bluebird bio (BLUE) 10-K 21 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	1,293,971
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		54,951,177
Entity Public Float			$ 7,882,817,517

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 402,579	$ 758,505
Marketable securities	982,725	531,604
Tenant improvement receivable	19	3,112
Prepaid expenses	19,762	21,171
Receivables and other current assets	13,912	8,377
Total current assets	1,418,997	1,322,769
Marketable securities	506,123	324,193
Property, plant and equipment, net	246,622	199,606
Intangible assets, net	13,169	16,931
Goodwill	13,128	13,128
Restricted cash and other non-current assets	44,805	23,940
Total assets	2,242,844	1,900,567
Current Liabilities:
Accounts payable	17,831	12,873
Accrued expenses and other current liabilities	99,393	57,065
Deferred revenue, current portion	18,602	25,674
Collaboration research advancement, current portion	10,605
Total current liabilities	146,431	95,612
Deferred revenue, net of current portion	16,338	21,763
Collaboration research advancement, net of current portion	33,349
Contingent consideration	5,230	2,231
Financing lease obligation, net of current portion	153,319	154,749
Other non-current liabilities	3,107	2,780
Total liabilities	357,774	277,135
Commitments and contingencies (Note 8)
Stockholders' Equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at December 31, 2018 and December 31, 2017
Common stock, $0.01 par value, 125,000 shares authorized; 54,738 and 49,406 shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	547	494
Additional paid-in capital	3,386,958	2,540,951
Accumulated other comprehensive loss	(3,627)	(4,205)
Accumulated deficit	(1,498,808)	(913,808)
Total stockholders' equity	1,885,070	1,623,432
Total liabilities and stockholders' equity	$ 2,242,844	$ 1,900,567

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 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	54,738,000	49,406,000
Common stock, shares outstanding	54,738,000	49,406,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Total revenues	$ 54,579	$ 35,427	$ 6,155
Operating expenses:
Research and development	448,589	273,040	204,775
General and administrative	174,129	93,550	$ 65,119
Cost of license and royalty revenue	$ 885	$ 1,527
Type of cost, good or service [Extensible List]	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember
Change in fair value of contingent consideration	$ 2,999	$ (525)	$ 4,091
Total operating expenses	626,602	367,592	273,985
Loss from operations	(572,023)	(332,165)	(267,830)
Interest income (expense), net	14,624	(2,001)	3,782
Other income (expense), net	1,961	(1,267)	(71)
Loss before income taxes	(555,438)	(335,433)	(264,119)
Income tax (expense) benefit	(187)	(210)	612
Net loss	$ (555,625)	$ (335,643)	$ (263,507)
Net loss per share - basic and diluted	$ (10.68)	$ (7.71)	$ (7.07)
Weighted-average number of common shares used in computing net loss per share - basic and diluted	52,032	43,535	37,284
Other comprehensive income (loss):
Other comprehensive income (loss), net of tax expense of $0.4, $0.0 and $0.6 million for the years ended December 31, 2018, 2017 and 2016, respectively	$ 578	$ (3,056)	$ 1,142
Total other comprehensive income (loss)	578	(3,056)	1,142
Comprehensive loss	(555,047)	(338,699)	(262,365)
Collaboration [Member]
Revenue:
Total revenues	52,353	22,207	$ 6,155
License and Royalty [Member]
Revenue:
Total revenues	$ 2,226	$ 13,220

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Other comprehensive income (loss) tax expense	$ 0.4	$ 0	$ 0.6

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ 850,496	$ 369	$ 1,166,585	$ (2,291)	$ (314,167)
Beginning balance, shares at Dec. 31, 2015		36,894
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		113
Issuance of common stock upon public offering, net of issuance costs	234,731	$ 33	234,698
Issuance of common stock upon public offering, net of issuance costs, shares		3,289
Exercise of stock options	6,145	$ 4	6,141
Exercise of stock options, shares		377
Purchase of common stock under ESPP	677		677
Purchase of common stock under ESPP, shares		18
Stock-based compensation	39,756		39,756
Unrealized gain on available-for-sale securities, net of tax	1,142			1,142
Net loss	(263,507)				(263,507)
Ending balance at Dec. 31, 2016	869,440	$ 407	1,447,856	(1,149)	(577,674)
Ending balance, shares at Dec. 31, 2016		40,691
Adjustment to beginning accumulated deficit from adoption of accounting standard update | ASU 2016-09 [Member]			491		(491)
Vesting of restricted stock units		$ 1	(1)
Vesting of restricted stock units, shares		88
Issuance of common stock upon public offering, net of issuance costs	1,006,570	$ 76	1,006,494
Issuance of common stock upon public offering, net of issuance costs, shares		7,625
Exercise of stock options	31,686	$ 10	31,676
Exercise of stock options, shares		981
Purchase of common stock under ESPP	1,153		1,153
Purchase of common stock under ESPP, shares		21
Stock-based compensation	53,282		53,282
Other comprehensive loss	(3,056)			(3,056)
Net loss	(335,643)				(335,643)
Ending balance at Dec. 31, 2017	$ 1,623,432	$ 494	2,540,951	(4,205)	(913,808)
Ending balance, shares at Dec. 31, 2017	49,406	49,406
Adjustment to beginning accumulated deficit from adoption of accounting standard update | ASU 2014-09 [Member]	$ (29,375)				(29,375)
Vesting of restricted stock units		$ 2	(2)
Vesting of restricted stock units, shares		152
Issuance of common stock upon public offering, net of issuance costs	649,368	$ 42	649,326
Issuance of common stock upon public offering, net of issuance costs, shares		4,169
Issuance of common stock to Regeneron	54,484	$ 4	54,480
Issuance of common stock to Regeneron, shares		420
Exercise of stock options	$ 29,768	$ 5	29,763
Exercise of stock options, shares	575	575
Purchase of common stock under ESPP	$ 1,604		1,604
Purchase of common stock under ESPP, shares		16
Stock-based compensation	110,836		110,836
Other comprehensive loss	578			578
Net loss	(555,625)				(555,625)
Ending balance at Dec. 31, 2018	$ 1,885,070	$ 547	$ 3,386,958	$ (3,627)	$ (1,498,808)
Ending balance, shares at Dec. 31, 2018	54,738	54,738

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Stockholders Equity [Abstract]
Costs from IPO	$ 34,588	$ 53,487	$ 15,269

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (555,625)	$ (335,643)	$ (263,507)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	2,999	(2,189)	2,675
Depreciation and amortization	17,158	13,538	9,648
Stock-based compensation expense	110,836	53,282	39,756
Unrealized gain on equity securities	(2,154)
Other non-cash items	(5,880)	3,153	2,825
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(24,288)	(20,092)	(14,318)
Accounts payable	3,614	526	6,658
Accrued expenses and other liabilities	37,832	5,848	22,051
Deferred revenue	(41,872)	1,024	4,565
Collaboration research advancement	43,954
Net cash used in operating activities	(413,426)	(280,553)	(189,647)
Cash flows from investing activities:
Purchase of property, plant and equipment, including assets under financing lease obligation	(55,737)	(62,242)	(28,029)
Purchases of marketable securities	(1,517,982)	(686,204)	(348,225)
Proceeds from maturities of marketable securities	894,284	431,816	443,364
Net cash (used in) provided by investing activities	(679,435)	(316,630)	67,110
Cash flows from financing activities:
Cash paid for contingent purchase price consideration		(1,074)	(2,025)
Reimbursement of assets under financing lease obligation	3,098	38,021	1,663
Payments on financing lease obligation	(1,017)	(574)
Proceeds from issuance of common stock, net of issuance costs	649,368	1,006,570	234,962
Proceeds from exercise of stock options and ESPP contributions	31,759	33,231	6,934
Net cash provided by financing activities	737,692	1,076,174	241,534
(Decrease) increase in cash, cash equivalents and restricted cash	(355,169)	478,991	118,997
Cash, cash equivalents and restricted cash at beginning of year	772,268	293,277	174,280
Cash, cash equivalents and restricted cash at end of year	417,099	772,268	293,277
Supplemental cash flow disclosures:
Cash paid for interest in connection with financing lease obligation	15,494	11,411
Supplemental cash flow disclosures from investing and financing activities:
Assets acquired under financing lease obligation		3,271	48,034
Purchases of property, plant and equipment included in accounts payable and accrued expenses	7,449	2,566	6,363
Tenant improvements under financing lease included in tenant improvements receivable	14	$ 3,112	$ 8,542
Regeneron Collaboration Agreement [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	$ 54,484

Description of the business

Description of the business	12 Months Ended
	Dec. 31, 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 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 0 0 Collaborative arrangements As of December 31, 2018, the Company had cash, cash equivalents and marketable securities of $1.9 billion. Although the Company has incurred recurring losses and expects to continue to incur losses for the foreseeable future, the Company expects its cash, cash equivalents and marketable securities will be sufficient to fund current planned operations for at least the next twelve months.

Summary of significant accounti

Summary of significant accounting policies and basis of presentation	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of significant accounting policies and basis of presentation	2. Summary of significant accounting policies and basis of presentation Basis of presentation The accompanying consolidated financial statements have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States GAAP as found in the ASC and ASUs of the FASB. Certain items in the prior year’s consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year consolidated financial statements were impacted. Amounts reported are computed based on thousands, except percentages, per share amounts, and as otherwise noted. As a result, certain totals may not sum due to rounding. Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Company continually assesses whether it is the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in consolidation or deconsolidation of one or more collaborators or partners. In determining whether it is the primary beneficiary of an entity in which the Company has a variable interest, management applies a qualitative approach that determines whether the Company has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates and judgments are used in the following areas, among others: fair value estimates used to assess potential impairment of long-lived assets, including goodwill and intangible assets, financing lease obligations, contingent consideration, stock-based compensation expense, accrued expenses, and income taxes. In addition, estimates and judgments are used in the Company’s accounting for its revenue-generating arrangements, in particular as it relates to determining the standalone selling price of performance obligations, evaluating whether an option to acquire additional goods and services represents a material right, estimating the total transaction price, including estimating variable consideration and the probability of achieving future potential development and regulatory milestones, and the period of performance over which revenue may be recognized. Foreign currency translation The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss) in stockholders’ equity. Foreign currency transaction gains and losses are included in other expense, net in the results of operations. Segment information The Company operates in a single segment, focusing on the development of potentially transformative gene therapies for severe genetic diseases and cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources at a global, consolidated level. Therefore, results of our operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States. Cash and cash equivalents The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. Cash and cash equivalents comprise marketable securities with maturities of less than 90 days when purchased. Cash equivalents are reported at fair value. Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. The Company’s marketable securities are maintained by investment managers and consist of U.S. Treasury securities, U.S. government agency securities, equity securities, certificates of deposit, and money market accounts. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Equity securities with readily determinable fair values are also carried at fair value with unrealized gains and losses included in other income (expense), net. Realized gains and losses on both debt and equity securities are determined using the specific identification method and are included in other income (expense), net. The Company classifies equity securities with readily determinable fair values, which would be available for use in its current operations, as current assets even though the Company may not dispose of such marketable securities within the next 12 months. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss. Concentrations of credit risk and off-balance sheet risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and available-for-sale securities. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s marketable securities primarily consist of U.S. Treasury securities, U.S. government agency securities and certificates of deposit, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure. Fair value of financial instruments The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Fair values are determined utilizing quoted prices for identical or similar assets or liabilities in active markets or other market observable inputs such as interest rates, yield curves and foreign currency spot rates. Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Items measured at fair value on a recurring basis include marketable securities (see Note 3, “Marketable securities” “Fair value measurements” “Fair value measurements” Business combinations Business combinations are accounted for using the acquisition method of accounting. Using this method, the tangible and intangible assets acquired and the liabilities assumed are recorded as of the acquisition date at their respective fair values. The Company evaluates a business as an integrated set of activities and assets that is capable of being managed for the purpose of providing a return in the form of dividends, lower costs or other economic benefits and consists of inputs and processes that provide or have the ability to provide outputs. In an acquisition of a business, the excess of the fair value of the consideration transferred over the fair value of the net assets acquired is recorded as goodwill. In an acquisition of net assets that does not constitute a business, no goodwill is recognized. The consolidated financial statements include the results of operations of an acquired business after the completion of the acquisition. See Note 4, “Fair value measurements,” Goodwill Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill to date. Intangible assets Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. Contingent consideration Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and records within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones may be achieved, and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, “Fair value measurements,” Property, plant and equipment Property, plant and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 2-5 years Leasehold improvements Shorter of the useful life or remaining lease term The Company records certain costs incurred and reported by a landlord as an asset and corresponding financing lease obligation on the consolidated balance sheets. See Note 8, “ Commitments and contingencies Impairment of long-lived assets The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. Financing lease obligation Beginning in 2015 and through construction completion in 2017, the Company recorded certain estimated construction costs incurred and reported to the Company by the landlord for its 60 Binney Street location as an asset and corresponding financing lease obligation on the consolidated balance sheets because it was deemed to be the owner of the building during the construction period for accounting purposes. Any costs incurred by the Company that have been reimbursed by the landlord or that qualify for reimbursement by the landlord are recorded as an asset and financing lease obligation. Any incremental costs incurred directly by the Company that do not qualify for reimbursement by the landlord are also capitalized. Upon completion of the construction of the building in the first quarter of 2017, the Company evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, the Company is depreciating the building over 40 years and incurring interest expense in its consolidated statement of operations and comprehensive loss related to the financing lease obligation recorded on its consolidated balance sheet. The Company bifurcates its lease payments pursuant to the lease into (i) a portion that is allocated to the financing obligation related to the building and (ii) a portion that is allocated to the land on which the building was constructed. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced in September 2015 and is recorded on a straight-line basis over the initial lease term. See Note 8, “Commitments and contingencies,” 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. he identification of material rights requires judgments related to the determination of the value of the underlying license relative to the option exercise price, including assumptions about technical feasibility and the probability of developing a candidate that would be subject to the option rights. 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. 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. 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 generated from its collaboration arrangements with Celgene and Regeneron Pharmaceuticals, Inc. (“Regeneron”), as further described in Note 10, “ Collaborative arrangements 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. Development and regulatory 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. 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 10, “Collaborative arrangements” “License and royalty revenue” Recent accounting pronouncements - Recently adopted Research and development expenses Research and development costs are charged to expense as costs are incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, clinical study and related clinical manufacturing costs, license and milestone fees, contract services and other related costs. Research and development costs, including up-front fees and milestones paid to collaborators, are also expensed as incurred. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense. The Company accrues costs for clinical trial activities based upon estimates of the services received and related expenses incurred that have yet to be invoiced by the contract research organizations, clinical study sites, laboratories, consultants, or other clinical trial vendors that perform the activities. The Company recognizes the reimbursement associated with collaborative activities to its collaborative partners as research and development expense in the period the services are provided. Cost of license and royalty revenue Cost of license and royalty revenue represents expense associated with amounts owed to third parties as a result of revenue recognized under the Company’s out-license arrangements. Stock-based compensation The Company’s share-based compensation programs grant awards that have included stock options, restricted stock units, restricted stock awards, and shares issued under its employee stock purchase plan. Grants are awarded to employees, and non-employees, including directors. The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees, directors, and non-employees with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees and non-employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees and non-employees based on the fair value of the award on a straight-line basis over the associated service period of the award. The Company estimates the fair value of its option awards to employees, directors, and non-employees using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of complete company-specific historical and implied volatility data for the full expected term of the stock-based awards, the Company bases its estimate of expected volatility on a representative group of publicly traded companies in addition to its own volatility data. For these analyses, the Company selected companies with comparable characteristics to its own, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. As a result of the adoption of ASU 2016-09, Improvements to Employee Share-Based Payment Accounting Prior to the adoption of Accounting Standards Update (“ASU”) No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Interest income (expense), net Interest income (expense), net consists primarily of interest income earned on investments, net of amortization of premium and accretion of discount which is partially offset by interest expense on the Company’s 60 Binney Street financing lease obligation. Interest income was approximately $30.1 million, $9.5 million, and $3.8 million for the years ended December 31, 2018, 2017, and 2016, respectively. Interest expense was $15.5 million, $11.4 million, and $0 for the years ended December 31, 2018, 2017, and 2016, respectively. Please refer to Note 8, “Commitments and contingencies,” Other income (expense), net Other income (expense), net consists primarily of unrealized gains on equity securities, gains and losses on the disposal of fixed assets, realized gains and losses on debt securities, and gains and losses on foreign currency transactions. Net loss per share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation. The Company follows the two-class method when computing net loss per share in periods when issued shares that meet the definition of participating securities are outstanding. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to received dividends as if all income for the period had been distributed. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders when participating securities are outstanding, losses are not allocated to the participating securities. Income taxes Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are ex

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Marketable securities	3. Marketable securities The following table summarizes the marketable securities held at December 31, 2018 and 2017 (in thousands): Description Amortized Cost / Cost Unrealized gains Unrealized losses Fair value December 31, 2018 U.S. government agency securities and treasuries $ 1,459,649 $ 963 $ (3,011 ) $ 1,457,601 Certificates of deposit 9,080 — — 9,080 Equity securities 20,017 2,150 — 22,167 Total $ 1,488,746 $ 3,113 $ (3,011 ) $ 1,488,848 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 No available-for-sale debt securities held as of December 31, 2018 or 2017 had remaining maturities greater than three years.

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 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 December 31, 2018 and 2017 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2018 Assets: Cash and cash equivalents $ 402,579 $ 348,638 $ 53,941 $ — Marketable securities: U.S. government agency securities and treasuries 1,457,601 — 1,457,601 — Certificates of deposit 9,080 — 9,080 — Equity securities 22,167 22,167 — — Total assets $ 1,891,427 $ 370,805 $ 1,520,622 $ — Liabilities: Contingent consideration $ 5,230 $ — $ — $ 5,230 Total liabilities $ 5,230 $ — $ — $ 5,230 December 31, 2017 Assets: Cash and cash equivalents $ 758,505 $ 758,505 $ — $ — Marketable securities: U.S. government agency securities 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 and cash equivalents The Company considers all highly liquid securities with original final maturities of 90 days or less from the date of purchase to be cash equivalents. As of December 31, 2018, cash and cash equivalents comprise funds in cash, U.S. treasury securities, U.S. government agency securities, and money market accounts. As of December 31, 2017, cash and cash equivalents comprise funds in cash and money market accounts. Marketable securities The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2018 and 2017, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s debt securities. The aggregate fair value of debt securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2018 and 2017 was $787.5 million and $704.1 million, respectively. As of December 31, 2018, and 2017, there were $315.3 million and $134.4 million in securities held by the Company in an unrealized loss position for more than twelve months, respectively. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of December 31, 2018 and 2017. The Company holds equity securities with an aggregate fair value of $22.2 million at December 31, 2018 within short-term marketable securities on its consolidated balance sheet. The Company has recorded a $2.2 million unrealized gain during the year ended December 31, 2018 related to its equity securities, which is included in other income (expense), net on the consolidated statements of operations and comprehensive loss. Contingent consideration On June 30, 2014, the Company acquired Pregenen. In connection with the acquisition of Pregenen, the Company recorded contingent consideration pertaining to the amounts potentially payable to Pregenen’s former equityholders. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the consolidated statements of operations and comprehensive loss. Contingent consideration may change significantly as development progresses and additional data are obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. In evaluating the fair value information, considerable judgment is required to interpret the market data used to develop the estimates. The estimates of fair value may not be indicative of the amounts that could be realized in a current market exchange. Accordingly, the use of different market assumptions and/or different valuation techniques could result in materially different fair value estimates. The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from 2021 to 2028 and discount rates ranging from 15.0% to 15.8%. Significant increases or decreases in any of these inputs would result in a significantly higher or lower fair value measurement. The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations which include Level 3 inputs (in thousands): Year ended December 31, 2018 2017 Beginning balance $ 2,231 $ 7,756 Additions — — Changes in fair value 2,999 (525 ) Payments — (5,000 ) Ending balance $ 5,230 $ 2,231 As of December 31, 2018 and 2017, the remaining $5.2 million, and $2.2 million, respectively, of contingent consideration obligation is reflected as a non-current liability in the consolidated balance sheet. A $5.0 million preclinical milestone was achieved and paid to the former equityholders of Pregenen in the year ended December 31, 2017. Please refer to Note 8, “Commitments and contingencies,”

Property, plant and equipment,

Property, plant and equipment, net	12 Months Ended
	Dec. 31, 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 December 31, 2018 2017 Land $ 1,210 $ 1,210 Building 180,094 164,414 Computer equipment and software 6,365 5,134 Office equipment 5,584 4,478 Laboratory equipment 35,693 24,914 Leasehold improvements 183 116 Construction-in-progress 46,669 15,189 Total property, plant and equipment 275,798 215,455 Less accumulated depreciation and amortization (29,176 ) (15,849 ) Property, plant and equipment, net $ 246,622 $ 199,606 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 December 31, 2018, and 2017, includes $40.4 million and $12.9 million related to the North Carolina manufacturing facility, respectively. As of December 31, 2018, and 2017, total property, plant and equipment includes $168.1 million, and $164.4 Depreciation and amortization expense related to property, plant and equipment was $13.4 “Commitments and contingencies,”

Restricted cash and other non-c

Restricted cash and other non-current assets	12 Months Ended
	Dec. 31, 2018
Other Assets Noncurrent [Abstract]
Restricted cash and other non-current assets	6. Restricted cash and other non-current assets Restricted cash As of December 31, 2018, and 2017, the Company maintained letters of credit of $ (in thousands) As of December 31, 2018 As of December 31, 2017 60 Binney Street lease $ 13,763 $ 13,763 Other leases 757 — Total restricted cash $ 14,520 $ 13,763 Subject to the terms of the 60 Binney Street Lease agreement and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease by $1.5 million on the fourth, fifth and sixth anniversaries of the date the Company occupies the building. Other non-current assets In August 2018, the Company entered into a license and collaboration agreement with a third-party collaborator and paid a non-refundable, non-creditable upfront payment of $20.0 million, $14.5 million of which was recorded as prepaid research and development services and the remaining $5.5 million of which was expensed within research and development expense in the third quarter of 2018, as this payment represented an access fee for technology that has no clear alternate future use under ASC 730-10, Research and Development eceivables and other current assets estricted cash and other non-current assets in the consolidated balance sheet as of December 31, 2018.

Accrued expenses and other curr

Accrued expenses and other current liabilities	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued expenses and other current liabilities	7. Accrued expenses and other current liabilities Accrued expenses and other current liabilities consist of the following (in thousands): As of December 31, 2018 2017 Employee compensation $ 28,567 $ 19,657 Accrued manufacturing costs 21,618 8,480 Accrued clinical and contract research organization costs 11,891 8,082 Accrued license and milestone fees 7,739 4,584 Accrued professional fees 1,830 1,402 Other accrued goods and services 25,051 12,971 Financing lease obligation, current portion 1,424 1,051 Other 1,273 838 Total accrued expenses and other current liabilities $ 99,393 $ 57,065

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	8. Commitments and contingencies Operating lease commitments On June 3, 2016, the Company entered into a 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 was 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 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. Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. The Company also executed a $9.2 million letter of credit upon signing the 60 Binney Street Lease, which was required to be collateralized with a bank account at a financial institution in accordance with the 60 Binney Street Lease agreement. This letter of credit was increased to $13.8 million during the third quarter of 2016 as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease back to $9.2 million over time. The 60 Binney Street Lease will continue until March 31, 2027. Pursuant to a work letter entered into in connection with the 60 Binney Street Lease, the landlord will contribute an aggregate of $42.4 million toward the cost of construction and tenant improvements for the Building. The purpose of the 60 Binney Street Lease was to replace the Company’s previously leased premises at 150 Second Street and 215 First Street in Cambridge, Massachusetts, both of which were fully exited in the first half of 2017. The Company has the option to extend the 60 Binney Street Lease for two successive five-year terms. The Company occupied the Building beginning on March 27, 2017. Because the Company is involved in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building, among other items, the Company is deemed for accounting purposes to be the owner of the Building during the construction period. Accordingly, construction costs that have been 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 “Financing lease obligation, net of current portion” on the Company’s consolidated balance sheets. Tenant improvement costs that are reimbursable by the landlord and have not yet been paid to the Company are recorded in “Tenant improvements receivable” on the Company’s consolidated balance sheets. Tenant improvement costs that are not reimbursable by the landlord are recorded in “Property, plant and equipment, net” on the Company’s consolidated balance sheets. The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the portion of the building in service over a useful life of 40 years and incurred interest expense related to the financing obligation of $15.5 million, and $11.4 million, for the years ended December 31, 2018 and 2017, respectively. The Company made $1.0 million and $0.6 million in principal payments, which are included in operating expense, for the years ended December 31, 2018 and 2017, respectively. The Company bifurcates its lease payments pursuant to the 60 Binney Street Lease into (i) a portion that is allocated to the Building and (ii) a portion that is allocated to the land on which the Building is located, which is recorded as rental expense. The Company began making lease payments pursuant to the 60 Binney Street Lease in March 2017. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced upon execution of the 60 Binney Street Lease in September 2015. During the years ended December 31, 2018, 2017 and 2016, the Company recognized $1.9 million, $1.9 million, and $1.9 million of rental expense attributable to the land, respectively. As of December 31, 2018, future minimum commitments under the 60 Binney Street Lease and facility operating leases were as follows (in thousands): Years ended December 31, 60 Binney Street lease Other operating leases (1) Total lease commitments 2019 $ 18,974 $ 17,511 $ 36,485 2020 19,306 18,727 38,033 2021 19,643 18,777 38,420 2022 19,987 13,332 33,319 2023 20,337 13,384 33,721 2024 and thereafter 68,551 29,313 97,864 Total minimum lease payments $ 166,798 $ 111,044 $ 277,842 (1) Includes the lease of the Company’s lab and office space in Seattle, Washington, office space in Zug, Switzerland and two embedded operating leases at contract manufacturing organizations. For the 60 Binney Street Lease, the table above sets forth the future minimum rental payments that the Company is obligated to pay, including amounts reflected on the consolidated balance sheet as part of the balance under the caption “Accrued expenses and other current liabilities” and “Financing lease obligation, net of current portion.” The Company commenced rental payments in April 2017. Rent expense is calculated on a straight-line basis over the term of the lease. Rent expense recognized under all leases, including additional rent charges for utilities, parking, maintenance, and real estate taxes, and including rental expense attributable to the 60 Binney Street Lease land was $9.8 Contingent consideration related to business combinations On June 30, 2014, the Company acquired Pregenen. During 2017, one milestone under the Stock Purchase Agreement was achieved, which resulted in a $5.0 million payment to the former equityholders of Pregenen during 2017. During 2016, two milestones were achieved, which resulted in a $5.0 million payment to the former equityholders of Pregenen. The Company may be required to make up to an additional $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. See 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 December 31, 2018 and December 31, 2017 or royalties on future sales of specified products, which includes the collaboration agreement entered into with Regeneron Pharmaceuticals, Inc. (“Regeneron”) during the year ended December 31, 2018. Additionally, the Company is party to various contracts with contract research organizations and contract manufacturers that generally provide for termination on notice, with the exact amounts in the event of termination to be based on the timing of the termination and the terms of the agreement. Please refer to Note 10, “Collaborative Arrangements,” Based on our development plans as of December 31, 2018, we may be obligated to make future development, regulatory and commercial milestone payments and royalty payments on future sales of specified products associated with our collaboration and license agreements. Payments under these agreements generally become due and payable upon achievement of such milestones or sales. When the achievement of these milestones or sales have not occurred, such contingencies are not recorded in the Company’s financial statements and are excluded from the table below. The Company has various manufacturing development and license agreements to support clinical and commercial product needs. The Company incurred $44.0 million of expense related to these agreements in 2018. The following table presents non-cancelable contractual obligations arising from these arrangements: Years ended December 31, Purchase commitment 2019 $ 69,827 2020 74,529 2021 14,358 2022 24,311 2023 25,040 2024 and thereafter 25,792 Total purchase commitments $ 233,857 The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Common stock and preferred stoc

Common stock and preferred stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Common stock and preferred stock	9. Common stock and preferred stock The Company is authorized to issue 125.0 million shares of common stock. Holders of common stock are entitled to one vote per share. Holders of common stock are entitled to receive dividends, if and when declared by the Company’s Board of Directors, and to share ratably in the Company’s assets legally available for distribution to the Company’s shareholders in the event of liquidation. Holders of common stock have no preemptive, subscription, redemption or conversion rights. As of December 31, 2018, and 2017, the Company had 54.7 million and 49.4 million shares of common stock issued and outstanding, respectively. In December 2016, the Company sold 3.3 million shares of common stock through an underwritten public offering at a price of $76.00 per share for aggregate net proceeds of $234.7 million. In June 2017, the Company sold 4.4 million shares of common stock (inclusive of 0.6 million shares of common stock sold by the Company pursuant to the full exercise of an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $105.00 per share for aggregate net proceeds of $436.8 million. In December 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 (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 $162.50 per share for aggregate net proceeds of $600.6 million. The Company is authorized to issue 5.0 million shares of preferred stock in one or more series and to fix the powers, designations, preferences and relative participating option or other rights thereof, including dividend rights, conversion rights, voting rights, redemption terms, liquidation preferences and the number of shares constituting any series, without any further vote or action by the Company’s shareholders. As of December 31, 2018 and 2017, the Company had no shares of preferred stock issued or outstanding. Reserved for future issuance The Company has reserved for future issuance the following number of shares of common stock (in thousands): As of December 31, 2018 2017 Options to purchase common stock 4,643 3,755 Restricted stock units 931 477 2013 Stock Option and Incentive Plan 1,458 1,550 2013 Employee Stock Purchase Plan 172 188 7,204 5,970

Collaborative Arrangements

Collaborative Arrangements	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborative Arrangements	10. Collaborative arrangements To date, the Company’s collaboration revenue has been generated from its collaboration arrangement with Celgene, which was originally entered into in March 2013 and was subsequently amended in June 2015, and from its collaboration arrangement with Regeneron, as further described below. Celgene Celgene Original Collaboration Agreement On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Celgene Collaboration Agreement”) with Celgene to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine, for use in the collaboration. Under the terms of the Celgene Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. The Company was responsible for conducting discovery, research and development activities through completion of phase 1 clinical studies, if any, during the initial term of the Celgene 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 Celgene 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. Celgene Amended Collaboration Agreement On June 3, 2015, the Company and Celgene amended and restated the Collaboration Agreement (the “Amended Celgene Collaboration Agreement”). Under the Amended Celgene Collaboration Agreement, the parties narrowed the focus of the collaboration to exclusively work on anti- B-cell maturation antigen (“BCMA”) product candidates for a new three-year term. In connection with the Amended Celgene Collaboration Agreement, the Company received an up-front, one-time, non-refundable, non-creditable payment of $25.0 million to fund research and development under the collaboration. The collaboration will continue to be governed by the JSC. Under the terms of the Amended Celgene 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 1 clinical study of such product candidates. On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial phase 1 clinical study for such product candidate (the “Option Period”), the Company has 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. Celgene 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 Celgene Collaboration Agreement, pursuant to an executed license agreement (“bb2121 License Agreement”) entered into by the parties on February 16, 2016 and paid the associated $10.0 million option fee. Pursuant to the bb2121 License Agreement, Celgene is responsible for development and related funding of bb2121 after the substantial completion of the phase 1 clinical trial. The Company is responsible for the manufacture of vector and associated payload throughout development and upon Celgene’s request, commercialization, the costs of which are reimbursed by Celgene in accordance with the terms of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement, as further described below. Celgene is responsible for the manufacture of drug product throughout development and commercialization. Celgene 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. Celgene is responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. 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 is entitled to receive tiered royalty payments ranging from the mid-single digits to low-teens based on a percentage of net sales generated outside of the U.S., subject to certain reductions. Celgene 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 Celgene 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 1 clinical trial. The Company is responsible for the manufacture of vector and associated payload throughout development and, upon Celgene’s request, commercialization. Expenses incurred by the Company are fully reimbursable by Celgene at cost plus a mark-up. Throughout both development and commercialization, Celgene is responsible for the manufacture of drug product. The Company currently expects it will exercise its option to co-develop and co-promote bb21217 within the U.S. The Company’s election to co-develop and co-promote bb21217 must be made by the substantial completion of the on-going phase 1 clinical trial of bb21217. If elected, the Company expects the responsibilities of the parties to remain largely unchanged, however, the Company expects it will share equally in all profits and losses relating to developing, commercializing and manufacturing bb21217 within the U.S. and to have the right to participate in the development and promotion of bb21217 in the U.S. Celgene would be responsible for the costs incurred to manufacture vector and associated payload for use outside of the U.S., plus a markup. 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 allows 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 Celgene Collaboration Agreement was executed in June 2015 given the product candidate had been formally nominated by the JSC and that substantially all investigational new drug application, or IND, enabling activities had been completed by that time. In making this determination, the Company also considered the option price relative to the value of the underlying license. Celgene’s exercise of this material right in February 2016 was determined to represent a contract modification and represents the last contract modification prior to the adoption of Topic 606. As a result, the Celgene Collaboration Agreement, Amended Celgene Collaboration Agreement, and bb2121 CCPS 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 due primarily to the significant uncertainty regarding whether any additional product candidates would be identified under the Amended Celgene Collaboration Agreement. Therefore, the license to the Company’s second product candidate, bb21217, which was executed in September 2017, is accounted for as a separate contract. Refer below for discussion of the bb21217 accounting analysis. As of the February 2016 contract modification date, the Company concluded the arrangement contained the following promised goods and services: (i) research and development services, (ii) a license to 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 that time Celgene had 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 given that Celgene can benefit from the research and development services on their own and such services are distinct within the context of the contract. bb2121 transaction price The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of December 31, 2018: (in thousands) bb2121 transaction price as of December 31, 2018 Up-front non-refundable payment - Celgene Collaboration Agreement $ 75,000 Up-front non-refundable payment - Amended Celgene Collaboration Agreement 25,000 bb2121 license fee - bb2121 License Agreement 10,000 Estimated variable consideration 85,723 $ 195,723 (in thousands) Allocation of transaction price to performance obligations Transaction price unsatisfied as of December 31, 2018 bb2121 research and development services $ 38,647 $ — bb2121 license and manufacturing services 157,076 50,599 $ 195,723 $ 50,599 The estimated variable consideration of $85.7 million relates 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. All of the clinical and regulatory milestones 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 clinical trials, the licensee’s efforts, or the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Celgene and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, at each reporting period and as uncertain events are resolved or other changes in circumstances occur. bb2121 research and development services The Company allocated $38.6 million of the transaction price to the research and development services. The Company satisfied this performance obligation as the research and development services were performed. The Company determined that the period of performance of the research and development services was three years through projected initial phase 1 clinical study substantial completion, or through May 2018. The Company recognized revenue related to research and development services performed using an input method by calculating costs incurred at each period end relative to total costs expected to be incurred. Although the Company fully satisfied this performance obligation during the second quarter of 2018, any changes to the total transaction price following the completion of this performance obligation in May 2018 will be allocated to the performance obligations under the arrangement based on a relative SSP basis and therefore the allocation of any changes to the total transaction price may impact the revenue recognized for this performance obligation in the period of change. The following table summarizes the revenue recognized related to the bb2121 research and development services for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, (in thousands) 2018 (1) 2017 (1) 2016 (1) bb2121 research and development services revenue $ 5,751 $ 6,208 $ 6,155 $ 5,751 $ 6,208 $ 6,155 (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” bb2121 license and manufacturing services The Company allocated $157.1 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 collaboration revenue for its U.S. manufacturing services by analogy to Topic 606. The portion of Celgene’s U.S. development costs that the Company 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. The following table summarizes the net collaboration revenue recognized or expense incurred related to the combined performance obligation for the license and vector manufacturing of bb2121 in the U.S. for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, bb2121 license and manufacturing services - U.S. (in thousands) 2018 (2) 2017 (2) 2016 (2)(3) ASC 808 bb2121 license and manufacturing revenue - U.S. (1) $ 6,255 $ 4,905 $ — ASC 808 bb2121 license and manufacturing research and development expense - U.S. (1) $ 8,689 $ 3,037 $ — (1) As noted above, the calculation of collaboration revenue or research and development expense to be recognized for the Company’s combined performance obligation for its license and vector manufacturing of bb2121 in the U.S. is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. (2) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (3) Revenue recognition for the bb2121 license and manufacturing services for Celgene commenced in 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016. Revenue related to the combined unit of accounting for its non-US license and vector manufacturing services is accounted for in accordance with Topic 606. The following table summarizes the revenue recognized related to the combined unit of accounting for the bb2121 non-US license and vector manufacturing services for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, bb2121 license and manufacturing services - outside of U.S. (in thousands) 2018 (1) 2017 (1) 2016 (1)(2) ASC 606 license and manufacturing revenue - outside of U.S. $ 35,900 $ 10,372 $ — (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (2) Revenue recognition for the bb2121 license and manufacturing services for Celgene commenced in 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016. As of December 31, 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 $50.6 million, which the Company expects to recognize as revenue as manufacturing services are provided through the remaining development period which is estimated to be through 2020. As of December 31, 2018, the Company had $23.0 million of deferred revenue 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 Celgene 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 given that Celgene can benefit from the research and development services on their own and such services are distinct within the context of the contract . bb21217 transaction price The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of December 31, 2018: (in thousands) bb21217 transaction price as of December 31, 2018 bb21217 license fee - bb21217 License Agreement $ 15,000 Estimated variable consideration 26,687 $ 41,687 (in thousands) Allocation of transaction price to performance obligations Transaction price unsatisfied as of December 31, 2018 bb21217 research and development services $ 5,444 $ 1,839 bb21217 license and manufacturing services 36,243 36,243 $ 41,687 $ 38,082 The estimated variable consideration of $26.7 million relates 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. All of the clinical and regulatory milestones 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, or the receipt of regulatory approval. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Celgene and therefore are recognized at the later of when the performance obligation is satisfied, or the related sales occur. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, each reporting period and as uncertain events are resolved or other changes in circumstances occur. bb21217 research and development services The Company allocated $5.4 million of the transaction price to the research and development services. 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 1 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 following table summarizes the revenue recognized related to the bb21217 research and development services for the years ended December 31, 2018, 2017, and 2016 For the years ended December 31, (in thousands) 2018 (1) 2017 (1) 2016 (1)(2) bb21217 research and development services revenue $ 2,884 $ 721 $ — $ 2,884 $ 721 $ — (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (2) Celgene in-licensed bb21217 in September 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016. As of December 31, 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 $1.8 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 December 31, 2018, the manufacturing services for bb21217 had not yet commenced. Therefore, no amounts have been recognized for the combined performance obligation in the consolidate statement of operations for the years ended December 31, 2018, 2017, and 2016. 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 of remaining deferred revenue as of December 31, 2018 associated with the combined performance obligation consisting of the bb21217 license and manufacturing services. Contract assets and liabilities – bb2121 and bb21217 The Company receives payments from its collaborative partners based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until such time as the Company satisfies its performance obligations under these arrangements. A contract asset is a conditional right to consideration in exchange for goods or services that the Company has transferred to a customer. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The following table presents changes in the balances of the Company’s Celgene receivables and contract liabilities during the twelve months ended December 31, 2018: (in thousands) Balance at beginning of period under Topic 606 Additions Deductions Balance at end of period under Topic 606 Receivables $ 4,635 $ 8,570 $ (6,677 ) $ 6,528 Contract liabilities: Payable included in accrued expenses $ — $ 8,031 $ (8,031 ) $ — Deferred revenue $ 76,812 $ — $ (41,873 ) $ 34,939 The change in the receivables balance for the year ended December 31, 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 December 31, 2018, the Company has a receivable given that the Company’s U.S. development costs incurred in the fourth quarter of 2018 for which Celgene is responsible are in excess of Celgene’s U.S. development costs for which the Company is responsible. The decrease in deferred revenue during the year ended December 31, 2018 is primarily driven by amounts recognized for the combined performance obligation consisting of the bb2121 license and manufacturing services. During the year ended December 31, 2018, $41.9 million of the deferred revenue balance at the beginning of the period was released from deferred revenue, of which $32.1 million was recognized as collaboration revenue and $9.8 million was recorded as contra-research and development expense. Regeneron Regeneron Collaboration Agreement On August 3, 2018, the Company entered into a Collaboration Agreement (the “Regeneron Collaboration Agreement”) with Regeneron pursuant to which the parties will apply their respective technology platforms to the discovery, development, and commercialization of novel immune cell therapies for cancer. On August 24, 2018, following the completion of required regulatory reviews, the Regeneron Collaboration Agreement became effective. Under the terms of the agreement, the parties will leverage Regeneron’s proprietary platform technologies for the discovery and characterization of fully human antibodies, as well as T cell receptors directed against tumor-specific proteins and peptides and the Company will contribute its field-leading expertise in gene therapy. In accordance with the Regeneron Collaboration Agreement, the parties jointly selected six initial targets and intend to equally share the costs of research up to the point of submitting an IND application for a potential gene therapy product directed to a particular target. Additional targets may be selected during the five-year research collaboration term as agreed to by the parties. Regeneron will accrue a certain number of option rights exercisable against targets as the parties reach certain milestones under the terms of the agreement. Upon the acceptance of an IND for the first product candidate directed to a target, Regeneron will have the right to exercise an option for co-development/co-commercialization of product candidates directed to such target on a worldwide or applicable opt-in territory basis, with certain exceptions. Where Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories. Outside of the applicable opt-in territories, the target becomes a licensed target and Regeneron would be eligible to receive, with respect to any resulting product, milestone payments of up to $130.0 million per product and royalties on net sales outside of the applicable opt-in territories at a rate ranging from the mid-single digits to low-double digits. Where Regeneron does not exercise its option, or does not have an option to a target, the target would also become a licensed target. Either party may terminate a given research program directed to a particular target for convenience, and the other party may elect to continue such research program at its expense, receiving applicable cross-licenses. The terminating party will receive licensed product royalties and milestone payments on the potential applicable gene therapy products. Where the Company terminates a given research program for convenience, and Regeneron elects to continue such research program, the parties will enter into a transitional services agreement. Under certain conditions, following its opt-in, Regeneron may terminate a given collaboration program and the Company may elect to continue the development and commercialization of the applicable potential gene therapy products as licensed products. Regeneron Share Purchase Agreement A Share Purchase Agreement (“SPA”) was entered into by the parties on August 3, 2018. On August 24, 2018, the closing date of the transaction,

License and Royalty Revenue

License and Royalty Revenue	12 Months Ended
	Dec. 31, 2018
License And Royalty Revenue [Abstract]
License and Royalty Revenue	11. Novartis Pharma AG On April 26, 2017, the Company entered into a worldwide license agreement with Novartis. Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including Kymriah (formerly known as CTL019), Novartis’s anti-CD19 CAR T therapy. At contract inception, financial terms of the agreement included a $7.5 million payment upon execution, $7.5 million of potential future milestone payments associated with regulatory 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 upon transition. The Company identified only one performance obligation, consisting of the license, which was satisfied at contract inception. Accordingly, the nonrefundable license fee of $7.5 million was recognized as revenue upon contract execution in the second quarter of 2017 and a $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. 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 until such time as regulatory approvals are received. 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. 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 year ended December 31, 2018, the Company recognized royalty revenue of $2.2 million. During the year ended December 31, 2017, the Company recognized license revenue of $10.0 million in connection with this arrangement, as there were no other undelivered elements in the arrangement. The Company also recognized $0.1 million of royalty revenue in connection with this arrangement. The associated cost of license and royalty revenue for the year ended December 31, 2018 was $0.9 million. For the year ended December 31, 2017, the cost of license and royalty revenue was $1.4 million. As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” Orchard Therapeutics Limited (assigned by GlaxoSmithKline Intellectual Property Development Limited) On April 28, 2017, the Company entered into a worldwide license agreement with GlaxoSmithKline Intellectual Property Development Limited (“GSK”). Under the terms of the agreement, GSK non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize gene therapies for Wiscott-Aldrich syndrome and metachromatic leukodystrophy, two rare genetic diseases. Financial terms of the agreement include a nonrefundable upfront payment of $3.0 million as well as $1.3 million of potential milestone payments for each marketing authorization for each indication in any country as well as low single digit royalties on net sales of covered products. 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 upon transition 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. 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 until such time as regulatory approvals are received. During the year ended December 31, 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 year ended December 31, 2018. 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. For the year ended December 31, 2017 the cost of license and royalty revenue was $0.1 million. Given there was no revenue recognized under this arrangement in the year ended December 31, 2018, there was no associated cost of license and royalty revenue. As noted in Note 2, “Summary of significant accounting policies and basis of presentation,”

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible Assets	12. Intangible assets The Company’s intangible assets consist entirely of developed technology, obtained through its acquisition of Pregenen, a privately-held biotechnology company. As a result, the Company obtained gene editing and cell signaling technology with a broad range of potential therapeutic applications. The Company considered the intangible asset acquired to be developed technology, as at the date of the acquisition it could be used the way it was intended to be used in certain ongoing research and development activities. The gene editing platform intangible asset is being amortized to research and development expense over its expected useful life of approximately eight years from the date of the acquisition. Amortization expense for the gene editing platform intangible asset was $3.8 2017 was $16.9 The intangible asset will continue to be amortized on a straight-line basis over its remaining useful life of 3.5 years.

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation	13. Stock-based compensation On June 3, 2013, the Company’s board of directors adopted its 2013 Stock Option and Incentive Plan (“2013 Plan”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on June 24, 2013. The 2013 Plan replaces the 2010 Stock Option and Grant Plan (“2010 Plan”). The 2013 Plan allows for the granting of incentive stock options, non-qualified stock options, restricted stock units and restricted stock awards to the Company’s employees, members of the board of directors, and consultants of the Company. The Company initially reserved 955,000 shares of its common stock for the issuance of awards under the 2013 Plan. The 2013 Plan provides that the number of shares reserved and available for issuance under the 2013 Plan will automatically increase each January 1, beginning on January 1, 2014, by four percent of the outstanding number of shares of common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. In January 2018 and January 2019, the number of common stock available for issuance under the 2013 Plan was increased by approximately 2.0 million and 2.2 million shares, respectively, as a result of this automatic increase provision. Any options or awards outstanding under the Company’s previous stock option plans, including both the 2010 Plan and the Second Amended and Restated 2002 Employee, Director and Consultant Stock Plan (“2002 Plan”), at the time of adoption of the 2013 Plan remain outstanding and effective. The shares of common stock underlying any awards that are forfeited, canceled, repurchased, expire or are otherwise terminated (other than by exercise) under the 2002 Plan and 2010 Plan are added to the shares of common stock available for issuance under the 2013 Plan. As of December 31, 2018, the total number of common stock that may be issued under all plans is 1.5 The Company does not currently hold any treasury shares. Upon stock option exercise, the Company issues new shares and delivers them to the participant. Stock-based compensation expense The Company recognized stock-based compensation expense totaling $110.8 million, $53.3 million, and $39.8 million during the years ended December 31, 2018, 2017 and 2016, respectively. Stock-based compensation expense recognized by award type is as follows (in thousands): Year ended December 31, 2018 2017 2016 Stock options $ 83,449 $ 42,262 $ 33,966 Restricted stock units 26,628 10,495 5,374 Employee stock purchase plan 759 525 416 $ 110,836 $ 53,282 $ 39,756 Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Year ended December 31, 2018 2017 2016 Research and development $ 54,422 $ 26,633 $ 19,690 General and administrative 56,414 26,649 20,066 $ 110,836 $ 53,282 $ 39,756 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 December 31, 2018 and none of which vested during the year ended December 31, 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 year ended December 31, 2018 related to these awards and may recognize up to $37.9 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 December 31, 2018, there was $199.8 million, $76.4 million and $0.1 million of unrecognized compensation expense related to unvested stock options, restricted stock units, exclusive of those with service and performance conditions described above, and the employee stock purchase plan, respectively, that is expected to be recognized over a weighted-average period of 2.8, 2.9, and 0.1 years. Stock options The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Year ended December 31, 2018 2017 2016 Expected volatility 75.5 % 78.1 % 74.3 % Expected term (in years) 6.0 6.0 6.0 Risk-free interest rate 2.7 % 2.1 % 1.5 % Expected dividend yield 0.0 % 0.0 % 0.0 % The following table summarizes the stock option activity under the Company’s equity awards plans: Shares (in thousands) Weighted- average exercise price per share Weighted- average contractual life (in years) Aggregate intrinsic value (a) (in Outstanding at December 31, 2017 3,755 $ 67.91 Granted 1,602 $ 185.45 Exercised (575 ) $ 51.82 Canceled or forfeited (139 ) $ 130.93 Outstanding at December 31, 2018 4,643 $ 108.56 7.5 $ 110,812 Exercisable at December 31, 2018 2,200 $ 64.80 6.1 $ 90,785 Vested and expected to vest at December 31, 2018 4,641 $ 108.53 7.5 $ 110,593 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2018. The weighted-average fair values of options granted during 2018, 2017 and 2016 was $125.12, $62.03, and $34.22, respectively. The intrinsic value of options exercised during the years ended December 31, 2018, 2017, and 2016, was $73.1 million, $91.0 million and $15.3 million, respectively. Restricted stock units The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average grant date fair value Unvested balance at December 31, 2017 477 $ 80.72 Granted 673 192.90 Vested (152 ) 86.60 Forfeited (67 ) 149.02 Unvested balance at December 31, 2018 931 $ 155.99 The intrinsic value of restricted stock units vested during the years ended December 31, 2018, 2017, and 2016 was $25.5 million, $8.1 million and $5.3 million, respectively. Employee Stock Purchase Plan On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 0.2 million shares of the Company’s common stock to participating employees. During the years ended December 31, 2018 and 2017, 16,026

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	14. 401(k) Savings plan In 1997, the Company established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“the 401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. In March 2019, the Company expects to make contributions of approximately $2.4 million related to employee contributions made during 2018. In March 2018, the Company made $1.5 million of contributions related to employee contributions made during 2017. The match contribution is included in accrued expenses and other current liabilities as of December 31, 2018 and 2017. Expense related to the 401(k) Plan totaled $2.4 million, $1.5 million, $1.0 million for the years ended December 31, 2018, 2017, and 2016, respectively.

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income taxes	15. Income taxes The components of loss before income taxes were as follows (in thousands): Year ended December 31, 2018 2017 2016 U.S. $ (440,473 ) $ (266,236 ) $ (210,188 ) Foreign (114,965 ) (69,197 ) (53,931 ) Total $ (555,438 ) $ (335,433 ) $ (264,119 ) The provision for (benefit from) income taxes were as follows (in thousands): Year ended December 31, 2018 2017 2016 Current: Federal $ — $ — $ — State 324 115 — Foreign 222 95 — Deferred: Federal (307 ) — (588 ) State (52 ) — (24 ) Foreign — — — Total income tax expense (benefit) $ 187 $ 210 $ (612 ) A reconciliation of income tax provision (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate (provision) benefit as reflected in the financial statements is as follows: Year ended December 31, 2018 2017 2016 Federal income tax expense at statutory rate 21.0 % 34.0 % 34.0 % State income tax, net of federal benefit 5.1 % 4.3 % 3.3 % Permanent differences 0.9 % 2.7 % (5.3 %) Research and development credit 6.5 % 12.8 % 15.0 % Foreign differential (4.4 %) (6.9 %) (7.0 %) Federal tax rate change 0.1 % (31.6 %) 0.0 % Other (0.1 %) (0.8 %) 0.0 % Change in valuation allowance (29.1 %) (14.6 %) (39.9 %) Effective income tax rate (expense) benefit 0.0 % (0.1 %) 0.1 % For the years ended December 31, 2018, 2017 and 2016, the Company recognized an income tax (expense) benefit of $(0.2) million or 0.0%, $(0.2) million or (0.1)%, and $0.6 million or 0.1%, respectively. The Company did not recognize any significant tax expense for the years ended December 31, 2018, 2017, or 2016 as the Company was subject to a full valuation allowance. Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands): Year ended December 31, 2018 2017 Deferred tax assets: U.S. net operating loss carryforwards (federal and state) $ 298,701 $ 194,160 Tax credit carryforwards (federal and state) 167,517 131,289 Capitalized license fees and research and development expenses 18,083 13,629 60 Binney Street lease 42,059 42,025 Deferred revenue 21,442 12,795 Stock-based compensation 31,858 19,519 Accruals and other 8,886 6,262 Total deferred tax assets 588,546 419,679 Intangible assets (3,579 ) (4,567 ) Fixed assets (41,472 ) (42,062 ) Less valuation allowance (543,495 ) (373,050 ) Net deferred taxes $ — $ — A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. The valuation allowance increased on a net basis by approximately $170.4 million during the year ended December 31, 2018 due primarily to net operating losses, tax credit carryforwards, and stock-based compensation. As of December 31, 2018, 2017 and 2016, the Company had U.S. federal net operating loss carryforwards of approximately $1,096.7 million, $716.1 million, and $466.8 million, respectively, which may be available to offset future income tax liabilities. Of the amount as of December 31, 2018, $386.0 million will carryforward indefinitely while $710.7 million will expire at various dates through 2037. As of December 31, 2018, 2017 and 2016, the Company also had U.S. state net operating loss carryforwards of approximately $1,082.1 million, $692.9 million, and $456.8 million, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2038. At December 31, 2016, $195.4 million and $195.4 million of federal and state net operating losses, respectively, related to excess equity based compensation tax deductions, the benefits for which will be recorded to additional paid-in capital when recognized through a reduction of cash taxes paid. As a result of adopting FASB ASU 2016-09 in the first quarter of 2017, the Company recorded a cumulative-effect adjustment to retained earnings of $76.7 million to record a net deferred tax asset relative to these tax attribute carryforwards. The deferred tax asset was offset by a corresponding adjustment to the valuation allowance. As of December 31, 2018, 2017 and 2016, the Company had federal research and development and orphan drug tax credit carryforwards of approximately $156.2 million, $124.1 million, and $83.2 million, respectively, available to reduce future tax liabilities which expire at various dates through 2038. As of December 31, 2018, 2017 and 2016, the Company had state credit carryforwards of approximately $14.3 million, $9.1 million, and $6.0 million, respectively, available to reduce future tax liabilities which expire at various dates through 2033. Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception which it believes has resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code. During the fourth quarter of 2018, the Company completed an analysis of prior year estimates of U.S. research and development and orphan drug tax credits for the years 2013 through 2017. The analysis resulted in an immaterial adjustment to our income tax benefit, which was offset by an adjustment to the valuation allowance. The Company or one of its subsidiaries files income tax returns in the United States, and various state and foreign jurisdictions. The federal, state and foreign income tax returns are generally subject to tax examinations for the tax years ended December 31, 2015 through December 31, 2017. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, state or foreign tax authorities to the extent utilized in a future period. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: (in thousands) Unrecognized tax benefits Balance as of December 31, 2016 $ — Increases for tax positions related to current period — Increases for tax positions related to prior periods — Balance as of December 31, 2017 — Increases for tax positions related to current period 3,370 Increases for tax positions related to prior periods 8,725 Balance as of December 31, 2018 12,095 The unrecognized tax benefits at December 31, 2018, if recognized, would not affect the Company’s effective tax rate due to its full valuation allowance position. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease within the next 12 months. The Company has elected to include interest and penalties related to uncertain tax positions as a component of its provision for income taxes. For the years ended December 31, 2018, 2017 and 2016, the Company’s accrued interest and penalties related to uncertain tax positions were not material. On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted. This law substantially amended the Internal Revenue Code and among other things, permanently reduced the U.S. corporate income tax rate from 35% to 21%. 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, there were no material adjustments recorded and the accounting for these changes has been completed as of December 31, 2018.

Net loss per share

Net loss per share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss per share	16. 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): Year ended December 31, 2018 2017 2016 Outstanding stock options 4,643 3,755 3,735 Restricted stock units 931 477 263 ESPP shares 10 9 11 5,584 4,241 4,009

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected quarterly financial data (Unaudited)	17. Selected quarterly financial data (unaudited) The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2018 First quarter Second quarter Third quarter Fourth quarter Total (in thousands, except per share data) Total revenues $ 15,957 $ 7,851 $ 11,528 $ 19,243 $ 54,579 Total operating expenses 132,586 156,465 161,347 176,204 626,602 Loss from operations (116,629 ) (148,614 ) (149,819 ) (156,961 ) (572,023 ) Net loss (115,126 ) (145,996 ) (145,480 ) (149,023 ) (555,625 ) Net loss per share applicable to common stockholders - basic and diluted $ (2.31 ) $ (2.91 ) $ (2.73 ) $ (2.72 ) $ (10.68 ) 2017 First quarter Second quarter Third quarter Fourth quarter Total (in thousands, except per share data) Total revenues $ 6,832 $ 16,716 $ 7,711 $ 4,168 $ 35,427 Total operating expenses 76,745 84,538 85,369 120,940 367,592 Loss from operations (69,913 ) (67,822 ) (77,658 ) (116,772 ) (332,165 ) Net loss (68,712 ) (70,898 ) (78,805 ) (117,228 ) (335,643 ) Net loss per share applicable to common stockholders - basic and diluted $ (1.68 ) $ (1.73 ) $ (1.73 ) $ (2.52 ) $ (7.71 )

Summary of significant accoun_2

Summary of significant accounting policies and basis of presentation (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying consolidated financial statements have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States GAAP as found in the ASC and ASUs of the FASB. Certain items in the prior year’s consolidated financial statements have been reclassified to conform to the current presentation. As a result, no subtotals in the prior year consolidated financial statements were impacted. Amounts reported are computed based on thousands, except percentages, per share amounts, and as otherwise noted. As a result, certain totals may not sum due to rounding.
Principles of consolidation	Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Company continually assesses whether it is the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in consolidation or deconsolidation of one or more collaborators or partners. In determining whether it is the primary beneficiary of an entity in which the Company has a variable interest, management applies a qualitative approach that determines whether the Company has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates and judgments are used in the following areas, among others: fair value estimates used to assess potential impairment of long-lived assets, including goodwill and intangible assets, financing lease obligations, contingent consideration, stock-based compensation expense, accrued expenses, and income taxes. In addition, estimates and judgments are used in the Company’s accounting for its revenue-generating arrangements, in particular as it relates to determining the standalone selling price of performance obligations, evaluating whether an option to acquire additional goods and services represents a material right, estimating the total transaction price, including estimating variable consideration and the probability of achieving future potential development and regulatory milestones, and the period of performance over which revenue may be recognized.
Foreign currency transaction	Foreign currency translation The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss) in stockholders’ equity. Foreign currency transaction gains and losses are included in other expense, net in the results of operations.
Segment information	Segment information The Company operates in a single segment, focusing on the development of potentially transformative gene therapies for severe genetic diseases and cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources at a global, consolidated level. Therefore, results of our operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. Cash and cash equivalents comprise marketable securities with maturities of less than 90 days when purchased. Cash equivalents are reported at fair value.
Marketable securities	Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. The Company’s marketable securities are maintained by investment managers and consist of U.S. Treasury securities, U.S. government agency securities, equity securities, certificates of deposit, and money market accounts. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Equity securities with readily determinable fair values are also carried at fair value with unrealized gains and losses included in other income (expense), net. Realized gains and losses on both debt and equity securities are determined using the specific identification method and are included in other income (expense), net. The Company classifies equity securities with readily determinable fair values, which would be available for use in its current operations, as current assets even though the Company may not dispose of such marketable securities within the next 12 months. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss.
Concentrations of credit risk and off-balance sheet risk	Concentrations of credit risk and off-balance sheet risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and available-for-sale securities. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s marketable securities primarily consist of U.S. Treasury securities, U.S. government agency securities and certificates of deposit, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure.
Fair value of financial instruments	Fair value of financial instruments The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Fair values are determined utilizing quoted prices for identical or similar assets or liabilities in active markets or other market observable inputs such as interest rates, yield curves and foreign currency spot rates. Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Items measured at fair value on a recurring basis include marketable securities (see Note 3, “Marketable securities” “Fair value measurements” “Fair value measurements”
Business combinations	Business combinations Business combinations are accounted for using the acquisition method of accounting. Using this method, the tangible and intangible assets acquired and the liabilities assumed are recorded as of the acquisition date at their respective fair values. The Company evaluates a business as an integrated set of activities and assets that is capable of being managed for the purpose of providing a return in the form of dividends, lower costs or other economic benefits and consists of inputs and processes that provide or have the ability to provide outputs. In an acquisition of a business, the excess of the fair value of the consideration transferred over the fair value of the net assets acquired is recorded as goodwill. In an acquisition of net assets that does not constitute a business, no goodwill is recognized. The consolidated financial statements include the results of operations of an acquired business after the completion of the acquisition. See Note 4, “Fair value measurements,”
Goodwill	Goodwill Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill to date.
Intangible assets	Intangible assets Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment.
Contingent consideration	Contingent consideration Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and records within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones may be achieved, and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, “Fair value measurements,”
Property, plant and equipment	Property, plant and equipment Property, plant and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 2-5 years Leasehold improvements Shorter of the useful life or remaining lease term The Company records certain costs incurred and reported by a landlord as an asset and corresponding financing lease obligation on the consolidated balance sheets. See Note 8, “ Commitments and contingencies
Impairment of long-lived assets	Impairment of long-lived assets The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets.
Financing lease obligation	Financing lease obligation Beginning in 2015 and through construction completion in 2017, the Company recorded certain estimated construction costs incurred and reported to the Company by the landlord for its 60 Binney Street location as an asset and corresponding financing lease obligation on the consolidated balance sheets because it was deemed to be the owner of the building during the construction period for accounting purposes. Any costs incurred by the Company that have been reimbursed by the landlord or that qualify for reimbursement by the landlord are recorded as an asset and financing lease obligation. Any incremental costs incurred directly by the Company that do not qualify for reimbursement by the landlord are also capitalized. Upon completion of the construction of the building in the first quarter of 2017, the Company evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, the Company is depreciating the building over 40 years and incurring interest expense in its consolidated statement of operations and comprehensive loss related to the financing lease obligation recorded on its consolidated balance sheet. The Company bifurcates its lease payments pursuant to the lease into (i) a portion that is allocated to the financing obligation related to the building and (ii) a portion that is allocated to the land on which the building was constructed. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced in September 2015 and is recorded on a straight-line basis over the initial lease term. See Note 8, “Commitments and contingencies,”
Revenue recognition	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. he identification of material rights requires judgments related to the determination of the value of the underlying license relative to the option exercise price, including assumptions about technical feasibility and the probability of developing a candidate that would be subject to the option rights. 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. 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. 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 generated from its collaboration arrangements with Celgene and Regeneron Pharmaceuticals, Inc. (“Regeneron”), as further described in Note 10, “ Collaborative arrangements 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. Development and regulatory 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. 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 10, “Collaborative arrangements” “License and royalty revenue” Recent accounting pronouncements - Recently adopted
Research and development expenses	Research and development expenses Research and development costs are charged to expense as costs are incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, clinical study and related clinical manufacturing costs, license and milestone fees, contract services and other related costs. Research and development costs, including up-front fees and milestones paid to collaborators, are also expensed as incurred. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense. The Company accrues costs for clinical trial activities based upon estimates of the services received and related expenses incurred that have yet to be invoiced by the contract research organizations, clinical study sites, laboratories, consultants, or other clinical trial vendors that perform the activities. The Company recognizes the reimbursement associated with collaborative activities to its collaborative partners as research and development expense in the period the services are provided.
Cost of license and royalty revenue	Cost of license and royalty revenue Cost of license and royalty revenue represents expense associated with amounts owed to third parties as a result of revenue recognized under the Company’s out-license arrangements.
Stock-based compensation	Stock-based compensation The Company’s share-based compensation programs grant awards that have included stock options, restricted stock units, restricted stock awards, and shares issued under its employee stock purchase plan. Grants are awarded to employees, and non-employees, including directors. The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees, directors, and non-employees with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees and non-employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees and non-employees based on the fair value of the award on a straight-line basis over the associated service period of the award. The Company estimates the fair value of its option awards to employees, directors, and non-employees using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of complete company-specific historical and implied volatility data for the full expected term of the stock-based awards, the Company bases its estimate of expected volatility on a representative group of publicly traded companies in addition to its own volatility data. For these analyses, the Company selected companies with comparable characteristics to its own, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. As a result of the adoption of ASU 2016-09, Improvements to Employee Share-Based Payment Accounting Prior to the adoption of Accounting Standards Update (“ASU”) No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting
Interest income (expense), net	Interest income (expense), net Interest income (expense), net consists primarily of interest income earned on investments, net of amortization of premium and accretion of discount which is partially offset by interest expense on the Company’s 60 Binney Street financing lease obligation. Interest income was approximately $30.1 million, $9.5 million, and $3.8 million for the years ended December 31, 2018, 2017, and 2016, respectively. Interest expense was $15.5 million, $11.4 million, and $0 for the years ended December 31, 2018, 2017, and 2016, respectively. Please refer to Note 8, “Commitments and contingencies,”
Other income (expense), net	Other income (expense), net Other income (expense), net consists primarily of unrealized gains on equity securities, gains and losses on the disposal of fixed assets, realized gains and losses on debt securities, and gains and losses on foreign currency transactions.
Net loss per share	Net loss per share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation. The Company follows the two-class method when computing net loss per share in periods when issued shares that meet the definition of participating securities are outstanding. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to received dividends as if all income for the period had been distributed. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders when participating securities are outstanding, losses are not allocated to the participating securities.
Income taxes	Income taxes Income taxes are recorded in accordance with FASB ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. The Company accrues for potential interest and penalties related to unrecognized tax benefits in income tax expense.
Comprehensive loss	Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on debt securities, foreign currency translation adjustments and other items.
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 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 consolidated balance sheet as of December 31, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 18,602 $ 7,772 $ 10,830 Deferred revenue, net of current portion $ 16,338 $ 3,094 $ 13,244 Accumulated deficit $ (1,498,808 ) $ (10,866 ) $ (1,487,942 ) Impact of Topic 606 adoption on consolidated statement of operations and comprehensive loss for the twelve months ended December 31, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 52,353 $ 13,917 $ 38,436 Research and development expense $ 448,589 $ (4,592 ) $ 453,181 Net loss $ (555,625 ) $ 18,509 $ (574,134 ) Net loss per share - basic and diluted: $ (10.68 ) $ 0.35 $ (11.03 ) Impact of Topic 606 adoption on consolidated statement of cash flows for the twelve months ended December 31, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Net loss $ (555,625 ) $ 18,509 $ (574,134 ) Changes in deferred revenue $ (41,872 ) $ (18,509 ) $ (23,363 ) The most significant change above relates to the Company’s collaboration revenue, which to date has been primarily 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 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 December 31, 2018 2017 2016 Cash and cash equivalents $ 402,579 $ 758,505 $ 278,887 Restricted cash included in receivables and other current assets 364 — 627 Restricted cash included in restricted cash and other non-current assets 14,156 13,763 13,763 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 417,099 $ 772,268 $ 293,277 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 ASU 2018-03, Technical Corrections and Improvements to Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities In February 2018, the FASB issued ASU 2018-03, Technical Corrections and Improvements to Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities (“ASU 2018-03”). The new standard amends the standard ASU 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities , which was issued by the FASB in January 2016 and adopted by the Company effective January 1, 2018. This amendment simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment. In addition, it requires enhanced disclosures about investments. The Company adopted ASU 2018-03 effective July 1, 2018. The adoption of this standard did not have an impact on the Company’s financial position or results of operations upon adoption. ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU 2018-07. The new standard simplifies the accounting for share-based payments to non-employees 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 early adopted ASU 2018-07 effective July 1, 2018 using a cumulative-effect adjustment as of the date of adoption. The impact of adopting this standard was not material. Not yet adopted ASU 2016-02, Leases and ASU 2018-11, Leases, Targeting Improvements 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 most 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. ASU 2016-02 will be effective beginning January 1, 2019 requiring the use of a modified retrospective transition approach applied at the beginning of the earliest comparative period presented in the financial statements. In July 2018, the FASB issued ASU 2018-11, Leases, Targeted Improvements , (“ASU 2018-11”), which contains certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. ASU 2018-11 provides registrants with an option to not restate comparative periods presented in the financial statements. The Company intends to elect this new transition approach using a cumulative-effect adjustment on the effective date of the standard, for which comparative periods will be presented in accordance with the previous guidance in ASC 840, Leases . The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position, results of operations, and related footnotes. The Company expects it will elect to utilize the available package of practical expedients permitted under the transition guidance within the new standard, which does not require the reassessment of the following: i) whether existing or expired arrangements are or contain a lease, ii) the lease classification of existing or expired leases, and iii) whether previous initial direct costs would qualify for capitalization under the new lease standard. Additionally, the Company expects it will make an accounting policy election to keep leases with an initial term of 12 months or less off of its balance sheet. 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 Street in Cambridge, Massachusetts, its laboratory space in Seattle, Washington, its office space in Zug, Switzerland, its equipment leases, and its embedded leases associated with the Company’s contract manufacturing agreements. 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 is effective beginning January 1, 2019. 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 The new standard will be effective beginning January 1, 2019. The adoption of this standard is not expected to have a material impact on the Company’s financial position and results of operations ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, (“ASU 2018-13”). ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, (“ASU 2018-15”) ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”)

Summary of significant accoun_3

Summary of significant accounting policies and basis of presentation (Tables)	12 Months Ended
	Dec. 31, 2018
Estimated Useful Lives of Assets	Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 2-5 years Leasehold improvements Shorter of the useful life or remaining lease term
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:
ASU 2014-09 [Member]
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 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 consolidated balance sheet as of December 31, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Deferred revenue, current portion $ 18,602 $ 7,772 $ 10,830 Deferred revenue, net of current portion $ 16,338 $ 3,094 $ 13,244 Accumulated deficit $ (1,498,808 ) $ (10,866 ) $ (1,487,942 ) Impact of Topic 606 adoption on consolidated statement of operations and comprehensive loss for the twelve months ended December 31, 2018 (in thousands, except per share data) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Collaboration revenue $ 52,353 $ 13,917 $ 38,436 Research and development expense $ 448,589 $ (4,592 ) $ 453,181 Net loss $ (555,625 ) $ 18,509 $ (574,134 ) Net loss per share - basic and diluted: $ (10.68 ) $ 0.35 $ (11.03 ) Impact of Topic 606 adoption on consolidated statement of cash flows for the twelve months ended December 31, 2018 (in thousands) As reported under Topic 606 Adjustments Balances without adoption of Topic 606 Net loss $ (555,625 ) $ 18,509 $ (574,134 ) Changes in deferred revenue $ (41,872 ) $ (18,509 ) $ (23,363 )

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Marketable Securities Held	The following table summarizes the marketable securities held at December 31, 2018 and 2017 (in thousands): Description Amortized Cost / Cost Unrealized gains Unrealized losses Fair value December 31, 2018 U.S. government agency securities and treasuries $ 1,459,649 $ 963 $ (3,011 ) $ 1,457,601 Certificates of deposit 9,080 — — 9,080 Equity securities 20,017 2,150 — 22,167 Total $ 1,488,746 $ 3,113 $ (3,011 ) $ 1,488,848 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)	12 Months Ended
	Dec. 31, 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 December 31, 2018 and 2017 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2018 Assets: Cash and cash equivalents $ 402,579 $ 348,638 $ 53,941 $ — Marketable securities: U.S. government agency securities and treasuries 1,457,601 — 1,457,601 — Certificates of deposit 9,080 — 9,080 — Equity securities 22,167 22,167 — — Total assets $ 1,891,427 $ 370,805 $ 1,520,622 $ — Liabilities: Contingent consideration $ 5,230 $ — $ — $ 5,230 Total liabilities $ 5,230 $ — $ — $ 5,230 December 31, 2017 Assets: Cash and cash equivalents $ 758,505 $ 758,505 $ — $ — Marketable securities: U.S. government agency securities 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): Year ended December 31, 2018 2017 Beginning balance $ 2,231 $ 7,756 Additions — — Changes in fair value 2,999 (525 ) Payments — (5,000 ) Ending balance $ 5,230 $ 2,231

Property, plant and equipment_2

Property, plant and equipment, net (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 2018 2017 Land $ 1,210 $ 1,210 Building 180,094 164,414 Computer equipment and software 6,365 5,134 Office equipment 5,584 4,478 Laboratory equipment 35,693 24,914 Leasehold improvements 183 116 Construction-in-progress 46,669 15,189 Total property, plant and equipment 275,798 215,455 Less accumulated depreciation and amortization (29,176 ) (15,849 ) Property, plant and equipment, net $ 246,622 $ 199,606

Restricted cash and other non_2

Restricted cash and other non-current assets (Tables)	12 Months Ended
	Dec. 31, 2018
Other Assets Noncurrent [Abstract]
Schedule of Collateralized Bank Account of Financial Institution	As of December 31, 2018, and 2017, the Company maintained letters of credit of $ (in thousands) As of December 31, 2018 As of December 31, 2017 60 Binney Street lease $ 13,763 $ 13,763 Other leases 757 — Total restricted cash $ 14,520 $ 13,763

Accrued expenses and other cu_2

Accrued expenses and other current liabilities (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 2018 2017 Employee compensation $ 28,567 $ 19,657 Accrued manufacturing costs 21,618 8,480 Accrued clinical and contract research organization costs 11,891 8,082 Accrued license and milestone fees 7,739 4,584 Accrued professional fees 1,830 1,402 Other accrued goods and services 25,051 12,971 Financing lease obligation, current portion 1,424 1,051 Other 1,273 838 Total accrued expenses and other current liabilities $ 99,393 $ 57,065

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Commitments	As of December 31, 2018, future minimum commitments under the 60 Binney Street Lease and facility operating leases were as follows (in thousands): Years ended December 31, 60 Binney Street lease Other operating leases (1) Total lease commitments 2019 $ 18,974 $ 17,511 $ 36,485 2020 19,306 18,727 38,033 2021 19,643 18,777 38,420 2022 19,987 13,332 33,319 2023 20,337 13,384 33,721 2024 and thereafter 68,551 29,313 97,864 Total minimum lease payments $ 166,798 $ 111,044 $ 277,842 (1) Includes the lease of the Company’s lab and office space in Seattle, Washington, office space in Zug, Switzerland and two embedded operating leases at contract manufacturing organizations.
Schedule of Non-cancelable Contractual Obligations	The following table presents non-cancelable contractual obligations arising from these arrangements: Years ended December 31, Purchase commitment 2019 $ 69,827 2020 74,529 2021 14,358 2022 24,311 2023 25,040 2024 and thereafter 25,792 Total purchase commitments $ 233,857

Common stock and preferred st_2

Common stock and preferred stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Future Issuance of Common Stock Shares	The Company has reserved for future issuance the following number of shares of common stock (in thousands): As of December 31, 2018 2017 Options to purchase common stock 4,643 3,755 Restricted stock units 931 477 2013 Stock Option and Incentive Plan 1,458 1,550 2013 Employee Stock Purchase Plan 172 188 7,204 5,970

Collaborative arrangements (Tab

Collaborative arrangements (Tables) - Celgene Corporation [Member]	12 Months Ended
	Dec. 31, 2018
Changes in Balances of Company's Receivables and Contract Liabilities	The following table presents changes in the balances of the Company’s Celgene receivables and contract liabilities during the twelve months ended December 31, 2018: (in thousands) Balance at beginning of period under Topic 606 Additions Deductions Balance at end of period under Topic 606 Receivables $ 4,635 $ 8,570 $ (6,677 ) $ 6,528 Contract liabilities: Payable included in accrued expenses $ — $ 8,031 $ (8,031 ) $ — Deferred revenue $ 76,812 $ — $ (41,873 ) $ 34,939
U.S. [Member]
Summary of Revenue Recognized or Expense Incurred Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121	The following table summarizes the net collaboration revenue recognized or expense incurred related to the combined performance obligation for the license and vector manufacturing of bb2121 in the U.S. for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, bb2121 license and manufacturing services - U.S. (in thousands) 2018 (2) 2017 (2) 2016 (2)(3) ASC 808 bb2121 license and manufacturing revenue - U.S. (1) $ 6,255 $ 4,905 $ — ASC 808 bb2121 license and manufacturing research and development expense - U.S. (1) $ 8,689 $ 3,037 $ — (1) As noted above, the calculation of collaboration revenue or research and development expense to be recognized for the Company’s combined performance obligation for its license and vector manufacturing of bb2121 in the U.S. is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. (2) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (3) Revenue recognition for the bb2121 license and manufacturing services for Celgene commenced in 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016.
Outside of U.S. [Member]
Summary of Revenue Recognized or Expense Incurred Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121	The following table summarizes the revenue recognized related to the combined unit of accounting for the bb2121 non-US license and vector manufacturing services for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, bb2121 license and manufacturing services - outside of U.S. (in thousands) 2018 (1) 2017 (1) 2016 (1)(2) ASC 606 license and manufacturing revenue - outside of U.S. $ 35,900 $ 10,372 $ — (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (2) Revenue recognition for the bb2121 license and manufacturing services for Celgene commenced in 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016.
bb2121 Transaction Price [Member]
Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied	The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of December 31, 2018: (in thousands) bb2121 transaction price as of December 31, 2018 Up-front non-refundable payment - Celgene Collaboration Agreement $ 75,000 Up-front non-refundable payment - Amended Celgene Collaboration Agreement 25,000 bb2121 license fee - bb2121 License Agreement 10,000 Estimated variable consideration 85,723 $ 195,723 (in thousands) Allocation of transaction price to performance obligations Transaction price unsatisfied as of December 31, 2018 bb2121 research and development services $ 38,647 $ — bb2121 license and manufacturing services 157,076 50,599 $ 195,723 $ 50,599
bb2121 Research and Development Services [Member]
Summary of Revenue Recognized Related to Research and Development Services	The following table summarizes the revenue recognized related to the bb2121 research and development services for the years ended December 31, 2018, 2017, and 2016: For the years ended December 31, (in thousands) 2018 (1) 2017 (1) 2016 (1) bb2121 research and development services revenue $ 5,751 $ 6,208 $ 6,155 $ 5,751 $ 6,208 $ 6,155 (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,”
bb21217 Transaction Price [Member]
Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied	The following tables summarize the total transaction price, the allocation of the total transaction price to the identified performance obligations under the arrangement, and the amount of the transaction price unsatisfied as of December 31, 2018: (in thousands) bb21217 transaction price as of December 31, 2018 bb21217 license fee - bb21217 License Agreement $ 15,000 Estimated variable consideration 26,687 $ 41,687 (in thousands) Allocation of transaction price to performance obligations Transaction price unsatisfied as of December 31, 2018 bb21217 research and development services $ 5,444 $ 1,839 bb21217 license and manufacturing services 36,243 36,243 $ 41,687 $ 38,082
bb21217 Research and Development Services [Member]
Summary of Revenue Recognized Related to Research and Development Services	The following table summarizes the revenue recognized related to the bb21217 research and development services for the years ended December 31, 2018, 2017, and 2016 For the years ended December 31, (in thousands) 2018 (1) 2017 (1) 2016 (1)(2) bb21217 research and development services revenue $ 2,884 $ 721 $ — $ 2,884 $ 721 $ — (1) As noted in Note 2, “Summary of significant accounting policies and basis of presentation,” (2) Celgene in-licensed bb21217 in September 2017 and as such there were no amounts recognized in the consolidated statement of operations in 2016.

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
	Dec. 31, 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 recognized by award type is as follows (in thousands): Year ended December 31, 2018 2017 2016 Stock options $ 83,449 $ 42,262 $ 33,966 Restricted stock units 26,628 10,495 5,374 Employee stock purchase plan 759 525 416 $ 110,836 $ 53,282 $ 39,756
Schedule of Stock-Based Compensation Expense by Classification	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Year ended December 31, 2018 2017 2016 Research and development $ 54,422 $ 26,633 $ 19,690 General and administrative 56,414 26,649 20,066 $ 110,836 $ 53,282 $ 39,756
Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted	The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Year ended December 31, 2018 2017 2016 Expected volatility 75.5 % 78.1 % 74.3 % Expected term (in years) 6.0 6.0 6.0 Risk-free interest rate 2.7 % 2.1 % 1.5 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity Under Plan	The following table summarizes the stock option activity under the Company’s equity awards plans: Shares (in thousands) Weighted- average exercise price per share Weighted- average contractual life (in years) Aggregate intrinsic value (a) (in Outstanding at December 31, 2017 3,755 $ 67.91 Granted 1,602 $ 185.45 Exercised (575 ) $ 51.82 Canceled or forfeited (139 ) $ 130.93 Outstanding at December 31, 2018 4,643 $ 108.56 7.5 $ 110,812 Exercisable at December 31, 2018 2,200 $ 64.80 6.1 $ 90,785 Vested and expected to vest at December 31, 2018 4,641 $ 108.53 7.5 $ 110,593 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2018.
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Restricted Common Stock Awards	The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average grant date fair value Unvested balance at December 31, 2017 477 $ 80.72 Granted 673 192.90 Vested (152 ) 86.60 Forfeited (67 ) 149.02 Unvested balance at December 31, 2018 931 $ 155.99

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule Components of Loss Before Income Taxes	The components of loss before income taxes were as follows (in thousands): Year ended December 31, 2018 2017 2016 U.S. $ (440,473 ) $ (266,236 ) $ (210,188 ) Foreign (114,965 ) (69,197 ) (53,931 ) Total $ (555,438 ) $ (335,433 ) $ (264,119 )
Summary of Provision for (Benefit from) Income Taxes	The provision for (benefit from) income taxes were as follows (in thousands): Year ended December 31, 2018 2017 2016 Current: Federal $ — $ — $ — State 324 115 — Foreign 222 95 — Deferred: Federal (307 ) — (588 ) State (52 ) — (24 ) Foreign — — — Total income tax expense (benefit) $ 187 $ 210 $ (612 )
Reconciliation of Income Tax Provision (Benefit) Computed at the Statutory Federal Income Tax Rate to Effective Income Tax Rate (Provision) Benefit as Reflected in the Financial Statements	A reconciliation of income tax provision (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate (provision) benefit as reflected in the financial statements is as follows: Year ended December 31, 2018 2017 2016 Federal income tax expense at statutory rate 21.0 % 34.0 % 34.0 % State income tax, net of federal benefit 5.1 % 4.3 % 3.3 % Permanent differences 0.9 % 2.7 % (5.3 %) Research and development credit 6.5 % 12.8 % 15.0 % Foreign differential (4.4 %) (6.9 %) (7.0 %) Federal tax rate change 0.1 % (31.6 %) 0.0 % Other (0.1 %) (0.8 %) 0.0 % Change in valuation allowance (29.1 %) (14.6 %) (39.9 %) Effective income tax rate (expense) benefit 0.0 % (0.1 %) 0.1 %
Components of Deferred Tax Assets and Liabilities	The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands): Year ended December 31, 2018 2017 Deferred tax assets: U.S. net operating loss carryforwards (federal and state) $ 298,701 $ 194,160 Tax credit carryforwards (federal and state) 167,517 131,289 Capitalized license fees and research and development expenses 18,083 13,629 60 Binney Street lease 42,059 42,025 Deferred revenue 21,442 12,795 Stock-based compensation 31,858 19,519 Accruals and other 8,886 6,262 Total deferred tax assets 588,546 419,679 Intangible assets (3,579 ) (4,567 ) Fixed assets (41,472 ) (42,062 ) Less valuation allowance (543,495 ) (373,050 ) Net deferred taxes $ — $ —
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: (in thousands) Unrecognized tax benefits Balance as of December 31, 2016 $ — Increases for tax positions related to current period — Increases for tax positions related to prior periods — Balance as of December 31, 2017 — Increases for tax positions related to current period 3,370 Increases for tax positions related to prior periods 8,725 Balance as of December 31, 2018 12,095

Net loss per share (Tables)

Net loss per share (Tables)	12 Months Ended
	Dec. 31, 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): Year ended December 31, 2018 2017 2016 Outstanding stock options 4,643 3,755 3,735 Restricted stock units 931 477 263 ESPP shares 10 9 11 5,584 4,241 4,009

Selected quarterly financial _2

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2018 First quarter Second quarter Third quarter Fourth quarter Total (in thousands, except per share data) Total revenues $ 15,957 $ 7,851 $ 11,528 $ 19,243 $ 54,579 Total operating expenses 132,586 156,465 161,347 176,204 626,602 Loss from operations (116,629 ) (148,614 ) (149,819 ) (156,961 ) (572,023 ) Net loss (115,126 ) (145,996 ) (145,480 ) (149,023 ) (555,625 ) Net loss per share applicable to common stockholders - basic and diluted $ (2.31 ) $ (2.91 ) $ (2.73 ) $ (2.72 ) $ (10.68 ) 2017 First quarter Second quarter Third quarter Fourth quarter Total (in thousands, except per share data) Total revenues $ 6,832 $ 16,716 $ 7,711 $ 4,168 $ 35,427 Total operating expenses 76,745 84,538 85,369 120,940 367,592 Loss from operations (69,913 ) (67,822 ) (77,658 ) (116,772 ) (332,165 ) Net loss (68,712 ) (70,898 ) (78,805 ) (117,228 ) (335,643 ) Net loss per share applicable to common stockholders - basic and diluted $ (1.68 ) $ (1.73 ) $ (1.73 ) $ (2.52 ) $ (7.71 )

Description of the business - A

Description of the business - Additional Information (Detail) $ in Billions	12 Months Ended
	Dec. 31, 2018USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992
Cash, cash equivalents and marketable securities	$ 1.9

Summary of significant accoun_4

Summary of significant accounting policies and basis of presentation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jan. 01, 2018USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment	1
Interest income net of amortization of premium and accretion of discount	$ 30,100	$ 9,500	$ 3,800
Interest expenses	15,500	11,400	$ 0
Accumulated deficit	$ 1,498,808	913,808		$ 943,183
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	$ 543,495	$ 373,050
ASU 2014-09 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred tax assets, valuation allowance	7,900
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Accumulated deficit	$ 10,866			$ 29,375
Building [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful life of asset	40 years	40 years
Interest expenses	$ 15,500	$ 11,400

Summary of significant accoun_5

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Building [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	40 years	40 years
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	3 years
Furniture and Fixtures [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	2 years
Furniture and Fixtures [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Laboratory Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	2 years
Laboratory Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Leasehold improvements	Shorter of the useful life or remaining lease term

Summary of significant accoun_6

Summary of significant accounting policies and basis of presentation - Impact of Topic 606 Adoption on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	$ 18,602	$ 45,344	$ 25,674
Deferred revenue, net of current portion	16,338	31,468	21,763
Accumulated deficit	(1,498,808)	(943,183)	$ (913,808)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	7,772	19,670
Deferred revenue, net of current portion	3,094	9,705
Accumulated deficit	(10,866)	(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	10,830	25,674
Deferred revenue, net of current portion	13,244	21,763
Accumulated deficit	$ (1,487,942)	$ (913,808)

Summary of significant accoun_7

Summary of significant accounting policies and basis of presentation - Impact of Topic 606 Adoption on Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 19,243	$ 11,528	$ 7,851	$ 15,957	$ 4,168	$ 7,711	$ 16,716	$ 6,832	$ 54,579	$ 35,427	$ 6,155
Research and development expense									448,589	273,040	204,775
Net loss	$ (149,023)	$ (145,480)	$ (145,996)	$ (115,126)	$ (117,228)	$ (78,805)	$ (70,898)	$ (68,712)	$ (555,625)	$ (335,643)	$ (263,507)
Net loss per share - basic and diluted:	$ (2.72)	$ (2.73)	$ (2.91)	$ (2.31)	$ (2.52)	$ (1.73)	$ (1.73)	$ (1.68)	$ (10.68)	$ (7.71)	$ (7.07)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense									$ (4,592)
Net loss									$ 18,509
Net loss per share - basic and diluted:									$ 0.35
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense									$ 453,181
Net loss									$ (574,134)
Net loss per share - basic and diluted:									$ (11.03)
Collaboration [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue									$ 52,353	$ 22,207	$ 6,155
Collaboration [Member] | ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue									13,917
Collaboration [Member] | ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue									$ 38,436

Summary of significant accoun_8

Summary of significant accounting policies and basis of presentation - Impact of Topic 606 Adoption on Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	$ (149,023)	$ (145,480)	$ (145,996)	$ (115,126)	$ (117,228)	$ (78,805)	$ (70,898)	$ (68,712)	$ (555,625)	$ (335,643)	$ (263,507)
Changes in deferred revenue									(41,872)	$ 1,024	$ 4,565
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss									18,509
Changes in deferred revenue									(18,509)
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss									(574,134)
Changes in deferred revenue									$ (23,363)

Summary of significant accoun_9

Summary of significant accounting policies and basis of presentation - Statement of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 402,579	$ 758,505	$ 278,887
Restricted cash included in receivables and other current assets	364		627
Restricted cash included in restricted cash and other non-current assets	14,156	13,763	13,763
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 417,099	$ 772,268	$ 293,277	$ 174,280

Marketable Securities - Summary

Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Equity Securities, Amortized Cost / Cost	$ 20,017
Equity Securities, Unrealized gains	2,150
Equity Securities, Fair value	22,167
Marketable Securities, Amortized Cost / Cost	1,488,746	$ 859,375
Marketable Securities, Unrealized gains	3,113	1
Marketable Securities, Unrealized losses	(3,011)	(3,579)
Marketable Securities, Fair value	1,488,848	855,797
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized Cost / Cost	1,459,649	841,895
Debt Securities, Unrealized gains	963
Debt Securities, Unrealized losses	(3,011)	(3,579)
Debt Securities, Fair value	1,457,601	838,316
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized Cost / Cost	9,080	17,480
Debt Securities, Unrealized gains		1
Debt Securities, Fair value	$ 9,080	$ 17,481

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Marketable Securities, Fair value	$ 1,488,848	$ 855,797
Equity Securities, Fair value	22,167
Liabilities:
Contingent consideration	5,230	2,231
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	402,579	758,505
Equity Securities, Fair value	22,167
Total assets	1,891,427	1,614,302
Liabilities:
Contingent consideration	5,230	2,231
Total liabilities	5,230	2,231
Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,457,601
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	9,080	17,481
Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Securities [Member]
Assets:
Marketable Securities, Fair value		838,316
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	348,638	758,505
Equity Securities, Fair value	22,167
Total assets	370,805	758,505
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	53,941
Total assets	1,520,622	855,797
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,457,601
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	9,080	17,481
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities [Member]
Assets:
Marketable Securities, Fair value		838,316
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	5,230	2,231
Total liabilities	$ 5,230	$ 2,231

Fair value measurements - Addit

Fair value measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	90 days or less
Unrealized loss on debt securities, less than twelve months	$ 787,500,000	$ 704,100,000
Unrealized loss on debt securities, more than twelve months	315,300,000	134,400,000
Investments with other-than-temporary impairment	0	0
Investment at fair value	22,167,000
Contingent consideration, non current	5,230,000	2,231,000
Achievement of research milestone	$ 2,999,000	(525,000)	$ 4,091,000
Preclinical Milestones [Member] | Pregenen [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Achievement of research milestone		$ 5,000,000	$ 5,000,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,021
Milestone discount rates	15.00%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,028
Milestone discount rates	15.80%
Other Income (Expense), Net [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gain on equity securities	$ 2,200,000

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] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 2,231	$ 7,756
Changes in fair value	2,999	(525)
Payments		(5,000)
Ending balance	$ 5,230	$ 2,231

Property, plant and equipment_3

Property, plant and equipment, net - Summary of Property, Plant and Equipment Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 275,798	$ 215,455
Less accumulated depreciation and amortization	(29,176)	(15,849)
Property, plant and equipment, net	246,622	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	180,094	164,414
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	6,365	5,134
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	5,584	4,478
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	35,693	24,914
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	183	116
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property, plant and equipment	$ 46,669	$ 15,189

Property, plant and equipment_4

Property, plant and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 275,798	$ 215,455
Depreciation and amortization expense	13,400	9,800	$ 5,900
Construction In Progress North Carolina Manufacturing Facility [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	40,400	12,900
60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	168,100	164,400
Construction cost incurred by landlord	$ 156,000	$ 156,000

Restricted cash and other non_3

Restricted cash and other non-current assets - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Restricted cash balance			$ 14,520,000	$ 13,763,000
Research and development expense			448,589,000	273,040,000	$ 204,775,000
License and Collaboration Agreement [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Upfront payment	$ 20,000,000
Prepaid research and development services		$ 14,500,000
Research and development expense		$ 5,500,000
60 Binney Street Lease [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Restricted cash balance			13,763,000	13,763,000
Decrease in restricted cash balance, year four			1,500,000
Decrease in restricted cash balance, year five			1,500,000
Decrease in restricted cash balance, year six			1,500,000
Letter of Credit [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Restricted cash balance			$ 14,500,000	$ 13,800,000

Restricted cash and other non_4

Restricted cash and other non-current assets - Schedule of Collateralized Bank Account of Financial Institution (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Total restricted cash	$ 14,520	$ 13,763
60 Binney Street Lease [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Total restricted cash	13,763	$ 13,763
Other Leases [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Total restricted cash	$ 757

Accrued expenses and other cu_3

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Employee compensation	$ 28,567	$ 19,657
Accrued manufacturing costs	21,618	8,480
Accrued clinical and contract research organization costs	11,891	8,082
Accrued license and milestone fees	7,739	4,584
Accrued professional fees	1,830	1,402
Other accrued goods and services	25,051	12,971
Financing lease obligation, current portion	1,424	1,051
Other	1,273	838
Total accrued expenses and other current liabilities	$ 99,393	$ 57,065

Commitments and contingencies -

Commitments and contingencies - Additional Information (Detail) € in Millions	Nov. 18, 2016EUR (€)	Jun. 03, 2016USD ($)	Sep. 21, 2015USD ($)ft²$ / ft²	Mar. 31, 2018USD ($)	Sep. 30, 2018EUR (€)	Jun. 30, 2018USD ($)	Dec. 31, 2016EUR (€)	Sep. 30, 2016USD ($)	Dec. 31, 2018USD ($)	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 | €					€ 1		€ 2
Annual fees									$ 885,000	$ 1,527,000
Type of cost, good or service [Extensible List]									blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember
Termination description									The Company may terminate this agreement with twelve months’ notice and a one-time termination fee.
Interest expense related to financing obligation									$ 15,500,000	$ 11,400,000	$ 0
Rent expense									9,800,000	9,000,000	8,300,000
Expense related to agreements									448,589,000	273,040,000	204,775,000
Manufacturing Development and License Agreements [Member]
Commitments And Contingencies Disclosure [Line Items]
Expense related to agreements									$ 44,000,000
Pregenen [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments											120,000,000
Settlement of contingent consideration liability										$ 5,000,000	5,000,000
Pregenen [Member] | Clinical Milestone Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments											20,100,000
Pregenen [Member] | Commercial Milestones Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments											99,900,000
Building [Member]
Commitments And Contingencies Disclosure [Line Items]
Useful life of asset									40 years	40 years
Interest expense related to financing obligation									$ 15,500,000	$ 11,400,000
Financing obligation, principal payments									1,000,000	600,000
Land [Member]
Commitments And Contingencies Disclosure [Line Items]
Rent expense									$ 1,900,000	1,900,000	1,900,000
Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Annual fees | €	€ 9.8
Type of cost, good or service [Extensible List]	us-gaap:MaintenanceMember
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				$ 2,000,000
Milestone paid						$ 1,500,000				$ 5,500,000	$ 5,000,000
Construction completed month and year									2018-03
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
Manufacturing Agreement [Member] | Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Additional contractual payables to milestones		$ 8,000,000
Lease starting on October 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease starting date			Oct. 1, 2016
Lease building space | ft²			253,108
Lease payments base annual rent			$ 18,400,000
Landlord contribution for cost of construction and tenant improvements			42,400,000
Capital lease description									Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes.
Cash-collateralized irrevocable standby letter of credit			$ 9,200,000
Increase in letter of credit under the terms of the lease								$ 13,800,000
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Annual lease rent per square foot | $ / ft²			72.50
Lease rent increase percentage			1.75%
Lease expiration date			Mar. 31, 2027
Option to extend capital lease			10 years

Commitments and contingencies_2

Commitments and contingencies - Future Minimum Commitments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Future Minimum Payments Due [Line Items]
2,019	$ 36,485
2,020	38,033
2,021	38,420
2,022	33,319
2,023	33,721
2024 and thereafter	97,864
Total minimum lease payments	277,842
Other Operating Lease [Member]
Future Minimum Payments Due [Line Items]
2,019	17,511
2,020	18,727
2,021	18,777
2,022	13,332
2,023	13,384
2024 and thereafter	29,313
Total minimum lease payments	111,044
60 Binney Street Lease [Member]
Future Minimum Payments Due [Line Items]
2,019	18,974
2,020	19,306
2,021	19,643
2,022	19,987
2,023	20,337
2024 and thereafter	68,551
Total minimum lease payments	$ 166,798

Commitments and contingencies_3

Commitments and contingencies - Future Minimum Commitments (Parenthetical) (Detail)	Dec. 31, 2018OperatingLease
Other Operating Lease [Member]
Future Minimum Payments Due [Line Items]
Number of embedded operating leases at contract manufacturing organizations	2

Commitments and contingencies_4

Commitments and contingencies - Schedule of Non-cancelable Contractual Obligations (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 69,827
2,020	74,529
2,021	14,358
2,022	24,311
2,023	25,040
2024 and thereafter	25,792
Total purchase commitments	$ 233,857

Common stock and preferred st_3

Common stock and preferred stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended						12 Months Ended
	Aug. 31, 2018	Jul. 31, 2018	Jan. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2018
Schedule Of Capitalization [Line Items]
Common stock, shares authorized				125,000,000			125,000,000
Common stock voting rights							Common stock are entitled to one vote per share
Shares of common stock issued				49,406,000			54,738,000
Shares of common stock outstanding				49,406,000			54,738,000
Number of shares issued in public offering	400,000	3,900,000	300,000	3,200,000	4,400,000	3,300,000
Proceeds from public offering of common stock, net of issuance costs	$ 100	$ 600.6	$ 48.7	$ 569.8	$ 436.8	$ 234.7
Shares issued, price per share	$ 238.10	$ 162.50	$ 185	$ 185	$ 105	$ 76
Common stock amount attributable to prepayment of joint research activities	$ 45.5
Preferred stock, shares authorized				5,000,000			5,000,000
Shares of preferred stock issued				0			0
Shares of preferred stock outstanding				0			0
Overallotment Option [Member]
Schedule Of Capitalization [Line Items]
Number of shares issued in public offering					600,000

Common stock and preferred st_4

Common stock and preferred stock - Summary of Future Issuance of Common Stock Shares (Detail) - shares shares in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	7,204	5,970
Options to Purchase Common Stock [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	4,643	3,755
Restricted Stock Units [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	931	477
2013 Employee Stock Purchase Plan [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	172	188
2013 Stock Option and Incentive Plan [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	1,458	1,550

Collaborative arrangements - Ad

Collaborative arrangements - Additional Information (Detail)	Aug. 24, 2018USD ($)Target$ / sharesshares	Sep. 28, 2017USD ($)	Feb. 16, 2016USD ($)	Mar. 19, 2013USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Aug. 03, 2018USD ($)	Mar. 28, 2018USD ($)	Jun. 03, 2015USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue released					$ 41,900,000
Investment in common stock					$ 649,368,000	$ 1,006,570,000	$ 234,962,000
Common Shares [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock shares issued | shares					420,000
Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues					$ 9,800,000
Research and Development Services [Member] | Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues					32,100,000
Celgene Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					34,939,000	76,812,000
Term of collaboration agreement				3 years
Deferred revenue balance recognized as gross revenues					41,873,000
Celgene Corporation [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability				$ 75,000,000
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability										$ 25,000,000
Estimated variable consideration					$ 85,700,000
Deferred revenue recognition period					3 years
Celgene Corporation [Member] | Amended Collaborative Arrangement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					$ 38,600,000
Celgene Corporation [Member] | bb2121 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received					10,000,000
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
Estimated variable consideration					$ 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
Regulatory milestone payments receivable					117,000,000
Commercial milestone payments receivable					78,000,000
Celgene Corporation [Member] | bb21217 License Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					1,800,000
Collaboration agreement, transaction price					5,400,000
Remaining performance obligation revenue					1,800,000
Celgene Corporation [Member] | bb21217 License Agreement [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					9,800,000
Remaining performance obligation revenue					36,200,000
Deferred revenue balance recognized as gross revenues					0	0	0
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
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					157,076,000
Remaining performance obligation revenue					50,599,000
Celgene Corporation [Member] | bb2121 License and Manufacturing Services [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with customer liability					23,000,000
Remaining performance obligation revenue					$ 50,600,000
Remaining performance obligation expected to be recognize as revenue, year					2,020
Regeneron Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation revenue					$ 44,000,000
Number of initial collaboration targets | Target	6
Research collaboration term	5 years
Colloaboration agreements costs and profits share description					Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories.
Investment in common stock					$ 54,484,000
Purchase price premium	$ 37,000,000
Regeneron Collaboration Agreement [Member] | Short Term [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation revenue					10,600,000
Regeneron Collaboration Agreement [Member] | Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue balance recognized as gross revenues					1,600,000	$ 0	$ 0
Regeneron Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments receivable								$ 130,000,000
Regeneron Collaboration Agreement [Member] | Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price					100,000,000
Purchase price premium					37,000,000
Collaborative arrangement amount attributed to equity sold					54,500,000
Collaborative arrangement amount attributed to joint research activities					$ 45,500,000
Collaborative arrangement amortization period					5 years
Regeneron Collaboration Agreement [Member] | Share Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock shares issued | shares	400,000
Investment in common stock	$ 100,000,000
Common stock price per share | $ / shares	$ 238.10
Purchase price premium	$ 37,000,000
Collaborative arrangement research initial funding obligation, percentage	50.00%
Regeneron Collaboration Agreement [Member] | Share Purchase Agreement [Member] | Common Shares [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Investment in common stock	$ 63,000,000

Collaborative arrangements - Su

Collaborative arrangements - Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied (Detail) - Celgene Corporation [Member] - USD ($) $ in Thousands	Sep. 28, 2017	Dec. 31, 2018
Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		$ 75,000
Amended Celgene Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front non-refundable payment		25,000
bb2121 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee		10,000
Vectors and Associated Payload [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		85,723
bb2121 Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		195,723
Transaction price unsatisfied		50,599
bb2121 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		38,647
bb2121 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		157,076
Transaction price unsatisfied		50,599
bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, license fee	$ 15,000	15,000
Estimated variable consideration		26,700
Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated variable consideration		26,687
bb21217 Transaction Price [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		41,687
Transaction price unsatisfied		38,082
bb21217 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		5,444
Transaction price unsatisfied		1,839
bb21217 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		36,243
Transaction price unsatisfied		$ 36,243

Collaborative arrangements - _2

Collaborative arrangements - Summary of Revenue Recognized Related to Research and Development Services (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 19,243	$ 11,528	$ 7,851	$ 15,957	$ 4,168	$ 7,711	$ 16,716	$ 6,832	$ 54,579	$ 35,427	$ 6,155
Celgene Corporation [Member] | bb2121 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue									5,751	6,208	$ 6,155
Celgene Corporation [Member] | bb21217 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue									$ 2,884	$ 721

Collaborative arrangements - _3

Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121 (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 19,243,000	$ 11,528,000	$ 7,851,000	$ 15,957,000	$ 4,168,000	$ 7,711,000	$ 16,716,000	$ 6,832,000	$ 54,579,000	$ 35,427,000	$ 6,155,000
Celgene Corporation [Member] | bb2121 Research and Development Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue									5,751,000	6,208,000	6,155,000
Celgene Corporation [Member] | U.S. [Member] | bb2121 License and Manufacturing Services [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue											0
Celgene Corporation [Member] | U.S. [Member] | bb2121 License and Manufacturing Services [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue									6,255,000	4,905,000
Celgene Corporation [Member] | U.S. [Member] | bb2121 Research and Development Services [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue									8,689,000	3,037,000
Celgene Corporation [Member] | Outside of U.S. [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue											$ 0
Celgene Corporation [Member] | Outside of U.S. [Member] | ASU 2014-09 [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License and manufacturing revenue (expense)									$ 35,900,000	$ 10,372,000

Collaborative arrangements - _4

Collaborative arrangements - Summary of Revenue Recognized or Expense Incurred Related to Combined Unit of Accounting for its License and Vector Manufacturing of bb2121 (Parenthetical) (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 19,243,000	$ 11,528,000	$ 7,851,000	$ 15,957,000	$ 4,168,000	$ 7,711,000	$ 16,716,000	$ 6,832,000	$ 54,579,000	$ 35,427,000	$ 6,155,000
Celgene Corporation [Member] | U.S. [Member] | License and Manufacturing Services [Member] | bb2121 License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue											0
Celgene Corporation [Member] | Outside of U.S. [Member] | License and Manufacturing Services [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue											$ 0

Collaborative arrangements - _5

Collaborative arrangements - Summary of Revenue Recognized Related to Research and Development Services (Parenthetical) (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 19,243,000	$ 11,528,000	$ 7,851,000	$ 15,957,000	$ 4,168,000	$ 7,711,000	$ 16,716,000	$ 6,832,000	$ 54,579,000	$ 35,427,000	$ 6,155,000
Celgene Corporation [Member] | bb21217 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue											$ 0

Collaborative arrangements - Ch

Collaborative arrangements - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Celgene Corporation [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Receivables
Balance at beginning of period under Topic 606	$ 4,635
Additions	8,570
Deductions	(6,677)
Balance at end of period under Topic 606	6,528
Contract liabilities:
Balance at beginning of period under Topic 606	76,812
Deductions	(41,873)
Balance at end of period under Topic 606	34,939
Accrued Expenses [Member]
Contract liabilities:
Additions	8,031
Deductions	$ (8,031)

License and Royalty Revenue - A

License and Royalty Revenue - Additional Information (Detail)

Apr. 28, 2017USD ($)PerformanceObligation

Apr. 26, 2017USD ($)PerformanceObligation

Aug. 31, 2017USD ($)

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Sep. 30, 2017USD ($)

Jun. 30, 2017USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

License And Royalty Revenue [Line Items]

Revenue

$ 19,243,000

$ 11,528,000

$ 7,851,000

$ 15,957,000

$ 4,168,000

$ 7,711,000

$ 16,716,000

$ 6,832,000

$ 54,579,000

$ 35,427,000

$ 6,155,000

Cost of license and royalty revenue

885,000

1,527,000

License and Royalty [Member]

License And Royalty Revenue [Line Items]

Revenue

2,226,000

13,220,000

Novartis Pharma AG [Member]

License And Royalty Revenue [Line Items]

License agreement upfront payment

$ 7,500,000

Revenue recognized upon achievement of related milestone

$ 2,500,000

Novartis Pharma AG [Member] | Royalty [Member]

License And Royalty Revenue [Line Items]

Revenue

2,200,000

100,000

Novartis Pharma AG [Member] | License [Member]

License And Royalty Revenue [Line Items]

Revenue

10,000,000

Novartis Pharma AG [Member] | License and Royalty [Member]

License And Royalty Revenue [Line Items]

Cost of license and royalty revenue

900,000

1,400,000

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

Cost of license and royalty revenue

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

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

Intangible Assets - Additional

Intangible Assets - Additional Information (Detail) - Pregenen [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Gene editing platform expected useful life	8 years
Amortization expense	$ 3.8	$ 3.8	$ 3.8
Accumulated Amortization	$ 16.9	$ 13.2
Amortization of intangible assets remaining useful life	3 years 6 months

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 24, 2013	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
Number of shares available for issuance			1,500,000
Stock-based compensation expense			$ 110,836,000	$ 53,282,000	$ 39,756,000
Weighted average grant date fair value of options granted			$ 125.12	$ 62.03	$ 34.22
Intrinsic value of stock options exercised			$ 73,100,000	$ 91,000,000	$ 15,300,000
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
Vested during period			0
Unvested restricted stock awards outstanding			200,000
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 83,449,000	42,262,000	33,966,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 199,800,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan			2 years 9 months 18 days
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 26,628,000	$ 10,495,000	5,374,000
Vested during period			152,000
Unvested restricted stock awards outstanding			931,000	477,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 76,400,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan			2 years 10 months 24 days
Intrinsic value of restricted stock units vested			$ 25,500,000	$ 8,100,000	5,300,000
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance							200,000
Stock-based compensation expense			759,000	$ 525,000	$ 416,000
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 100,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan			1 month 6 days
Shares of common stock issued under plan			16,026	20,773
Maximum [Member] | Performance And Service Condition Based Restricted Stock Unit [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense yet to recognize			$ 37,900,000
Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan	2,200,000
2013 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance						955,000
2013 Stock Option and Incentive Plan [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of automatic increase in shares reserved and available for issuance determined based on shares outstanding			4.00%

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 110,836	$ 53,282	$ 39,756
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	83,449	42,262	33,966
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	26,628	10,495	5,374
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 759	$ 525	$ 416

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 110,836	$ 53,282	$ 39,756
Research And Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	54,422	26,633	19,690
General And Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 56,414	$ 26,649	$ 20,066

Stock-based compensation - Assu

Stock-based compensation - Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected volatility	75.50%	78.10%	74.30%
Expected term (in years)	6 years	6 years	6 years
Risk-free interest rate	2.70%	2.10%	1.50%
Expected dividend yield	0.00%	0.00%	0.00%

Stock-based compensation - Su_2

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Shares
Outstanding at beginning of period | shares	3,755
Granted | shares	1,602
Exercised | shares	(575)
Canceled or forfeited | shares	(139)
Outstanding at end of period | shares	4,643
Exercisable at end of period | shares	2,200
Vested and expected to vest at end of period | shares	4,641
Weighted-average exercise price per share
Outstanding at beginning of period | $ / shares	$ 67.91
Granted | $ / shares	185.45
Exercised | $ / shares	51.82
Canceled or forfeited | $ / shares	130.93
Outstanding at end of period | $ / shares	108.56
Exercisable at end of period | $ / shares	64.80
Vested and expected to vest at end of period | $ / shares	$ 108.53
Outstanding at December 31, 2018	7 years 6 months
Exercisable at December 31, 2018	6 years 1 month 6 days
Vested and expected to vest at December 31, 2018	7 years 6 months
Aggregate intrinsic value
Outstanding at end of period | $	$ 110,812	[1]
Exercisable at end of period | $	90,785	[1]
Vested and expected to vest at end of period | $	$ 110,593	[1]
[1]	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2018.

Stock-based compensation - Su_3

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Shares
Unvested balance at beginning of period | shares	477
Granted | shares	673
Vested | shares	(152)
Forfeited | shares	(67)
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	192.90
Vested | $ / shares	86.60
Forfeited | $ / shares	149.02
Unvested balance at end of period | $ / shares	$ 155.99

401(k) Savings plan - Additiona

401(k) Savings plan - Additional Information (Detail) - 401 (k) [Member] - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) Plan		$ 1.5
Expenses related to 401(k) Plan			$ 2.4	$ 1.5	$ 1
Scenario Forecast [Member]
Defined Contribution Plan Disclosure [Line Items]
Contributions to the 401(k) Plan	$ 2.4

Income taxes - Schedule of Comp

Income taxes - Schedule of Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S.	$ (440,473)	$ (266,236)	$ (210,188)
Foreign	(114,965)	(69,197)	(53,931)
Loss before income taxes	$ (555,438)	$ (335,433)	$ (264,119)

Income taxes - Summary of Provi

Income taxes - Summary of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
State	$ 324	$ 115
Foreign	222	95
Deferred:
Federal	(307)		$ (588)
State	(52)		(24)
Total income tax expense (benefit)	$ 187	$ 210	$ (612)

Income taxes - Reconciliation o

Income taxes - Reconciliation of Income Tax Provision (Benefit) Computed at the Statutory Federal Income Tax Rate to Effective Income Tax Rate (Provision) Benefit as Reflected in the Financial Statements (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	34.00%	34.00%
State income tax, net of federal benefit	5.10%	4.30%	3.30%
Permanent differences	0.90%	2.70%	(5.30%)
Research and development credit	6.50%	12.80%	15.00%
Foreign differential	(4.40%)	(6.90%)	(7.00%)
Federal tax rate change	0.10%	(31.60%)	0.00%
Other	(0.10%)	(0.80%)	0.00%
Change in valuation allowance	(29.10%)	(14.60%)	(39.90%)
Effective income tax rate (expense) benefit	0.00%	(0.10%)	0.10%

Income taxes - Additional Infor

Income taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components Of Income Tax Expense Benefit [Line Items]
Income tax (expense) benefit	$ (187,000)	$ (210,000)	$ 612,000
Effective income tax rate (expense) benefit	0.00%	(0.10%)	0.10%
Approximately valuation allowance increased	$ 170,400,000
Cumulative-effect adjustment to retained earnings	$ 298,701,000	$ 194,160,000
U.S. corporate income tax rate	21.00%	34.00%	34.00%
Earlier Tax Year [Member]
Components Of Income Tax Expense Benefit [Line Items]
Federal, state and foreign income tax returns	Dec. 31, 2015
Latest Tax Year [Member]
Components Of Income Tax Expense Benefit [Line Items]
Federal, state and foreign income tax returns	Dec. 31, 2017
ASU 2016-09 [Member]
Components Of Income Tax Expense Benefit [Line Items]
Cumulative-effect adjustment to retained earnings	$ 76,700,000
Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Limitations on use of net operating losses and tax credit carryforwards, percentage	50.00%
Limitations on use of net operating losses and tax credit carryforwards, period	3 years
U.S. [Member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 1,096,700,000	$ 716,100,000	$ 466,800,000
Operating loss carryforwards expiration year	2,037	2,037	2,037
Credit carryforward expiration year	2,038	2,038	2,038
U.S. [Member] | Excess Equity Based Compensation Tax Deductions
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards			$ 710,700,000
U.S. [Member] | Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 156,200,000	$ 124,100,000	83,200,000
State and Local Jurisdiction [Member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 1,082,100,000	$ 692,900,000	$ 456,800,000
Operating loss carryforwards expiration year	2,038	2,038	2,038
Credit carryforward expiration year	2,033	2,033	2,033
State and Local Jurisdiction [Member] | Excess Equity Based Compensation Tax Deductions
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards			$ 195,400,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 14,300,000	$ 9,100,000	$ 6,000,000
Foreign Tax Authority [Member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 386,000

Income taxes - Components of De

Income taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$ 298,701	$ 194,160
Tax credit carryforwards (federal and state)	167,517	131,289
Capitalized license fees and research and development expenses	18,083	13,629
60 Binney Street lease	42,059	42,025
Deferred revenue	21,442	12,795
Stock-based compensation	31,858	19,519
Accruals and other	8,886	6,262
Total deferred tax assets	588,546	419,679
Intangible assets	(3,579)	(4,567)
Fixed assets	(41,472)	(42,062)
Less valuation allowance	$ (543,495)	$ (373,050)

Income taxes - Reconciliation_2

Income taxes - Reconciliation of Unrecognized Tax Benefits (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Income Tax Disclosure [Abstract]
Balance as of December 31, 2016	$ 12,095
Increases for tax positions related to current period	3,370
Increases for tax positions related to prior periods	$ 8,725

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	5,584	4,241	4,009
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	4,643	3,755	3,735
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	477	263
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	10	9	11

Selected quarterly financial _3

Selected quarterly financial data - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 19,243	$ 11,528	$ 7,851	$ 15,957	$ 4,168	$ 7,711	$ 16,716	$ 6,832	$ 54,579	$ 35,427	$ 6,155
Total operating expenses	176,204	161,347	156,465	132,586	120,940	85,369	84,538	76,745	626,602	367,592	273,985
Loss from operations	(156,961)	(149,819)	(148,614)	(116,629)	(116,772)	(77,658)	(67,822)	(69,913)	(572,023)	(332,165)	(267,830)
Net loss	$ (149,023)	$ (145,480)	$ (145,996)	$ (115,126)	$ (117,228)	$ (78,805)	$ (70,898)	$ (68,712)	$ (555,625)	$ (335,643)	$ (263,507)
Net loss per share - basic and diluted	$ (2.72)	$ (2.73)	$ (2.91)	$ (2.31)	$ (2.52)	$ (1.73)	$ (1.73)	$ (1.68)	$ (10.68)	$ (7.71)	$ (7.07)