“Subtopic 310-20”) Subtopic 310-20 calls for a modified retrospective application under which . The new standard will be effective beginning January 1, 2019 and early adoption is permitted for public entities. financial position or results of operations upon adoption. ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued ASU 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (“ASU 2018-02”) The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s financial position and results of operations ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”) simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The new standard will be effective beginning January 1, 2019 and early adoption is permitted. The Company is currently evaluating the potential impact ASU 2018-07 may have on its results of operations

Basis of presentation, princi22

Basis of presentation, principles of consolidation and significant accounting policies (Tables)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Summary of Impact of Topic 606 Adoption on Financial Statement	Refer below for a summary of t he amount by which each financial statement line item was affected by Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of January 1, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 45,344 $ 19,670 $ 25,674 Deferred revenue, net of current portion $ 31,468 $ 9,705 $ 21,763 Accumulated deficit $ (943,183 ) $ (29,375 ) $ (913,808 ) The amount by which each financial statement line item is affected in the current reporting period by Topic 606 as compared with the guidance that was in effect prior to adoption is disclosed below. Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheet as of June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 27,323 $ 12,010 $ 15,313 Deferred revenue, net of current portion $ 27,900 $ 8,545 $ 19,355 Accumulated deficit $ (1,204,303 ) $ (20,555 ) $ (1,183,748 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Three Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 7,437 $ 887 $ 6,550 Research and development expense $ 115,014 $ (1,800 ) $ 116,814 Net loss $ (145,996 ) $ 2,687 $ (148,683 ) Net loss per share - basic and diluted: $ (2.91 ) $ 0.05 $ (2.96 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Operations and Comprehensive Loss for the Six Months Ended June 30, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 23,045 $ 7,020 $ 16,025 Research and development expense $ 212,123 $ (1,800 ) $ 213,923 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Net loss per share - basic and diluted: $ (5.22 ) $ 0.17 $ (5.39 ) Impact of Topic 606 Adoption on Condensed Consolidated Statement of Cash Flows for the Six Months Ended June 30, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Net loss $ (261,120 ) $ 8,820 $ (269,940 ) Changes in deferred revenue $ (21,589 ) $ (8,820 ) $ (12,769 )
Statement of Reconciliation of Cash Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the statement of financial position that sum to the total of the same such amounts shown in the statement of cash flows: (in thousands) As of June 30, 2018 As of June 30, 2017 Cash and cash equivalents $ 333,949 $ 686,334 Restricted cash included in receivables and other current assets 100 627 Restricted cash included in restricted cash and other non-current assets 13,763 13,763 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 347,812 $ 700,724

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Available for Sale Securities Held	The following table summarizes the available-for-sale securities held at June 30, 2018 and December 31, 2017 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value June 30, 2018 U.S. government agency securities and treasuries $ 1,116,012 $ 36 $ (4,714 ) $ 1,111,334 Certificates of deposit 11,960 — — 11,960 Total $ 1,127,972 $ 36 $ (4,714 ) $ 1,123,294 December 31, 2017 U.S. government agency securities and treasuries $ 841,895 $ — $ (3,579 ) $ 838,316 Certificates of deposit 17,480 1 — 17,481 Total $ 859,375 $ 1 $ (3,579 ) $ 855,797

Fair value measurements (Tables

Fair value measurements (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2018 and December 31, 2017 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) June 30, 2018 Assets: Cash and cash equivalents $ 333,949 $ 333,949 $ — $ — Marketable securities: U.S. government agency securities and treasuries 1,111,334 — 1,111,334 — Certificates of deposit 11,960 — 11,960 — Total assets $ 1,457,243 $ 333,949 $ 1,123,294 $ — Liabilities: Contingent consideration $ 3,027 $ — $ — $ 3,027 Total liabilities $ 3,027 $ — $ — $ 3,027 December 31, 2017 Assets: Cash and cash equivalents $ 758,505 $ 758,505 $ — $ — Marketable securities: U.S. government agency securities and treasuries 838,316 — 838,316 — Certificates of deposit 17,481 — 17,481 — Total assets $ 1,614,302 $ 758,505 $ 855,797 $ — Liabilities: Contingent consideration $ 2,231 $ — $ — $ 2,231 Total liabilities $ 2,231 $ — $ — $ 2,231
Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands): For the six months ended June 30, 2018 Beginning balance $ 2,231 Additions — Changes in fair value 796 Payments — Ending balance $ 3,027

Property, plant and equipment25

Property, plant and equipment, net (Tables)	6 Months Ended
	Jun. 30, 2018
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 As of June 30, 2018 December 31, 2017 Land $ 1,210 $ 1,210 Building 167,155 164,414 Computer equipment and software 5,551 5,134 Office equipment 5,036 4,478 Laboratory equipment 30,293 24,914 Leasehold improvements 272 116 Construction-in-progress 31,876 15,189 Total property, plant and equipment 241,393 215,455 Less accumulated depreciation and amortization (22,167 ) (15,849 ) Property, plant and equipment, net $ 219,226 $ 199,606

Accrued expenses and other cu26

Accrued expenses and other current liabilities (Tables)	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): As of As of June 30, 2018 December 31, 2017 Accrued goods and services $ 52,683 $ 29,533 Employee compensation 13,383 19,657 Accrued professional fees 1,748 1,402 Financing lease obligation, current portion 1,214 1,051 Accrued license and milestone fees 914 4,584 Other 1,061 838 Total accrued expenses and other current liabilities $ 71,003 $ 57,065

Collaboration revenue (Tables)

Collaboration revenue (Tables)	6 Months Ended
	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Changes in Balances of Company's Celgene Receivables and Contract Liabilities	The following table presents changes in the balances of the Company’s Celgene receivables and contract liabilities during the six months ended June 30, 2018: For the six months ended June 30, 2018 (in thousands) Balance at beginning of period under Topic 606 Additions Deductions Balance at end of period under Topic 606 Receivables $ 4,635 $ 1,641 (6,276 ) $ — Contract liabilities: Payable included in accrued expenses $ — $ 2,824 $ — $ 2,824 Deferred revenue $ 76,812 — (21,589 ) $ 55,223

Stock-based compensation (Table

Stock-based compensation (Tables)	6 Months Ended
	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock-Based Compensation Expense by Award Type	Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): For the For the three months ended June 30, six months ended June 30, 2018 2017 2018 2017 Stock options $ 20,932 $ 10,764 $ 38,227 $ 20,107 Restricted stock units 6,953 2,636 12,494 4,592 Employee stock purchase plan 171 89 330 272 $ 28,056 $ 13,489 $ 51,051 $ 24,971
Schedule of Stock-Based Compensation Expense by Classification	Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): For the For the three months ended June 30, six months ended June 30, 2018 2017 2018 2017 Research and development $ 14,196 $ 6,797 $ 25,820 $ 12,451 General and administrative 13,860 6,692 25,231 12,520 $ 28,056 $ 13,489 $ 51,051 $ 24,971
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, 2017 3,755 $ 67.91 Granted 1,220 $ 198.41 Exercised (426 ) $ 59.45 Canceled or forfeited (44 ) $ 120.49 Outstanding at June 30, 2018 4,505 $ 103.55 Exercisable at June 30, 2018 1,954 $ 56.65 Vested and expected to vest at June 30, 2018 4,497 $ 103.61
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Restricted Common Stock Awards	The following table summarizes the restricted stock unit activity under the Company’s equity award plans: Shares (in thousands) Weighted- average grant date fair value Unvested balance at December 31, 2017 477 $ 80.72 Granted 575 $ 201.53 Vested (107 ) $ 77.53 Forfeited (14 ) $ 142.86 Unvested balance at June 30, 2018 931 $ 154.73

Net loss per share (Tables)

Net loss per share (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 2017 Outstanding stock options 4,505 4,191 Restricted stock units 931 460 ESPP shares 5 11 5,441 4,662

Description of the business - A

Description of the business - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992
Cash, cash equivalents and marketable securities	$ 1,460

Basis of presentation, princi31

Basis of presentation, principles of consolidation and significant accounting policies - Additional Information (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)Segment	Jan. 01, 2018USD ($)	Dec. 31, 2017USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment	1
Accumulated deficit	$ 1,204,303	$ 943,183	$ 913,808
Description of performance obligations	Under ASC 605, the Company accounted for contract modifications to the Celgene collaboration as they occurred and the accounting for those changes was prospective in nature. Through the application of the practical expedient discussed above in connection with the adoption of Topic 606, the Company reflected the aggregate effect of all modifications to the Celgene collaboration when identifying the satisfied and unsatisfied performance obligations, determining the transaction price, and allocating the transaction price. As a result, although the performance obligations identified under Topic 606 were generally consistent with the units of account identified under ASC 605, the timing of the allocation of the transaction price to the identified performance obligations under Topic 606 differed from the allocations of consideration under ASC 605. Accordingly, the transaction price ultimately allocated to each performance obligation under Topic 606 differed from the amounts allocated under ASC 605.
Deferred tax assets, valuation allowance	$ 7,900
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Accumulated deficit	$ 20,555	$ 29,375

Basis of presentation, princi32

Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	$ 27,323	$ 45,344	$ 25,674
Deferred revenue, net of current portion	27,900	31,468	21,763
Accumulated deficit	(1,204,303)	(943,183)	$ (913,808)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	12,010	19,670
Deferred revenue, net of current portion	8,545	9,705
Accumulated deficit	(20,555)	(29,375)
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	15,313	25,674
Deferred revenue, net of current portion	19,355	21,763
Accumulated deficit	$ (1,183,748)	$ (913,808)

Basis of presentation, princi33

Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 7,851	$ 16,716	$ 23,808	$ 23,548
Research and development expense	115,014	63,891	212,123	118,919
Net loss	$ (145,996)	$ (70,898)	$ (261,120)	$ (139,611)
Net loss per share - basic and diluted:	$ (2.91)	$ (1.73)	$ (5.22)	$ (3.41)
Collaboration [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 7,437	$ 6,146	$ 23,045	$ 12,978
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense	(1,800)		(1,800)
Net loss	$ 2,687		$ 8,820
Net loss per share - basic and diluted:	$ 0.05		$ 0.17
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense	$ 116,814		$ 213,923
Net loss	$ (148,683)		$ (269,940)
Net loss per share - basic and diluted:	$ (2.96)		$ (5.39)
ASU 2014-09 [Member] | Collaboration [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 887		$ 7,020
ASU 2014-09 [Member] | Collaboration [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 6,550		$ 16,025

Basis of presentation, princi34

Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	$ (145,996)	$ (70,898)	$ (261,120)	$ (139,611)
Changes in deferred revenue			(21,589)	$ (7,682)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	2,687		8,820
Changes in deferred revenue			(8,820)
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	$ (148,683)		(269,940)
Changes in deferred revenue			$ (12,769)

Basis of presentation, princi35

Basis of presentation, principles of consolidation and significant accounting policies - Statement of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 333,949	$ 758,505	$ 686,334
Restricted cash included in receivables and other current assets	100		627
Restricted cash included in restricted cash and other non-current assets	13,763		13,763
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 347,812	$ 772,268	$ 700,724	$ 293,277

Marketable securities - Summary

Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	$ 1,127,972	$ 859,375
Unrealized gains	36	1
Unrealized losses	(4,714)	(3,579)
Fair value	1,123,294	855,797
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	1,116,012	841,895
Unrealized gains	36
Unrealized losses	(4,714)	(3,579)
Fair value	1,111,334	838,316
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	11,960	17,480
Unrealized gains		1
Fair value	$ 11,960	$ 17,481

Marketable securities - Additio

Marketable securities - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Realized gain (loss) on available-for-sale securities	$ 0	$ 0
Reclassification out of accumulated other comprehensive income (loss)	0	$ 0
Unrealized Loss on Securities	807,200,000		$ 704,100,000
Unrealized loss on securities, more than twelve months	101,200,000		134,400,000
Unrealized loss on securities for less than twelve months	4,300,000		3,300,000
Unrealized loss on securities for more than twelve months	400,000		300,000
Investments with other-than-temporary impairment	$ 0		$ 0

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 333,949	$ 758,505
Total assets	1,457,243	1,614,302
Liabilities:
Contingent consideration	3,027	2,231
Total liabilities	3,027	2,231
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	1,111,334	838,316
Certificates of Deposit [Member]
Assets:
Marketable securities	11,960	17,481
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	333,949	758,505
Total assets	333,949	758,505
Significant other observable inputs (Level 2) [Member]
Assets:
Total assets	1,123,294	855,797
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	1,111,334	838,316
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable securities	11,960	17,481
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	3,027	2,231
Total liabilities	$ 3,027	$ 2,231

Fair value measurements - Addit

Fair value measurements - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	90 days or less
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,021
Milestone discount rates	15.60%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,028
Milestone discount rates	16.50%

Fair value measurements - Roll-

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) - Significant unobservable inputs (Level 3) [Member] - Contingent consideration obligations [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 2,231
Changes in fair value	796
Ending balance	$ 3,027

Property, plant and equipment41

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 241,393	$ 215,455
Less accumulated depreciation and amortization	(22,167)	(15,849)
Property, plant and equipment, net	219,226	199,606
Land [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	1,210	1,210
Building [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	167,155	164,414
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	5,551	5,134
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	5,036	4,478
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	30,293	24,914
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	272	116
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 31,876	$ 15,189

Property, plant and equipment42

Property, plant and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 241,393	$ 215,455
Construction In Progress North Carolina Manufacturing Facility [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	28,100	12,900
60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	167,200	164,400
Construction cost incurred by landlord	$ 156,000	$ 156,000

Accrued expenses and other cu43

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued goods and services	$ 52,683	$ 29,533
Employee compensation	13,383	19,657
Accrued professional fees	1,748	1,402
Financing lease obligation, current portion	1,214	1,051
Accrued license and milestone fees	914	4,584
Other	1,061	838
Total accrued expenses and other current liabilities	$ 71,003	$ 57,065

Commitments and contingencies -

Commitments and contingencies - Additional Information (Detail) € in Millions	Nov. 18, 2016EUR (€)	Jun. 03, 2016USD ($)	Sep. 21, 2015	Mar. 31, 2018USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2016EUR (€)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Commitments And Contingencies Disclosure [Line Items]
Term of agreement	5 years
Renewal period of agreement	3 years
Up-front fee | €	€ 3
Up-front fee paid | €							€ 2
Up-front fee payable in third quarter of 2018 | €	1
Annual fees					$ 21,000	$ 420,000		$ 36,000	$ 420,000
Termination description								The Company may terminate this agreement with twelve months’ notice and a one-time termination fee.
Decrease in letter of credit under the terms of the lease								$ 9,200,000
Estimated future payments for 2018					12,000,000			12,000,000
Estimated future payments for 2019					12,000,000			12,000,000
Pregenen [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments					120,000,000			120,000,000
Pregenen [Member] | Clinical Milestone Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments					20,100,000			20,100,000
Pregenen [Member] | Commercial Milestones Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments					99,900,000			$ 99,900,000
Building [Member]
Commitments And Contingencies Disclosure [Line Items]
Useful life of asset								40 years
Interest expense related to financing obligation					3,900,000	$ 3,700,000		$ 7,800,000	$ 3,700,000
Letter of credit under the terms of lease								$ 13,800,000
Maintenance and Production [Member]
Commitments And Contingencies Disclosure [Line Items]
Annual fees | €	€ 9.8
Strategic manufacturing agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period		12 years
Amount payable upon achievement of certain contractual milestones		$ 12,500,000
Milestone payments				$ 1,500,000	$ 2,000,000					$ 5,500,000	$ 5,000,000
Lease payments		$ 5,100,000
Agreement termination description								The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.
Early termination lease term		24 months
Strategic manufacturing agreement [Member] | Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Additional contractual payables to milestones		$ 8,000,000
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Mar. 31, 2027

Collaboration revenue - Additio

Collaboration revenue - Additional Information (Detail) - USD ($)	Sep. 28, 2017	Feb. 16, 2016	Jun. 03, 2015	Mar. 19, 2013	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 28, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue					$ 7,851,000		$ 16,716,000	$ 23,808,000	$ 23,548,000
Research and development expense					115,014,000		63,891,000	212,123,000	118,919,000
Deferred revenue balance recognized as gross revenues								21,589,000
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of collaboration agreement				3 years
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received								10,000,000
Collaboration agreement, transaction price					182,500,000			182,500,000
Estimated variable consideration					72,500			72,500
Up-front non-refundable fees								$ 100,000,000
Deferred revenue recognition period								3 years
Percentage of revenue related to worldwide development cost								67.50%
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					36,000,000			$ 36,000,000
Revenue					1,000,000		1,600,000	3,100,000	3,100,000
Celgene Corporation [Member] | bb2121 License Agreement [Member] | First Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received		$ 10,000,000
Celgene Corporation [Member] | bb2121 Co-Development, Co-Promote and Profit Share Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable										$ 70,000,000
Celgene Corporation [Member] | bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received	$ 15,000,000							15,000,000
Additional fee receivable if option to co-develop and co-promote is not exercised	$ 10,000,000
Collaboration agreement, transaction price					41,700,000			41,700,000
Estimated variable consideration					26,700,000			$ 26,700,000
Deferred revenue recognition period								2 years
Celgene Corporation [Member] | bb21217 License Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical milestone payments receivable					10,000,000			$ 10,000,000
Regulatory milestone payments receivable					117,000,000			117,000,000
Commercial milestone payments receivable					78,000,000			78,000,000
Celgene Corporation [Member] | bb21217 License Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					5,400,000			5,400,000
Revenue					700,000		0	1,400,000	0
Remaining performance obligation revenue					3,600,000			3,600,000
Remaining deferred revenue					3,600,000			3,600,000
Celgene Corporation [Member] | bb21217 License Agreement [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue					0		0	0	0
Remaining performance obligation revenue					36,200,000			36,200,000
Remaining deferred revenue					9,800,000			9,800,000
Celgene Corporation [Member] | bb21217 License Agreement, Co-promotion and Development [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments receivable					70,000,000			70,000,000
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					146,400,000			146,400,000
Revenue						$ 3,900,000
Research and development expense					3,300,000
Gross revenue					8,500,000	11,800,000
Cost reimbursement					11,800,000	$ 7,900,000
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member] | Rest of the World [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue					5,800,000			14,700,000
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member] | ASC 605 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue							$ 6,000,000		$ 10,900,000
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation revenue					$ 80,800,000			$ 80,800,000
Remaining performance obligation expected to be recognize as revenue, year					2,020			2,020
Remaining deferred revenue					$ 41,800,000			$ 41,800,000
Up-front Payment Arrangement [Member] | Celgene Corporation [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received				$ 75,000,000
Up-front Payment Arrangement [Member] | Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received			$ 25,000,000

Collaboration revenue - Changes

Collaboration revenue - Changes in Balances of Company's Celgene Receivables and Contract Liabilities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Receivables
Balance at beginning of period under Topic 606	$ 4,635
Additions	1,641
Deductions	(6,276)
Contract liabilities:
Balance at beginning of period under Topic 606	76,812
Deductions	(21,589)
Balance at end of period under Topic 606	55,223
Accrued Expenses [Member]
Contract liabilities:
Additions	2,824
Balance at end of period under Topic 606	$ 2,824

License and royalty revenue - A

License and royalty revenue - Additional Information (Detail)	Apr. 28, 2017USD ($)PerformanceObligation	Apr. 26, 2017USD ($)PerformanceObligation	Aug. 31, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)
License And Royalty Revenue [Line Items]
Revenue				$ 7,851,000	$ 16,716,000	$ 23,808,000	$ 23,548,000
Cost of license and royalty revenue				21,000	420,000	36,000	420,000
License and Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				414,000	10,570,000	763,000	10,570,000
Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
License agreement upfront payment		$ 7,500,000
Revenue recognized upon achievement of a related milestone			$ 2,500,000
Novartis Pharma AG [Member] | Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				400,000	0	700,000	0
Novartis Pharma AG [Member] | License and Royalty [Member]
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue					300,000		300,000
Novartis Pharma AG [Member] | License and Royalty [Member] | Maximum [Member]
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				100,000		100,000
Novartis Pharma AG [Member] | Topic 606 [Member]
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception | PerformanceObligation		1
Revenue					7,500,000
Novartis Pharma AG [Member] | Topic 606 [Member] | Regulatory Milestones Payments [Member]
License And Royalty Revenue [Line Items]
Revenue					2,500,000
Orchard Therapeutics Limited [Member]
License And Royalty Revenue [Line Items]
License agreement upfront payment	$ 3,000,000
Orchard Therapeutics Limited [Member] | License and Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				0		0
Cost of license and royalty revenue				0	100,000	0	100,000
Orchard Therapeutics Limited [Member] | Topic 606 [Member]
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception | PerformanceObligation	1
Revenue					3,000,000
Number of unsatisfied performance obligation identified at date of contract inception | PerformanceObligation	0
Orchard Therapeutics Limited [Member] | Topic 606 [Member] | License [Member]
License And Royalty Revenue [Line Items]
Revenue				$ 0	$ 3,000,000	$ 0	$ 3,000,000
Regulatory Milestones Payments [Member] | Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 7,500,000
Each Subsequently Licensed Product [Member] | Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 1,100,000
Potential Milestones Payments [Member] | Orchard Therapeutics Limited [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 1,300,000

Equity - Additional Information

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Dec. 15, 2017	Jul. 31, 2018	Jan. 31, 2018
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	3.2		0.3
Shares issued, price per share	$ 185		$ 185
Proceeds from public offering of common stock, net of issuance costs	$ 569.8		$ 48.7
Subsequent Event [Member] | Inclusive of Shares Pursuant to Over Allotment Option Granted [Member]
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering		3.9
Shares issued, price per share		$ 162.50
Proceeds from public offering of common stock, net of issuance costs		$ 600.6

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 03, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan	2,000,000	1,600,000
Number of shares available for issuance			1,800,000		1,800,000
Stock-based compensation expense			$ 28,056,000	$ 13,489,000	$ 51,051,000	$ 24,971,000
Stock option share exercised					426,000
Proceed from option share exercised					$ 25,300,000
Performance And Service Condition Based Restricted Stock Unit [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 0		$ 0
Vested during period			0		0
Unvested restricted stock awards outstanding			200,000		200,000
Performance And Service Condition Based Restricted Stock Unit [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense yet to recognize			$ 40,400,000		$ 40,400,000
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			20,932,000	10,764,000	38,227,000	20,107,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			298,500,000		$ 298,500,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan					3 years 1 month 6 days
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 6,953,000	2,636,000	$ 12,494,000	4,592,000
Vested during period					107,000
Unvested restricted stock awards outstanding			931,000		931,000		477,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 298,500,000		$ 298,500,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan					3 years 1 month 6 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			171,000	$ 89,000	$ 330,000	$ 272,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 298,500,000		$ 298,500,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan					3 years 1 month 6 days
Common shares reserved for future issuance								238,000
Shares of common stock issued under plan					9,055	11,079

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 28,056	$ 13,489	$ 51,051	$ 24,971
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	20,932	10,764	38,227	20,107
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	6,953	2,636	12,494	4,592
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 171	$ 89	$ 330	$ 272

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 28,056	$ 13,489	$ 51,051	$ 24,971
Research And Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	14,196	6,797	25,820	12,451
General And Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 13,860	$ 6,692	$ 25,231	$ 12,520

Stock-based compensation - Su52

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	6 Months Ended
	Jun. 30, 2018$ / sharesshares
Shares
Outstanding at beginning of period | shares	3,755
Granted | shares	1,220
Exercised | shares	(426)
Canceled or forfeited | shares	(44)
Outstanding at end of period | shares	4,505
Exercisable at end of period | shares	1,954
Vested and expected to vest at end of period | shares	4,497
Weighted-average exercise price per share
Outstanding at beginning of period | $ / shares	$ 67.91
Granted | $ / shares	198.41
Exercised | $ / shares	59.45
Canceled or forfeited | $ / shares	120.49
Outstanding at end of period | $ / shares	103.55
Exercisable at end of period | $ / shares	56.65
Vested and expected to vest at end of period | $ / shares	$ 103.61

Stock-based compensation - Su53

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2018$ / sharesshares
Shares
Unvested balance at beginning of period | shares	477
Granted | shares	575
Vested | shares	(107)
Forfeited | shares	(14)
Unvested balance at end of period | shares	931
Weighted-average grant date fair value
Unvested balance at beginning of period | $ / shares	$ 80.72
Granted | $ / shares	201.53
Vested | $ / shares	77.53
Forfeited | $ / shares	142.86
Unvested balance at end of period | $ / shares	$ 154.73

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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	5,441	4,662
Outstanding Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,505	4,191
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	931	460
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	5	11

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Dec. 15, 2017	Jul. 31, 2018	Jan. 31, 2018
Subsequent Event [Line Items]
Number of shares issued in public offering	3.2		0.3
Shares issued, price per share	$ 185		$ 185
Proceeds from public offering of common stock, net of issuance costs	$ 569.8		$ 48.7
Inclusive of Shares Pursuant to Over Allotment Option Granted [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares issued in public offering		3.9
Shares issued, price per share		$ 162.50
Proceeds from public offering of common stock, net of issuance costs		$ 600.6