Voyager Therapeutics (VYGR) 10-Q 9 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 02, 2019
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Jun. 30, 2019
Entity File Number	001-37625
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-3003182
Entity Address, Address Line One	75 Sidney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	857
Local Phone Number	259-5340
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Central Index Key	0001640266
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Title of 12(b) Security	Common Stock, $0.001 par value
Trading Symbol	VYGR
Security Exchange Name	NASDAQ
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,933,937
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 79,885	$ 46,859
Marketable securities, current	247,577	108,947
Collaboration receivable	8,242
Unbilled accounts receivable	1,821
Prepaid expenses and other current assets	3,250	6,675
Total current assets	340,775	162,481
Property and equipment, net	14,493	12,771
Deposits and other non-current assets	2,003	1,149
Operating lease, right-of-use asset	30,041
Marketable securities, non-current	1,280	628
Total assets	388,592	177,029
Current liabilities:
Accounts payable	2,230	1,038
Accrued expenses	11,782	9,788
Other current liabilities	2,843
Deferred revenue, current portion	52,291	20,847
Total current liabilities	69,146	31,673
Deferred revenue, non-current	167,436	92,199
Other non-current liabilities	33,606	6,711
Total liabilities	270,188	130,583
Commitments and contingencies (see note 8)
Stockholders' equity:
Preferred stock $0.001 par value: 5,000,000 shares authorized at June 30, 2019 and December 31, 2018; no shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.001 par value: 120,000,000 shares authorized at June 30, 2019 and December 31, 2018; 36,677,608 and 32,364,895 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	37	32
Additional paid-in capital	403,334	315,598
Accumulated other comprehensive income (loss)	101	(133)
Accumulated deficit	(285,068)	(269,051)
Total stockholders' equity	118,404	46,446
Total liabilities and stockholders' equity	$ 388,592	$ 177,029

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Condensed Consolidated Balance Sheets
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
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 (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	36,677,608	32,364,895
Common Stock, Shares, Outstanding	36,677,608	32,364,895

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Collaboration revenue	$ 46,087	$ 2,575	$ 51,284	$ 3,517
Operating expenses:
Research and development	28,576	16,507	53,407	31,360
General and administrative	8,322	11,762	17,981	18,945
Total operating expenses	36,898	28,269	71,388	50,305
Operating income (loss)	9,189	(25,694)	(20,104)	(46,788)
Other income:
Interest income	2,097	870	3,242	1,458
Other (expense) income, net	(133)	(717)	845	(318)
Total other income	1,964	153	4,087	1,140
Income (loss) before income taxes	11,153	(25,541)	(16,017)	(45,648)
Income tax benefit				180
Net income (loss)	11,153	(25,541)	(16,017)	(45,468)
Other comprehensive income
Net unrealized gain on available-for-sale securities	176	37	234	42
Total other comprehensive income	176	37	234	162
Comprehensive income (loss)	$ 11,329	$ (25,504)	$ (15,783)	$ (45,306)
Net income (loss) per share, basic	$ 0.30	$ (0.80)	$ (0.46)	$ (1.43)
Net income (loss) per share, diluted	$ 0.29	$ (0.80)	$ (0.46)	$ (1.43)
Weighted-average common shares outstanding, basic	36,610,918	31,976,922	34,990,989	31,868,995
Weighted-average common shares outstanding, diluted	37,941,257	31,976,922	34,990,989	31,868,995
ASU 2016-01
Other comprehensive income
Cumulative effect adjustment resulting from ASU No. 2016-01				$ 120

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Stockholders' Equity, Beginning Balance at Dec. 31, 2017	$ 32	$ 295,019	$ (287)	$ (160,713)	$ 134,051
Shares, Outstanding, Beginning Balance at Dec. 31, 2017	31,572,044
Vesting of restricted stock		3			3
Vesting of restricted stock (in shares)	145,829
Exercises of vested stock options		1,412			1,412
Exercises of vested stock options (in shares)	132,733
Issuance of common stock under ESPP		418			418
Issuance of common stock under ESPP (in shares)	38,392
Stock based compensation expense		2,260			2,260
Life-to-date cumulative catchup on equity securities			120		120
Unrealized gain on available for-sale securities, net of tax			5		5
Impact of adopting new accounting standards | ASU 2014-09				(20,050)	(20,050)
Net loss				(19,926)	(19,926)
Stockholders' Equity, Ending Balance at Mar. 31, 2018	$ 32	299,112	(162)	(200,689)	98,293
Shares, Outstanding, Ending Balance at Mar. 31, 2018	31,888,998
Stockholders' Equity, Beginning Balance at Dec. 31, 2017	$ 32	295,019	(287)	(160,713)	134,051
Shares, Outstanding, Beginning Balance at Dec. 31, 2017	31,572,044
Unrealized gain on available for-sale securities, net of tax					42
Impact of adopting new accounting standards | ASU 2016-01					120
Impact of adopting new accounting standards | ASU 2014-09					(20,050)
Net loss					(45,468)
Stockholders' Equity, Ending Balance at Jun. 30, 2018	$ 32	308,296	(125)	(226,230)	81,973
Shares, Outstanding, Ending Balance at Jun. 30, 2018	32,063,948
Stockholders' Equity, Beginning Balance at Mar. 31, 2018	$ 32	299,112	(162)	(200,689)	98,293
Shares, Outstanding, Beginning Balance at Mar. 31, 2018	31,888,998
Vesting of restricted stock		3			3
Vesting of restricted stock (in shares)	61,632
Exercises of vested stock options		1,116			1,116
Exercises of vested stock options (in shares)	113,318
Stock based compensation expense		8,065			8,065
Unrealized gain on available for-sale securities, net of tax			37		37
Net loss				(25,541)	(25,541)
Stockholders' Equity, Ending Balance at Jun. 30, 2018	$ 32	308,296	(125)	(226,230)	81,973
Shares, Outstanding, Ending Balance at Jun. 30, 2018	32,063,948
Stockholders' Equity, Beginning Balance at Dec. 31, 2018	$ 32	315,598	(133)	(269,051)	46,446
Shares, Outstanding, Beginning Balance at Dec. 31, 2018	32,364,895
Exercises of vested stock options	$ 1	283			284
Exercises of vested stock options (in shares)	31,360
Issuance of common stock in connection with Neurocrine Collaboration Agreement	$ 4	77,613			77,617
Issuance of common stock in connection with Neurocrine Collaboration Agreement (in shares)	4,179,728
Stock based compensation expense		3,459			3,459
Unrealized gain on available for-sale securities, net of tax			58		58
Net loss				(27,170)	(27,170)
Stockholders' Equity, Ending Balance at Mar. 31, 2019	$ 37	396,953	(75)	(296,221)	100,694
Shares, Outstanding, Ending Balance at Mar. 31, 2019	36,575,983
Stockholders' Equity, Beginning Balance at Dec. 31, 2018	$ 32	315,598	(133)	(269,051)	$ 46,446
Shares, Outstanding, Beginning Balance at Dec. 31, 2018	32,364,895
Exercises of vested stock options (in shares)					88,821
Unrealized gain on available for-sale securities, net of tax					$ 234
Net loss					(16,017)
Stockholders' Equity, Ending Balance at Jun. 30, 2019	$ 37	403,334	101	(285,068)	118,404
Shares, Outstanding, Ending Balance at Jun. 30, 2019	36,677,608
Stockholders' Equity, Beginning Balance at Mar. 31, 2019	$ 37	396,953	(75)	(296,221)	100,694
Shares, Outstanding, Beginning Balance at Mar. 31, 2019	36,575,983
Exercises of vested stock options		714			714
Exercises of vested stock options (in shares)	57,461
Issuance of common stock under ESPP		471			471
Issuance of common stock under ESPP (in shares)	44,164
Stock based compensation expense		5,196			5,196
Unrealized gain on available for-sale securities, net of tax			176		176
Net loss				11,153	11,153
Stockholders' Equity, Ending Balance at Jun. 30, 2019	$ 37	$ 403,334	$ 101	$ (285,068)	$ 118,404
Shares, Outstanding, Ending Balance at Jun. 30, 2019	36,677,608

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flow from operating activities
Net loss	$ (16,017)	$ (45,468)
Adjustments to reconcile net loss to net cash provided by operating activities:
Stock-based compensation expense	8,771	10,436
Depreciation	1,292	972
Amortization of premiums and discounts on marketable securities	(1,844)	(908)
In-kind research and development expenses	616	176
Other non-cash items	(832)	355
Changes in operating assets and liabilities:
Collaboration receivable	(8,242)
Unbilled accounts receivable	(1,821)
Prepaid expenses and other current assets	2,966	(1,384)
Operating lease, right-of-use asset	1,382
Other non-current assets	(674)	(180)
Accounts payable	1,192	538
Accrued expenses	1,977	(3,911)
Operating lease liabilities	(1,226)
Deferred revenue	106,065	65,482
Net cash provided by operating activities	93,605	26,108
Cash flow from investing activities
Purchases of property and equipment	(3,168)	(2,933)
Proceeds from sale of equipment	171
Purchases of marketable securities	(314,352)	(205,037)
Proceeds from maturities of marketable securities	177,800	207,000
Net cash used in investing activities	(139,549)	(970)
Cash flow from financing activities
Proceeds from the exercise of stock options	998	2,528
Proceeds from the issuance of common stock in connection with the Neurocrine Collaboration Agreement, net	77,617
Proceeds from the purchase of common stock under ESPP	355	418
Net cash provided by financing activities	78,970	2,946
Net increase in cash, cash equivalents, and restricted cash	33,026	28,084
Cash, cash equivalents, and restricted cash beginning of period	47,594	32,265
Cash, cash equivalents, and restricted cash end of period	80,620	60,349
Supplemental disclosure of cash and non-cash activities
Capital expenditures incurred but not yet paid	$ 17
ASU 2014-09
Supplemental disclosure of cash and non-cash activities
Impact of adopting new accounting standards		$ 20,050

Nature of business

Nature of business	6 Months Ended
	Jun. 30, 2019
Nature of business
Nature of business	VOYAGER THERAPEUTICS, INC. NOTE S TO UNAUDITED CONDENSED CONSOLIDATED FINANCIAL STATEMENTS ​ 1. Nature of business Voyager Therapeutics, Inc. (the “Company”) is a clinical-stage gene therapy company focused on developing life-changing treatments for patients suffering from severe neurological diseases. The Company is focused on neurological diseases where it believes an adeno-associated virus (“AAV”) gene therapy approach that either increases or decreases the production of a specific protein can slow or reduce the symptoms experienced by patients, and therefore have a clinically meaningful impact. The Company has built a gene therapy platform that it believes positions itself to be a leading company at the intersection of AAV gene therapy and severe neurological disease. The Company’s gene therapy platform enables it to engineer, optimize, manufacture and deliver its AAV-based gene therapies that have the potential to provide durable efficacy following a single administration. Additionally, the Company is working to identify novel AAV capsids, which are the outer viral protein shells that enclose the genetic material of the virus payload. The Company’s team of experts in the fields of AAV gene therapy and neuroscience first identifies and selects severe neurological diseases that are well-suited for treatment using AAV gene therapy. The Company then engineers and optimizes AAV vectors for delivery of the virus payload to the targeted tissue or cells. The Company’s manufacturing process employs an established system that it believes will enable production of high quality AAV vectors at commercial scale. I The Company’s business strategy focuses on discovering, developing, manufacturing, and commercializing its gene therapy programs. As part of this strategy, the Company has developed core competencies specific to AAV gene therapy development and manufacturing and is beginning to build its commercial infrastructure. This business strategy also includes business development activities that may include in-licensing activities or partnering certain programs in certain geographies with collaborators, as the Company has demonstrated through i ts collaboration with AbbVie Tau Collaboration”), its collaboration with AbbVie Ireland Unlimited Company (the “AbbVie Alpha-Synuclein Collaboration”), and its collaboration with Neurocrine Biosciences, Inc. (the “Neurocrine Collaboration”), and its collaboration with Sanofi Genzyme (the “Sanofi Genzyme Collaboration”) which was terminated in June 2019. The Company has incurred annual net operating losses in every year since inception. As of June 30, 2019, the Company had an accumulated deficit of $285.1 million. The Company has not generated any product revenue and has financed its operations primarily through public offerings, private placements of its equity securities, and funding from its collaborations. Through June 30, 2019, the Company has raised approximately $640.0 million of proceeds from sales of convertible preferred stock and common stock, including its initial public offering and follow-on public offering, and proceeds from collaboration agreements. The Company believes that its cash, cash equivalents, and marketable debt securities of $327.5 million as of June 30, 2019, as well as amounts expected to be received for reimbursement of development costs from the Neurocrine Collaboration, are sufficient to fund its operating expenses and capital expenditure requirements debt or equity financing or generate product revenue or revenue from collaborative partners on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition.

Summary of significant accounti

Summary of significant accounting policies and basis of presentation	6 Months Ended
	Jun. 30, 2019
Summary of significant accounting policies and basis of presentation
Summary of significant accounting policies and basis of presentation	2. Summary of significant accounting policies and basis of presentation Basis of Presentation The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 as filed with the Securities and Exchange Commission (“SEC”) on February 26, 2019. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the periods presented. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). See “ Recently Adopted Accounting Pronouncements ” below for discussion of the Company’s adoption of new guidance effective January 1, 2019. All amounts and disclosures set forth in this Quarterly Report on Form 10-Q reflect adoption of these changes. Principles of Consolidation The unaudited interim consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018. Intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock-based compensation expense, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions. Certain reclassifications have been made to prior periods to conform to current period presentation. Leases Effective January 1, 2019, the Company accounted for its operating leases in accordance with ASC Topic 842 Leases (“ASC 842”). ASC 842 provides guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. The Company determines if an arrangement is a lease at inception. The Company’s operating leases are included in operating lease right-of-use (“ROU”) assets, other current liabilities, and other non-current liabilities on the condensed consolidated balance sheets. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at adoption date based on the present value of lease payments over the lease term. As the Company’s leases do not provide an implicit rate, an incremental borrowing rate was used based on the information available at the lease commencement date in determining the present value of lease payments. On future lease obligations, the implicit rate will be set when readily determinable. The operating lease ROU assets also include the effect of any lease payments made and exclude lease incentives. The Company’s lease terms may include options to extend or terminate the lease even when it is not reasonably certain that the option will be exercised. Lease expense for lease payments is recognized on a straight-line basis over the lease term. The Company has lease agreements with lease and non-lease components, which are accounted for separately. Non-lease components generally refer to common area maintenance charges related to the premises. Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in each arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development milestones 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, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestones resulting from any of the Company’s collaboration arrangements. For arrangements 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 revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. Stock-based Compensation Expense The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718 Compensation—Stock Compensation The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to a lack of company-specific historical and implied volatility data, the Company bases the estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded, blended with the most recent period of historic volatility of its common stock. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. Share-Based Payment The Company expenses the fair value of its stock-based compensation awards on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company records the expense for stock-based compensation awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates if the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date. Net Income (Loss) per Share Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases-Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). The new standard largely aligns the accounting for share-based payment awards issued to employees and nonemployees by expanding the scope of ASC 718 to apply to nonemployee share-based transactions, as long as the transaction is not effectively a form of financing. The adoption of ASU 2018-07 on January 1, 2019 did not have a material impact on the Company's consolidated financial statements. Recent Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments ​

Fair value measurements

Fair value measurements	6 Months Ended
	Jun. 30, 2019
Fair value measurements
Fair value measurements	3. Fair value measurements Assets and liabilities measured at fair value on a recurring basis as of June 30, 2019 and December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted Prices ​ Significant ​ ​ ​ ​ ​ ​ ​ ​ in Active ​ Other ​ Significant ​ ​ ​ ​ ​ Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ Assets Total (Level 1) (Level 2) (Level 3) June 30, 2019 ​ (in thousands) Money market funds included in cash and cash equivalents $ 79,108 $ 79,108 $ — $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 247,577 ​ 247,577 ​ — ​ — ​ Equity securities ​ ​ 1,280 ​ ​ 1,280 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 248,857 ​ $ 248,857 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 414 ​ ​ — ​ ​ 414 ​ ​ — ​ Total ​ $ 328,379 ​ $ 327,965 ​ $ 414 ​ $ — ​ December 31, 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents $ 46,173 $ 46,173 $ — $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 108,947 ​ 108,947 ​ — ​ — ​ Equity securities ​ ​ 628 ​ ​ 628 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 109,575 ​ $ 109,575 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 234 ​ ​ — ​ ​ 234 ​ ​ — ​ Total ​ $ 155,982 ​ $ 155,748 ​ $ 234 ​ $ — ​ ​ The Company measures the fair value of money market funds, U.S. Treasury notes and equity securities based on quoted prices in active markets for identical securities. The Level 2 equity securities include warrants used to purchase equity securities that are valued using the Black-Scholes model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the awards, (c) the risk-free interest rate, and (d) expected dividends. The assumptions utilized to value the warrants to purchase equity securities as of June 30, 2019 and December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ Risk-free interest rate 1.7 % 2.5 % ​ Expected dividend yield — % — % ​ Expected term (in years) 2.2 ​ 2.7 ​ ​ Expected volatility 88.8 % 112.7 % ​ ​ The expected volatility is based on the historic volatility for the equity securities underlying the warrants and is calculated based on a period of time commensurate with the expected term assumption. The expected term is based on the remaining contractual life of the warrants on each measurement date. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the warrants. The expected dividend yield is assumed to be zero as the entity that issued the warrants has never paid and has not indicated any intention to pay dividends.

Cash, cash equivalents, and ava

Cash, cash equivalents, and available-for-sale marketable securities	6 Months Ended
	Jun. 30, 2019
Cash, cash equivalents, and available for sale marketable securities
Cash, cash equivalents, and available for sale marketable securities	4. Cash, cash equivalents, and available-for-sale marketable securities Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Marketable securities consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. The Company classifies marketable debt securities with a remaining maturity when purchased of greater than three months as available - - - - - All available - - - - Cash, cash equivalents, and marketable securities included the following at June 30, 2019 and December 31, 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ ​ Cost Gains Losses Value ​ ​ ​ (in thousands) ​ As of June 30, 2019 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 79,108 ​ $ — ​ $ — ​ $ 79,108 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 247,348 ​ 230 ​ 1 ​ 247,577 ​ Equity securities ​ ​ 1,220 ​ ​ 60 ​ ​ — ​ ​ 1,280 ​ Total marketable securities ​ $ 248,568 ​ $ 290 ​ $ 1 ​ $ 248,857 ​ Total money market funds and marketable securities ​ $ 327,676 ​ $ 290 ​ $ 1 ​ $ 327,965 ​ As of December 31, 2018 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 46,173 ​ $ — ​ $ — ​ $ 46,173 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 108,951 ​ ​ 1 ​ ​ 5 ​ 108,947 ​ Equity securities ​ ​ 1,220 ​ ​ — ​ ​ 592 ​ ​ 628 ​ Total marketable securities ​ $ 110,171 ​ $ 1 ​ $ 597 ​ $ 109,575 ​ Total money market funds and marketable securities ​ $ 156,344 ​ $ 1 ​ $ 597 ​ $ 155,748 ​ ​ All of the Company’s marketable debt securities at June 30, 2019, have a contractual maturity of one year or less. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the condensed consolidated balance sheets that sum to the total of the same such amounts shown in the condensed consolidated statement of cash flows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ ​ 2019 2018 ​ ​ ​ (in thousands) ​ Cash and cash equivalents ​ $ 79,885 ​ $ 46,859 ​ Restricted cash included in deposits and other non-current assets ​ ​ 735 ​ ​ 735 ​ Total cash, cash equivalents, and restricted cash ​ $ 80,620 ​ $ 47,594 ​ ​

Accrued expenses

Accrued expenses	6 Months Ended
	Jun. 30, 2019
Accrued expenses
Accrued expenses	5. Accrued expenses Accrued expenses as of June 30, 2019 and December 31, 2018 consist of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ ​ (in thousands) ​ Research and development costs ​ $ 6,908 ​ $ 3,555 ​ Employee compensation costs ​ 2,319 ​ 3,780 ​ Professional services ​ ​ 1,301 ​ ​ 1,448 ​ Accrued goods and services ​ 976 ​ 784 ​ Patent costs ​ ​ 162 ​ ​ 120 ​ Other ​ 116 ​ 101 ​ Total ​ $ 11,782 ​ $ 9,788 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Right-of-use asset and lease li

Right-of-use asset and lease liabilities	6 Months Ended
	Jun. 30, 2019
Right-of-use asset and lease liabilities
Right-of-use asset and lease liabilities	6. Right-of-use asset and lease liabilities Operating Leases During April 2014, the Company entered into an agreement to lease its 75 Sidney Street facility under a non-cancelable operating lease that would have expired, if not subsequently extended, on December 15, 2019. The lease includes two renewal options, each for five-year terms and at fair market value upon exercise. The lease contains escalating rent clauses which require higher rent payments in future years. The Company expenses rent on a straight-line basis over the term of the lease, including any rent-free periods. In December 2015, the Company executed an amendment to extend the term of the 75 Sidney Street lease and also executed an agreement to lease an additional facility at 64 Sidney Street until December 31, 2024. The facility at 64 Sidney Street includes laboratory and office space and was ready for occupancy in early 2017. In February 2018, the Company executed a second amendment to the 75 Sidney Street lease to lease additional space to support its continued growth. The additional facility includes laboratory and office space and was ready for occupancy in mid-2018. In June 2018, the Company executed a third amendment to the 75 Sidney Street lease to lease additional space to further support its continued growth. The additional facility includes laboratory and office space, and was ready for occupancy in late 2018. The third amendment extended the term of the 75 Sidney Street lease to November 30, 2026. Additionally, the Company executed a second amendment to the 64 Sidney Street lease to extend that lease to November 30, 2026. The Company has received leasehold improvement incentives from the landlord totaling $1.3 million and $3.5 million for 75 Sidney Street and 64 Sidney Street, respectively. The Company recorded these incentives as a component of its ROU asset and is amortizing these incentives as a reduction of rent expense over the life of the leases. The leasehold improvements have been capitalized as fixed assets. The Company is entitled to receive approximately $0.3 million of leasehold improvements for the additional space at 75 Sidney Street acquired under the third amendment to the 75 Sidney Street lease. The Company’s lease agreements require the Company to maintain a cash deposit or irrevocable letter of credit of $0.7 million payable to the landlord as security for the performance of its obligations under the leases. These amounts are recorded as restricted cash and included in deposits and other non-current assets in the accompanying condensed consolidated balance sheets. ​ ​ ​ ​ ​ ​ ​ Total Minimum ​ Lease Payments ​ ​ (in thousands) ​ 2019 (remainder of year) ​ $ 2,756 ​ 2020 ​ 5,960 ​ 2021 ​ 6,138 ​ 2022 ​ 6,323 ​ 2023 ​ 6,512 ​ 2024+ ​ ​ 20,897 ​ Total future minimum lease payments ​ $ 48,586 ​ Less: imputed interest ​ ​ (13,138) ​ Total lease liability ​ $ 35,448 ​ Reported as: ​ ​ ​ ​ Other current liabilities ​ $ 2,843 ​ Other non-current liabilities ​ ​ 32,605 ​ Total lease liability ​ $ 35,448 ​ ​

Other liabilities

Other liabilities	6 Months Ended
	Jun. 30, 2019
Other liabilities
Other liabilities	7. Other liabilities ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ ​ (in thousands) ​ Other current liabilities ​ ​ ​ ​ ​ ​ Lease liability ​ 2,843 ​ ​ — ​ Total other current liabilities $ 2,843 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Other non-current liabilities ​ ​ ​ ​ ​ ​ Lease liability $ 32,605 ​ $ — ​ Deferred rent ​ — ​ ​ 5,710 ​ Other ​ 1,001 ​ ​ 1,001 ​ Total other non-current liabilities $ 33,606 ​ $ 6,711 ​ ​ ​

Commitments and contingencies

Commitments and contingencies	6 Months Ended
	Jun. 30, 2019
Commitments and contingencies
Commitments and contingencies	8. Commitments and contingencies Significant Agreements Sanofi Genzyme Collaboration Agreement Summary of Agreement In February 2015, the Company entered into a collaboration agreement with Sanofi Genzyme (the “Sanofi Genzyme Collaboration Agreement”), which included a non-refundable upfront payment of $65.0 million. In addition, contemporaneously with entering into the Sanofi Genzyme Collaboration Agreement, Sanofi Genzyme entered into a Series B Stock Purchase Agreement, under which Sanofi Genzyme purchased 10,000,000 shares of Series B Preferred Stock for $30.0 million. The fair value of the Series B Preferred Stock at the time of issuance was approximately $25.0 million. The $5.0 million premium over the fair value is accounted for as additional consideration under the Sanofi Genzyme Collaboration Agreement. Under the Sanofi Genzyme Collaboration Agreement, the Company granted Sanofi Genzyme an exclusive option to license, develop and commercialize (i) ex-U.S. rights to the following programs, which are referred to as “Split Territory Programs”; VY-AADC (“Parkinson’s Program”), VY-FXN01 (“Friedreich’s ataxia Program”), a future program to be designated by Sanofi Genzyme (“Future Program”), and VY-HTT01 (“Huntington’s Program”) with an incremental option to co-commercialize VY-HTT01 in the United States and (ii) worldwide rights to VY-SMN101 (“Spinal Muscular Atrophy Program”). Sanofi Genzyme’s option for the Split Territory Programs and the Spinal Muscular Atrophy Program is triggered following the completion of the first proof of principle human clinical study (“POP Study”), on a program-by-program basis. Prior to any option exercise by Sanofi Genzyme, the Company would collaborate with Sanofi Genzyme in the development of products under each Split Territory Program and/or the Spinal Muscular Atrophy Program pursuant to a written development plan and under the guidance of an Alliance Joint Steering Committee (“AJSC”), comprised of an equal number of employees from the Company and Sanofi Genzyme. Under the Sanofi Genzyme Collaboration Agreement, the Company was required to use commercially reasonable efforts to develop products under each Split Territory Program and the Spinal Muscular Atrophy Program through the completion of the applicable POP Study. During the development of these joint programs, the activities would be guided by a Development Advisory Committee (“DAC”). The DAC could elect to utilize certain Sanofi Genzyme technology relating to the Parkinson’s Program, the Huntington’s Program, or generally with the manufacture of Split Territory Program products. The Company was solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program products prior to the exercise of an option by Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme With the exception of the Parkinson’s Program, Sanofi Genzyme Upon Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Split Territory Licensed Products in the United States, including all of the associated costs. Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Upon exercise of its option to license a product in the Spinal Muscular Atrophy Program (the “Spinal Muscular Atrophy Licensed Product”), Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme In October 2017, Sanofi Genzyme decided not to exercise its option for the Parkinson’s Program (the “PD Opt-Out”). Sanofi Genzyme . In addition, to the extent any Split Territory Licensed Products or the Spinal Muscular Atrophy Licensed Product were commercialized, the Company would be entitled to tiered royalty payments ranging from the mid-single digits to mid-teens based on a percentage of net sales by Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme The Sanofi Genzyme Collaboration Agreement was to continue in effect until the later of (i) the expiration of the last to expire of the option rights and (ii) the expiration of all payment obligations unless sooner terminated by the Company or Sanofi Genzyme Sanofi Genzyme Sanofi Genzyme Termination of Agreement an Sanofi Genzyme has granted the Company exclusive, irrevocable, perpetual, royalty-free, fully-paid sublicensable (through multiple tiers), non-transferable, worldwide licenses in Sanofi Genzyme’s interests in the collaboration technology generated under or used in the Huntington’s Program and the Friedreich’s ataxia Program with respect to those programs pursuant to the Sanofi Genzyme Collaboration Agreement. In addition, Sanofi Genzyme has granted the Company non-exclusive, irrevocable, perpetual, royalty-free, fully-paid, sublicensable (through multiple tiers), non-transferable, worldwide licenses to the Sanofi Genzyme technology that was contributed to the Sanofi Genzyme Collaboration Agreement and was used in the development or manufacture of product candidates prior to the termination date. ​ Under the Sanofi Genzyme Termination Agreement, the Company paid Sanofi Genzyme $10.0 million up-front and has agreed to pay Sanofi Genzyme a $10.0 million milestone within fifteen days of the filing of an investigational new drug (“IND”) application for a product candidate incorporating certain intellectual property rights developed under or substantially related to the Huntington’s Program (a “Post-Termination HD Product”). The Company has agreed to pay Sanofi Genzyme (i) 50% of any income received from sublicensing arrangements related to Post-Termination HD Products in excess of specified thresholds and entered into prior to (x) the filing of an IND application for a Post-Termination HD Product or (y) the dosing of the first patient in a clinical trial for a Post-Termination HD Product in the United States or certain European countries, respectively and (ii) a low-double digit percentage of any income received from sublicensing arrangements outside the United States related to products incorporating intellectual property rights developed under, or substantially related to, the Friedreich’s ataxia Program (the “Post-Termination FA Products”) that are in excess of a specified threshold and entered into prior to the dosing of the first patient in a clinical trial for a Post-Termination FA Product in the United States or certain European countries, in each case, subject to certain limitations. The Company has also agreed to pay low-single digit royalties on net sales of Post-Termination HD Products. Under the Sanofi Genzyme Collaboration Agreement, the Company had rights to certain in-kind services. As of the termination date the Company waived its right to approximately $0.4 million in unused in-kind services, and the Company no longer has the right to receive any option payments, regulatory or commercial milestone payments or royalties from Sanofi Genzyme under the Sanofi Genzyme Collaboration Agreement. ​ The Company has granted Sanofi Genzyme an exclusive royalty-free, fully-paid, sublicensable (through multiple tiers), non-transferable, worldwide license under the Company’s interest in the collaboration technology generated under or used in the Spinal Muscular Atrophy Program pursuant to the Sanofi Genzyme Collaboration Agreement to manufacture, develop, and commercialize any Spinal Muscular Atrophy product. Under the Amended Capsid Agreement, the Company has granted Sanofi Genzyme an exclusive option to evaluate up to four capsids for no consideration. During the capsid evaluation period, the Company has granted Sanofi Genzyme a non-exclusive license to the capsid intellectual property to conduct evaluation studies. In addition, Sanofi Genzyme is able to evaluate up to two additional capsids for a low six-figure payment per additional capsid. The Company is not obligated to perform any additional research on the capsids. Sanofi Genzyme shall have the right to obtain an exclusive license for up to two capsids, each in a specified non-CNS indication. At its discretion, Sanofi Genzyme may exercise both its options for the same capsid for different specified non-CNS indications. Upon its exercise of each option, Sanofi Genzyme has agreed to pay the Company a $1.0 million option exercise fee. Under the Amended Capsid Agreement, the Company is also entitled to receive potential development and regulatory milestone payments upon the achievement of certain milestone events for products containing licensed capsids (“Licensed Products”) of up to an aggregate of $15.0 million per Licensed Product. In addition, for each specified indication, Sanofi Genzyme has agreed to pay to the Company a one-time sales milestone payment of $20.0 million, if aggregate worldwide net sales for all Licensed Products for such specified indication surpass a specified amount, and low-to-mid single-digit tiered royalty payments on worldwide net sales of Licensed Products, on a Licensed Product-by-Licensed Product basis. ​ Accounting Analysis At inception, the Sanofi Genzyme Collaboration Agreement included the following performance obligations: (i) research and development services for each of the Split Territory Programs and the Spinal Muscular Atrophy Program, (ii) participation in the AJSC, (iii) participation in the DAC and (iv) a material right associated with an option to obtain a development and commercial license in the Parkinson’s Program (“PD Material Right”). The Company determined that the option to obtain a development and commercial license in the Parkinson’s Program was a material right under ASC 606 primarily because there were no additional option exercise payments payable by Sanofi Genzyme at the time of option exercise. Therefore, the PD Material Right was considered a performance obligation at the inception of the arrangement. The options in the other Split Territory Programs and the Spinal Muscular Atrophy Program do not provide a material right to the customer that it would receive without entering into the contract principally because the option fees are at least equal to the standalone selling price for the underlying goods. Therefore, the other Split Territory Programs and the Spinal Muscular Atrophy Program options are not performance obligations at inception. The Company has identified $74.6 million of total transaction price consisting of the $65.0 million upfront fee, the $5.0 million premium paid in excess of fair value of the Series B Preferred Stock and $4.6 million of Sanofi Genzyme “in-kind” funding, which represents the transaction price at adoption. Additional consideration to be paid to the Company upon the exercise of the license options by Sanofi Genzyme or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contract term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. For all performance obligations, the Company determined the standalone selling price at contract inception based on each obligation’s estimated standalone selling price (“ESP”). The Company determined the ESP for the service related deliverable for the research and development activities based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, marked up to include a reasonable profit margin and adjusted for the scope of the potential license. Significant inputs used to determine the total expense of the research and development activities include, the length of time required and the number and costs of various studies that will be performed to complete the applicable POP Study. The ESP for the AJSC and DAC have been estimated based on the costs incurred to participate in the committees, marked up to include a reasonable profit margin. The ESP for the PD Material Right was determined based on the estimated value of the license adjusted for the estimated probability that the option would be exercised by Sanofi Genzyme. Based on the relative standalone selling price allocation, the allocation of the transaction price to the separate performance obligations was as follows: ​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Research and Development Services for: ​ ​ ​ ​ Huntington’s Program ​ $ 14,228 ​ Parkinson’s Program ​ ​ 6,040 ​ Friedreich’s ataxia Program ​ ​ 14,821 ​ Spinal Muscular Atrophy Program ​ ​ 29,116 ​ Future Program ​ ​ 2,239 ​ Committee Obligations: ​ ​ ​ ​ AJSC ​ ​ 133 ​ DAC ​ ​ 207 ​ PD Material Right ​ ​ 7,855 ​ Total ​ $ 74,639 ​ ​ The Company recognizes the amounts associated with research and development services and committee obligations on a proportional performance basis over the period of service using input-based measurements such as costs incurred to date, to estimate proportion performed, and remeasures its progress towards completion at the end of each reporting period. Contingent consideration related to the performance of in-kind services is considered in the transaction price based on the most-likely amount, which is the full amount of the services that the Company can require Sanofi Genzyme to complete. The amount allocated to the PD Material Right was initially deferred and recognized in full prior to the adoption of ASC 606. During the three and six months ended June 30, 2019, the Company recognized $1.5 million and $2.9 million, respectively, of revenue related to research and development services and committee obligations performed under the Sanofi Genzyme Collaboration prior to the termination of the collaboration on June 14, 2019 (the “Termination Date”). During the three and six months ended June 30, 2018, the Company recognized $0.7 million and $1.2 million, respectively, of revenue to research and development services and committee obligations performed under the Sanofi Genzyme Collaboration. Costs incurred relating to the programs that Sanofi Genzyme has the option to license under the Sanofi Genzyme Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies and preclinical research studies. These costs are included in research and development expenses in the Company’s condensed consolidated statement of operations during the three and six months ended June 30, 2019 and 2018. The Sanofi Genzyme Termination Agreement modifies both the pricing and scope of the Sanofi Genzyme Collaboration Agreement. As the modification does not add distinct goods or services to the Sanofi Genzyme Collaboration Agreement, the agreement is considered a modification of the original contract. ​ The Sanofi Genzyme Termination Agreement includes the following performance obligations: (i) worldwide license to collaboration technology and Sanofi Genzyme technology for the development, manufacturing and commercialization of the Huntington’s Program and (ii) worldwide license to collaboration technology and Sanofi Genzyme technology for the development, manufacturing and commercialization for the Friedreich’s ataxia Program. Such performance obligations were satisfied upon the Termination Date as control had transferred upon execution of the Sanofi Genzyme Termination Agreement. Therefore, the remainder of the transaction price under the Sanofi Genzyme Collaboration Agreement, which had not yet been recognized, was recognized as revenue upon the Termination Date. The Company has recognized $28.7 million of revenue upon the Termination Date . This amount consists of $48.7 million of deferred revenue related to the original agreement as of the Termination Date , offset by (x ) $10.0 million related to the fee paid by the Company to Sanofi Genzyme on the Termination Date, and (y ) $10.0 million related to the milestone payment which the Company expects to pay to Sanofi Genzyme upon the potential filing of an IND for a product candidate in connection with the Huntington’s Program. Further preclinical studies are underway with VY-HTT01 which, if successful, are expected to support a potential filing of an IND application in 2019. The Company has constrained $10.0 million of the remaining deferred revenue balance at the Termination Date as it expects to pay the milestone payment related to the potential filing of an IND for a product candidate in connection with the Huntington’s Program in 2019. As a result, the Company will maintain a $ 10.0 million deferred revenue balance associated with the potential milestone payment. This deferral will be reversed upon payment of the milestone to Sanofi Genzyme. If the Company decides not to file an IND for a product candidate in connection with the Huntington’s Program, the Company will recognize that amount as revenue upon determining that the IND filing is no longer likely. The $20.0 million payable by the Company to Sanofi Genzyme is treated as consideration payable to a customer and therefore accounted as a reduction of the transaction price. In the three and six months ended June 30, 2019, the Company recognized total revenue of $30.2 million and $31.6 million related to services performed and the subsequent termination of the Sanofi Genzyme Collaboration Agreement. At June 30, 2019, $ 10.0 million remains in deferred revenue in the accompanying condensed consolidated balance sheet. AbbVie Tau Collaboration Agreement Summary of Agreement ​ In February 2018, the Company entered into an exclusive collaboration and option agreement (the “AbbVie Tau Collaboration Agreement”) with AbbVie for the research, development and commercialization of AAV and other virus-based gene therapy products for the treatment of diseases of the central nervous system and other neurodegenerative diseases related to defective or excess aggregation of tau protein in the human brain, including Alzheimer’s disease. Under the AbbVie Tau Collaboration Agreement, the Company and AbbVie have agreed to collaborate on the research and development of specified vectorized antibody compounds comprised of an AAV or other viral capsid and a virus vector genome that encodes one or more antibodies that target and bind to a tau protein. The collaboration is comprised of a research period (the “Research Period”), a development period (the “Development Period”), and an exclusive license option (the “License Option”). The AbbVie Tau Collaboration Agreement included a non-refundable upfront payment of $69.0 million for services during the Research Period. During the Research Period, each party has agreed to identify up to five antibodies for inclusion in the collaboration. Subject to certain conditions and exceptions, the parties will select up to three antibodies (each, a “Research Antibody”) as candidates for creation of research compounds (each, a “Research Compound”), with AbbVie having the right to select two of the three Research Antibodies. The Company is required to use diligent efforts to conduct antibody engineering and other research activities to create Research Compounds and to develop product candidates containing or comprised of such Research Compounds (“Product Candidates”). The Company is solely responsible for its costs and expenses during the Research Period. During a specified portion of the Research Period, AbbVie may exercise one or more of its exclusive development options (each, a “Development Option”) to select up to a total of three Research Compounds (the “Selected Research Compounds”) and their corresponding Product Candidates (the “Selected Product Candidates”) to proceed to the Development Period. ​ Upon AbbVie’s exercise of a Development Option, AbbVie will pay the Company $80.0 million for the first Selected Research Compound and $30.0 million each for up to two additional Selected Research Compounds. During the Development Period, the Company is obligated to use diligent efforts to conduct development activities, including Investigational New Drug application-enabling and Phase 1 clinical trial activities, for the Selected Research Compounds and corresponding Selected Product Candidates. The Company will be solely responsible for the costs and expenses during the Development Period. During a specified portion of the Development Period (the “License Option Period”), AbbVie may exercise its License Option to further develop and commercialize all of the Research Compounds (the “Licensed Compounds”), and corresponding product candidates (the “Licensed Products”). Upon AbbVie’s exercise of its License Option, AbbVie will provide a one-time payment of $75.0 million to the Company, and the Company will grant to AbbVie an exclusive, worldwide license, with the right to sublicense, under certain of the Company’s intellectual property rights to develop and commercialize the Licensed Compounds and the Licensed Products for all human diagnostic, prophylactic and therapeutic uses. In addition, after AbbVie’s exercise of the License Option, the Company has certain obligations to complete any remaining research and development activities that have not been completed for any Research Compounds and Product Candidates. The Company’s research and development activities will be conducted pursuant to the plans agreed to by the parties and overseen by a joint governance committee (“JGC”) as detailed in the AbbVie Tau Collaboration Agreement. Any material amendment to the research or development plans must be mutually agreed to by the Company and AbbVie, which may be through the JGC. Under the AbbVie Tau Collaboration Agreement, AbbVie is required to use commercially reasonable efforts to develop and commercialize at least one Licensed Product in each of the United States, Japan, the United Kingdom, Germany, France, Italy, and Spain. After exercise of the License Option, AbbVie is solely responsible for all development and commercialization activities relating to Licensed Compounds and Licensed Products at its sole cost and expense, subject to the agreed-upon research and development plans. The Company may elect to share in AbbVie’s development costs relating to a Licensed Product on an indication-by-indication basis in exchange for a specified increase in royalties (a “Cost-Sharing Option”). If the Company exercises a Cost-Sharing Option, the Company may either reimburse AbbVie for AbbVie’s applicable development costs or, in the case of certain budget overruns, AbbVie may instead deduct applicable development costs, up to a specified cap, from milestone and royalty payments owed by AbbVie to the Company. Under the AbbVie Tau Collaboration Agreement, the Company is eligible to receive specified development and first-sale milestone payments for each Licensed Compound of up to an aggregate of $550.0 million in the case of an Alzheimer’s disease indication, up to $230.0 million in the case of the first indication other than Alzheimer’s disease and up to $115.0 million for a subsequent non-Alzheimer’s disease indication. Additionally, the Company is eligible to receive tiered, escalating royalties, in a range from a high-single digit to a mid-to-high teen (or, if the Company has exercised its Cost-Sharing Option, low-twenties) percentage of aggregate net sales of Licensed Products on a Licensed Compound by Licensed Compound basis, subject to potential reductions in certain circumstances. For each Licensed Product, AbbVie also has the right to decrease or eliminate its royalty payments on such Licensed Product in exchange for a one-time payment by AbbVie at a fair market value to be negotiated by the parties or determined pursuant to dispute resolution procedures specified in the AbbVie Collaboration Agreement. Unless earlier terminated, the AbbVie Tau Collaboration Agreement will expire on the earliest to occur of the expiration of (i) the Development Option Period, without AbbVie’s exercise of a Development Option; (ii) the License Option Period, without AbbVie’s exercise of its License Option; and (iii) the last-to-expire royalty term with respect to all Licensed Products in all countries. The Company and AbbVie have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party, and AbbVie has the right to terminate for convenience. ​ Accounting Analysis The Company assessed the promised goods and services under the AbbVie Tau Collaboration Agreement, in accordance with ASC 606, and determined that the AbbVie Tau Collaboration Agreement includes the following performance obligations: (i) research services during the Research Period (through the delivery of the final research report) including the identification of the Research Antibodies, conduct of research activities and provision of information to AbbVie to allow AbbVie to determine whether to exercise up to three development options to be rendered (“Research Services”), and (ii) a material right associated with the Development Option on the first Research Compound and associated Product Candidates (“First Development Option Material Right”). The first Development Option provides AbbVie with (i) additional development services on a selected Research Compound and (ii) the ability to exercise the License Option. The Company has concluded the option provides a material right as the consideration paid by AbbVie upon exercise of the first Development Option is less than the amount that the Company would otherwise expect to receive outside the context of the contract. The Company has concluded that the First Development Option Material Right is a separate performance obligation under ASC 606 as AbbVie is provided additional services and a License Option for additional consideration that represents a significant discount from amounts that would otherwise be offered outside the context of the contract. The First Development Option Material Right is distinct from the other performance obligations in the arrangement as it is an option in the contract that is not required for AbbVie to obtain the benefit of the other promised goods or services in the arrangement. The First Development Option Material Right does not include the underlying goods or services that are delivered upon exercise of the option, but rather represents the value to the customer of having the right to obtain development services and the right to the License Option at an advantageous price. The Company received a nonrefundable, upfront payment of $69.0 million as consideration under the AbbVie Collaboration Agreement, which represents the transaction price at inception. Additional consideration to be paid to the Company upon the exercise of the Development and License Options by AbbVie or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contact term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. The Company determined the standalone selling price at contract inception based on each obligation’s ESP. The Company determined the ESP for the research services obligation based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, inclusive of a reasonable profit margin. Significant inputs used to determine the total expense of the research services include the length of time required, the internal hours expected to be incurred on the services and the number and costs of various studies that will be performed to complete the Research Plan. The ESP for the First Development Option Material Right was determined based on the fees AbbVie would pay to exercise the Development and License Options, the estimated costs to perform the development services, inclusive of a reasonable profit margin, the estimated value of the License Option using comparable transactions, and the probability that the Development and License Options would be exercised by AbbVie. Based on the relative standalone selling price, the allocation of the transaction price to the separate performance obligations was as follows: ​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Research Services ​ $ 34,482 ​ First Development Option Material Right ​ ​ 34,518 ​ Total ​ $ 69,000 ​ ​ The Company recognizes the amounts associated with Research Services on a proportional performance basis over the period of service using input-based measurements of total cost of research incurred to estimate proportion performed and remeasures its progress towards completion at the end of each reporting period. The amount allocated to the First Development Option Material Right is recorded as deferred revenue and will be recognized either over the period in which goods and services underlying the option are transferred or upon expiry of the option. During the three and six months ended June 30, 2019, the Company recognized $3.2 million and $4.9 million, respectively, of revenue associated with the AbbVie Tau Collaboration related to the Research Services performed during the period. During the three and six months ended June 30, 2018, the Company recognized $1.9 million and $2.3 million, respectively, of revenue associated with the AbbVie Tau Collaboration related to the Research Services performed during the period. As of June 30, 2019, there was $57.1 million of deferred revenue related to the AbbVie Tau Collaboration Agreement, which is classified as either current or non-current in the accompanying condensed consolidated balance sheet based on the period the goods or services are expected to be delivered. Costs incurred relating to the AbbVie Tau Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies, and preclinical research studies. All of these costs are included in research and development expenses in the Company’s condensed consolidated statement of operations during the three and six months ended June 30, 2019. AbbVie Alpha Synuclein Collaboration Agreement Summary of Agreement In February 2019, the Company entered into an exclusive collaboration and option agreement (“the AbbVie Alpha-Synuclein Collaboration Agreement”) with AbbVie, for the research, development and commercialization of AAV and other virus-based gene therapy products directed against pathological species of alpha-synuclein for the potential treatment of Parkinson’s disease and other diseases characterized by the abnormal accumulation of misfolded alpha-synuclein protein (“synucleinopathies”). Under the AbbVie Alpha-Synuclein Collaboration Agreement, the Company and AbbVie have agreed to collaborate on the research and development of specified vectorized antibody compounds comprised of an AAV or other viral capsid and a virus vector genome that encodes one or more antibodies that target and bind to the alpha-synuclein protein. The collaboration is comprised of a research period (the “ASN Research Period”), an optional development period (the “ASN Development Period”), and an exclusive license option (the “ASN License Option”). The AbbVie Alpha-Synuclein Collaboration Agreement included a non-refundable upfront payment of $65.0 million for services during the ASN Research Period. During the ASN Research Period, the Company is obligated to conduct research activities directed to constructing one or more virus vectors that encode antibodies designated by AbbVie (the “AbbVie Designated Antibodies”) which initially are to be antibodies provided by AbbVie. The Company is obligated to use diligent efforts to conduct research activities to create research compounds (“ASN Research Compounds”) and to develop product candidates containing or comprised of the ASN Research Compounds (“ASN Product Candidates”). The Company is solely responsible for the costs and expenses during the ASN Research Period. During a specified portion of the ASN Research Period, AbbVie may exercise one or more of its exclusive development options to select up to a total of four ASN Research Compounds and their corresponding ASN Product Candidates to proceed to the ASN Development Period. Upon AbbVie’s exercise of an option to proceed to the ASN Development Period (an “ASN Development Option”), AbbVie will pay the Company $80.0 million for the first ASN Research Compound and $30.0 million each for up to three additional ASN Research Compounds. During the ASN Development Period, the Company is obligated to use diligent efforts to condu

Preferred stock

Preferred stock	6 Months Ended
	Jun. 30, 2019
Preferred stock
Preferred stock	9. Preferred stock The Company has authorized preferred stock amounting to 5,000,000 shares as of June 30, 2019 and December 31, 2018. The authorized preferred stock was classified under stockholders’ equity at June 30, 2019 and December 31, 2018.

Stock based compensation

Stock based compensation	6 Months Ended
	Jun. 30, 2019
Stock based compensation
Stock based compensation	10. Stock-based compensation 2014 Stock Option and Grant Plan In January 2014, the Company adopted the 2014 Stock Option and Grant Plan (the “2014 Plan”), under which it could grant incentive stock options, non-qualified stock options, restricted stock awards, unrestricted stock awards, or restricted stock units to purchase up to 823,529 shares of common stock to employees, officers, directors and consultants of the Company. In April 2014, the Company amended the 2014 Plan to allow for the issuance of up to 1,411,764 shares of common stock. In August 2014, April 2015, August 2015, and October 2015, the Company further amended the 2014 Plan to allow for the issuance of up to 2,000,000, 2,047,058, 2,669,411, and 2,998,823 shares of common stock, respectively. During 2014 the Company issued only restricted stock awards under the 2014 Plan and during 2015 the Company only granted stock options under the 2014 Plan. The terms of stock option agreements, including vesting requirements, were determined by the Board of Directors and were subject to the provisions of the 2014 Plan. Restricted stock awards granted by the Company generally vest based on each grantee’s continued service with the Company during a specified period following grant. Stock options granted to employees generally vest over four years, with 25% vesting on the one year anniversary and 75% vesting ratably, on a monthly basis, over the remaining three years. Stock options granted to non-employee consultants generally vest monthly over a period of one to four years. Founder Awards In January 2014, the Company issued 1,188,233 shares of restricted stock to its founders at an original issuance price of $0.0425 per share. Of the total restricted shares awarded to the founders, 835,292 shares generally vested over one to four years , based on each founder’s continued service to the Company in varying capacity as a Scientific Advisory Board member, consultant, director, officer or employee, as set forth in each grantee’s individual restricted stock purchase agreement. These shares were fully vested as of June 30, 2019. Stock-based compensation expense associated with these time-based awards was recognized over the vesting period. The remaining 352,941 of the shares issued begin vesting upon the achievement of certain performance objectives as well as continued service to the Company, as set forth in the agreements. These performance conditions are tied to certain milestone events specific to the Company’s corporate goals, including but not limited to preclinical and clinical development milestones related to the Company’s product candidates. Stock-based compensation expense associated with these performance-based awards is recognized when the achievement of the performance condition is considered probable, using management’s best estimates. During 2016, management determined that the achievement of the performance milestone for one of the three performance-based awards had become probable and began recognizing stock-based compensation accordingly. The Company recorded $0.1 million and $0.3 million in stock-based compensation expense related to this award during the three and six months ended June 30, 2018, respectively. No stock-based compensation expense was recorded in 2019 related to this award as it was fully vested during 2018. No stock-based compensation expense was recorded for the remaining two founders’ awards with performance-based vesting as of June 30, 2019 as the performance-based milestones related to these awards were not probable. 2015 Stock Option Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan (the “2015 Stock Option Plan”), which became effective upon the completion of the Company’s initial public offering (the “IPO”). The 2015 Stock Option Plan provides the Company with the flexibility to use various equity-based incentive and other awards as compensation tools to motivate its workforce. These tools include stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash-based awards. The 2015 Stock Option Plan replaced the 2014 Plan. Any options or awards outstanding under the 2014 Plan remained outstanding and effective. The number of shares initially reserved for issuance under the 2015 Stock Option Plan is the sum of (i) 1,311,812 shares of common stock and (ii) the number of shares under the 2014 Plan that are not needed to fulfill the Company’s obligations for awards issued under the 2014 Plan as a result of forfeiture, expiration, cancellation, termination or net issuances of awards thereunder. The number of shares of common stock that may be issued under the 2015 Stock Option Plan is also subject to increase on the first day of each fiscal year by up to 4% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31. Effective January 1, 2016, 2017, 2018, and 2019, an additional 1,069,971, 1,070,635, 1,285,200, and 1,302,830 shares, respectively, were added to the Company’s 2015 Stock Option Plan for future issuance. As of June 30, 2019, there were 2,166,946 shares of common stock available for future award grants under the 2015 Stock Option Plan. During the three and six months ended June 30, 2019, the Company granted a total of 557,580 and 1,310,920 stock options, respectively, to employees and directors under the 2015 Stock Option Plan. During the three and six months ended June 30, 2019, the Company awarded a total of 65,052 and 419,457 restricted stock units, respectively, to employees under the 2015 Stock Option Plan. During the three and six months ended June 30, 2019, there were no new stock options issued to non-employees under the 2015 Stock Option Plan. 2015 Employee Stock Purchase Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). The 2015 ESPP became effective upon the closing of the IPO. Under the 2015 ESPP, all full-time employees of the Company are eligible to purchase common stock of the Company twice per year, at the end of each six-month payment period. During each payment period, eligible employees who so elect, may authorize payroll deductions in an amount of 1% to 10% (whole percentages only) of the employee’s base pay for each payroll period. At the end of each payment period, the accumulated deductions are used to purchase shares of common stock from the Company at a discount. A total of 262,362 shares of common stock were initially authorized for issuance under this plan. The number of shares of common stock that may be issued under the 2015 ESPP is also subject to increase on the first day of each fiscal year by up to 1% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31. Effective January 1, 2016, 2017, 2018, and 2019, 267,492, 267,658, 321,300, and 325,707 shares of common stock, respectively, were added to the 2015 ESPP. Inducement Awards In the three and six months ended June 30, 2019, the Company issued to two executives non-statutory stock options to purchase an aggregate of 338,750 shares of the Company’s common stock and restricted stock unit awards for an aggregate of 58,125 units of the Company’s common stock, in each case outside of the Company’s 2015 Stock Option Plan as an inducement material to such executive’s acceptance of an offer of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). The stock options will vest over a four-year period, with 25% of the shares underlying the option award vesting on the first anniversary of the award and the remaining 75% of the shares underlying the award vesting monthly thereafter over the subsequent 36-month period. The restricted stock units will vest over a three-year period, with 33% of the restricted stock units vesting on the first anniversary, 33% of the restricted stock units vesting on the second anniversary, and the remaining restricted stock units vesting on the third anniversary. ​ Stock-Based Compensation Expense Total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive income (loss) is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, June 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) Research and development ​ $ 3,256 ​ $ 1,332 ​ $ 4,671 ​ $ 2,530 ​ General and administrative ​ 1,998 ​ 6,782 ​ 4,100 ​ 7,906 ​ Total stock-based compensation expense ​ $ 5,254 ​ $ 8,114 ​ $ 8,771 ​ $ 10,436 ​ ​ Stock-based compensation expense by type of award included within the consolidated statements of operations and comprehensive income (loss) was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, ​ June 30, ​ ​ 2019 2018 ​ 2019 2018 ​ ​ ​ (in thousands) ​ Stock options ​ $ 4,528 ​ $ 7,902 ​ $ 7,304 ​ $ 9,868 ​ Restricted stock awards and units ​ ​ 668 ​ ​ 163 ​ ​ 1,351 ​ ​ 457 ​ Employee stock purchase plan awards ​ 58 ​ 49 ​ ​ 116 ​ ​ 111 ​ Total stock-based compensation expense ​ $ 5,254 ​ $ 8,114 ​ ​ 8,771 ​ ​ 10,436 ​ ​ million of stock-based compensation expense related to the non-substantice service period of the Sah Agreement. Restricted Stock Units A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plans for the six months ended June 30, 2019 was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ Fair Value ​ ​ Units Per Unit ​ Unvested restricted stock units as of December 31, 2018 — ​ ​ — ​ Granted 419,457 ​ $ 10.91 ​ Vested — ​ ​ — ​ Forfeited (16,675) ​ $ 9.09 ​ Unvested restricted stock units as of June 30, 2019 402,782 ​ $ 10.98 ​ ​ Stock-based compensation of restricted stock units is based on the fair value of the Company’s common stock on the date of grant and recognized over the vesting period. The weighted average fair value of restricted stock units granted to employees during the three and six months ended June 30, 2019 was $20.74 and $10.91 per share, respectively. The restricted stock units granted in the three and six months ended June 30, 2019 vest in equal amounts, annually over three years. There were no restricted stock units granted to employees in the three and six months ended June 30, 2018. The expense related to restricted stock units granted to employees was $0.7 million and $1.4 million for the three and six months ended June 30, 2019, respectively. As of June 30, 2019, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock units of $3.6 million, which is expected to be recognized over the remaining average vesting period of 2.6 years. ​ Stock Options The following is a summary of stock option activity for the six months ended June 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Remaining Aggregate ​ ​ ​ ​ ​ Average ​ Contractual ​ Intrinsic ​ ​ ​ ​ ​ Exercise ​ Life ​ Value ​ ​ Shares Price (in years) (in thousands) ​ Outstanding at December 31, 2018 4,225,152 ​ $ 15.91 ​ ​ ​ ​ ​ ​ Granted 1,310,920 ​ $ 14.70 ​ ​ ​ ​ ​ ​ Exercised (88,821) ​ $ 11.24 ​ ​ ​ ​ ​ ​ Cancelled or forfeited (207,716) ​ $ 14.05 ​ ​ ​ ​ ​ ​ Outstanding at June 30, 2019 5,239,535 ​ $ 15.76 8.3 ​ $ 60,649 ​ Exercisable at June 30, 2019 1,972,340 ​ $ 13.87 7.2 ​ $ 26,574 ​ Vested and expected to vest at June 30, 2019 5,239,535 ​ $ 15.76 8.3 ​ $ 60,649 ​ ​ Using the Black ‑ The weighted-average fair value of options granted to employees and directors during the three and six months ended June 30, 2018 was $13.88 and $15.92 per share, respectively. The expense related to awards granted to employees and directors was $7.9 million and $9.8 million for the three and six months ended June 30, 2018, respectively. ​ The fair value of each option issued to employees and directors was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, ​ June 30, ​ ​ ​ 2019 2018 ​ 2019 2018 ​ Risk-free interest rate 2.2 % 2.8 % 2.4 % 2.7 % ​ Expected dividend yield — % — % — % — % ​ Expected term (in years) 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ ​ Expected volatility 75.1 % 74.7 % 75.1 % 74.4 % ​ ​ There were no new equity awards granted to non-employees during the three and six months ended June 30, 2019. The expense related to stock option awards previously granted to non ‑ ​ As of June 30, 2019, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $30.5 million which is expected to be recognized over the remaining weighted-average vesting period of 2.8 years.

Income taxes

Income taxes	6 Months Ended
	Jun. 30, 2019
Income taxes
Income taxes	11. Income taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. For the three and six months ended June 30, 2019 and 2018, the Company recognized a de minimis tax expense in other comprehensive income related to the unrealized gain on available-for-sale securities. ​

Net income (loss) per share

Net income (loss) per share	6 Months Ended
	Jun. 30, 2019
Net income (loss) per share
Net income (loss) per share	12. Net income (loss) per share The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net income (loss) per share because to include them would be anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Unvested restricted common stock awards ​ 235,294 ​ ​ 347,724 ​ 235,294 ​ 347,724 ​ Unvested restricted common stock units ​ ​ 154,781 ​ ​ — ​ ​ 402,782 ​ ​ — ​ Outstanding stock options ​ 4,157,197 ​ ​ 6,700,282 ​ 5,239,535 ​ 6,700,282 ​ Total ​ 4,547,272 ​ ​ 7,048,006 ​ 5,877,611 ​ 7,048,006 ​ ​ Basic net income (loss) and diluted weighted-average shares outstanding are as follows for the three and six months ended June 30, 2019 and 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ ​ ​ 2019 2018 ​ 2019 2018 ​ ​ ​ (in thousands, except share data) ​ Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ $ 11,153 ​ $ (25,541) ​ $ (16,017) ​ $ (45,468) ​ Denominator for basic net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 36,610,918 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​ Denominator for diluted net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 36,610,918 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​ Common stock options and restricted stock units ​ ​ 1,330,339 ​ ​ — ​ ​ — ​ ​ — ​ Weighted average shares and conversions ​ ​ 37,941,257 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​ ​ Basic net loss per share for the six months ended June 30, 2019 and 2018, is the same as diluted net loss per share as shown on the Company’s condensed consolidated statement of operations.

Related party transactions

Related party transactions	6 Months Ended
	Jun. 30, 2019
Related party transactions
Related party transactions	13. Related-party transactions During the three and six months ended June 30, 2019 and 2018, the Company received consulting and management services from one of its investors. The total amount of the services provided by this investor was de minimis during the three and six months ended June 30, 2019 and 2018. ​

Summary of significant accoun_2

Summary of significant accounting policies and basis of presentation (Policies)	6 Months Ended
	Jun. 30, 2019
Summary of significant accounting policies and basis of presentation
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 as filed with the Securities and Exchange Commission (“SEC”) on February 26, 2019. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the periods presented. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). See “ Recently Adopted Accounting Pronouncements ” below for discussion of the Company’s adoption of new guidance effective January 1, 2019. All amounts and disclosures set forth in this Quarterly Report on Form 10-Q reflect adoption of these changes.
Principles of Consolidation	Principles of Consolidation The unaudited interim consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018. Intercompany accounts and transactions have been eliminated.
Use of estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock-based compensation expense, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions. Certain reclassifications have been made to prior periods to conform to current period presentation.
Leases	Leases Effective January 1, 2019, the Company accounted for its operating leases in accordance with ASC Topic 842 Leases (“ASC 842”). ASC 842 provides guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. The Company determines if an arrangement is a lease at inception. The Company’s operating leases are included in operating lease right-of-use (“ROU”) assets, other current liabilities, and other non-current liabilities on the condensed consolidated balance sheets. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at adoption date based on the present value of lease payments over the lease term. As the Company’s leases do not provide an implicit rate, an incremental borrowing rate was used based on the information available at the lease commencement date in determining the present value of lease payments. On future lease obligations, the implicit rate will be set when readily determinable. The operating lease ROU assets also include the effect of any lease payments made and exclude lease incentives. The Company’s lease terms may include options to extend or terminate the lease even when it is not reasonably certain that the option will be exercised. Lease expense for lease payments is recognized on a straight-line basis over the lease term. The Company has lease agreements with lease and non-lease components, which are accounted for separately. Non-lease components generally refer to common area maintenance charges related to the premises.
Revenue Recognition	Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in each arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development milestones 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, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestones resulting from any of the Company’s collaboration arrangements. For arrangements 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 revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Stock-based Compensation Expense	Stock-based Compensation Expense The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718 Compensation—Stock Compensation The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to a lack of company-specific historical and implied volatility data, the Company bases the estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded, blended with the most recent period of historic volatility of its common stock. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. Share-Based Payment The Company expenses the fair value of its stock-based compensation awards on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company records the expense for stock-based compensation awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates if the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date.
Net Income (Loss) per Share	Net Income (Loss) per Share
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases-Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). The new standard largely aligns the accounting for share-based payment awards issued to employees and nonemployees by expanding the scope of ASC 718 to apply to nonemployee share-based transactions, as long as the transaction is not effectively a form of financing. The adoption of ASU 2018-07 on January 1, 2019 did not have a material impact on the Company's consolidated financial statements. Recent Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments

Fair value measurements (Tables

Fair value measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair value measurements
Assets and liabilities measured at fair value on a recurring basis	Assets and liabilities measured at fair value on a recurring basis as of June 30, 2019 and December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Quoted Prices ​ Significant ​ ​ ​ ​ ​ ​ ​ ​ in Active ​ Other ​ Significant ​ ​ ​ ​ ​ Markets for ​ Observable ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Inputs ​ Inputs ​ Assets Total (Level 1) (Level 2) (Level 3) June 30, 2019 ​ (in thousands) Money market funds included in cash and cash equivalents $ 79,108 $ 79,108 $ — $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 247,577 ​ 247,577 ​ — ​ — ​ Equity securities ​ ​ 1,280 ​ ​ 1,280 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 248,857 ​ $ 248,857 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 414 ​ ​ — ​ ​ 414 ​ ​ — ​ Total ​ $ 328,379 ​ $ 327,965 ​ $ 414 ​ $ — ​ December 31, 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents $ 46,173 $ 46,173 $ — $ — ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 108,947 ​ 108,947 ​ — ​ — ​ Equity securities ​ ​ 628 ​ ​ 628 ​ ​ — ​ ​ — ​ Total marketable securities ​ $ 109,575 ​ $ 109,575 ​ $ — ​ $ — ​ Warrants to purchase equity securities ​ ​ 234 ​ ​ — ​ ​ 234 ​ ​ — ​ Total ​ $ 155,982 ​ $ 155,748 ​ $ 234 ​ $ — ​
Schedule of fair value assumptions	The assumptions utilized to value the warrants to purchase equity securities as of June 30, 2019 and December 31, 2018 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ Risk-free interest rate 1.7 % 2.5 % ​ Expected dividend yield — % — % ​ Expected term (in years) 2.2 ​ 2.7 ​ ​ Expected volatility 88.8 % 112.7 % ​

Cash, cash equivalents, and a_2

Cash, cash equivalents, and available-for-sale marketable securities (Tables)	6 Months Ended
	Jun. 30, 2019
Cash, cash equivalents, and available for sale marketable securities
Cash, cash equivalents and marketable securities	Cash, cash equivalents, and marketable securities included the following at June 30, 2019 and December 31, 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ ​ Cost Gains Losses Value ​ ​ ​ (in thousands) ​ As of June 30, 2019 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 79,108 ​ $ — ​ $ — ​ $ 79,108 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 247,348 ​ 230 ​ 1 ​ 247,577 ​ Equity securities ​ ​ 1,220 ​ ​ 60 ​ ​ — ​ ​ 1,280 ​ Total marketable securities ​ $ 248,568 ​ $ 290 ​ $ 1 ​ $ 248,857 ​ Total money market funds and marketable securities ​ $ 327,676 ​ $ 290 ​ $ 1 ​ $ 327,965 ​ As of December 31, 2018 ​ ​ ​ ​ ​ Money market funds included in cash and cash equivalents ​ $ 46,173 ​ $ — ​ $ — ​ $ 46,173 ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ U.S. Treasury notes ​ 108,951 ​ ​ 1 ​ ​ 5 ​ 108,947 ​ Equity securities ​ ​ 1,220 ​ ​ — ​ ​ 592 ​ ​ 628 ​ Total marketable securities ​ $ 110,171 ​ $ 1 ​ $ 597 ​ $ 109,575 ​ Total money market funds and marketable securities ​ $ 156,344 ​ $ 1 ​ $ 597 ​ $ 155,748 ​
Reconciliation of cash, cash equivalents, and restricted cash	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ ​ 2019 2018 ​ ​ ​ (in thousands) ​ Cash and cash equivalents ​ $ 79,885 ​ $ 46,859 ​ Restricted cash included in deposits and other non-current assets ​ ​ 735 ​ ​ 735 ​ Total cash, cash equivalents, and restricted cash ​ $ 80,620 ​ $ 47,594 ​ ​

Accrued expenses (Tables)

Accrued expenses (Tables)	6 Months Ended
	Jun. 30, 2019
Accrued expenses
Schedule of accrued expenses	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ ​ (in thousands) ​ Research and development costs ​ $ 6,908 ​ $ 3,555 ​ Employee compensation costs ​ 2,319 ​ 3,780 ​ Professional services ​ ​ 1,301 ​ ​ 1,448 ​ Accrued goods and services ​ 976 ​ 784 ​ Patent costs ​ ​ 162 ​ ​ 120 ​ Other ​ 116 ​ 101 ​ Total ​ $ 11,782 ​ $ 9,788 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Right-of-use asset and lease _2

Right-of-use asset and lease liabilities (Tables)	6 Months Ended
	Jun. 30, 2019
Right-of-use asset and lease liabilities
Summary of significant contractual obligations under operating leases	​ ​ ​ ​ ​ ​ ​ Total Minimum ​ Lease Payments ​ ​ (in thousands) ​ 2019 (remainder of year) ​ $ 2,756 ​ 2020 ​ 5,960 ​ 2021 ​ 6,138 ​ 2022 ​ 6,323 ​ 2023 ​ 6,512 ​ 2024+ ​ ​ 20,897 ​ Total future minimum lease payments ​ $ 48,586 ​ Less: imputed interest ​ ​ (13,138) ​ Total lease liability ​ $ 35,448 ​ Reported as: ​ ​ ​ ​ Other current liabilities ​ $ 2,843 ​ Other non-current liabilities ​ ​ 32,605 ​ Total lease liability ​ $ 35,448 ​

Other liabilities (Tables)

Other liabilities (Tables)	6 Months Ended
	Jun. 30, 2019
Other liabilities
Summary of other current and non-current liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ As of June 30, ​ As of December 31, ​ ​ 2019 2018 ​ ​ (in thousands) ​ Other current liabilities ​ ​ ​ ​ ​ ​ Lease liability ​ 2,843 ​ ​ — ​ Total other current liabilities $ 2,843 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Other non-current liabilities ​ ​ ​ ​ ​ ​ Lease liability $ 32,605 ​ $ — ​ Deferred rent ​ — ​ ​ 5,710 ​ Other ​ 1,001 ​ ​ 1,001 ​ Total other non-current liabilities $ 33,606 ​ $ 6,711 ​

Commitments and contingencies (

Commitments and contingencies (Tables)	6 Months Ended
	Jun. 30, 2019
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	​ ​ ​ ​ ​ ​ ​ Total Minimum ​ Lease Payments ​ ​ (in thousands) ​ 2019 (remainder of year) ​ $ 2,756 ​ 2020 ​ 5,960 ​ 2021 ​ 6,138 ​ 2022 ​ 6,323 ​ 2023 ​ 6,512 ​ 2024+ ​ ​ 20,897 ​ Total future minimum lease payments ​ $ 48,586 ​ Less: imputed interest ​ ​ (13,138) ​ Total lease liability ​ $ 35,448 ​ Reported as: ​ ​ ​ ​ Other current liabilities ​ $ 2,843 ​ Other non-current liabilities ​ ​ 32,605 ​ Total lease liability ​ $ 35,448 ​
Allocable arrangement consideration allocation	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Research and Development Services for: ​ ​ ​ ​ Huntington’s Program ​ $ 14,228 ​ Parkinson’s Program ​ ​ 6,040 ​ Friedreich’s ataxia Program ​ ​ 14,821 ​ Spinal Muscular Atrophy Program ​ ​ 29,116 ​ Future Program ​ ​ 2,239 ​ Committee Obligations: ​ ​ ​ ​ AJSC ​ ​ 133 ​ DAC ​ ​ 207 ​ PD Material Right ​ ​ 7,855 ​ Total ​ $ 74,639 ​
Schedule of allocation of transaction price	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Research Services ​ $ 34,482 ​ First Development Option Material Right ​ ​ 34,518 ​ Total ​ $ 69,000 ​
AbbVie Alpha Synuclein Collaboration Agreement
Schedule of allocation of transaction price	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ ASN Research Services ​ $ 23,768 ​ ASN First Development Option Material Right ​ ​ 41,232 ​ Total ​ $ 65,000 ​
Schedule of allocation of fixed consideration	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Fixed Consideration ​ ​ ​ AADC Program ​ $ 80,373 ​ FA Program ​ ​ 6,005 ​ Discovery Program 1 ​ ​ 3,002 ​ Discovery Program 2 ​ ​ 3,002 ​ Total ​ $ 92,382 ​
Neurocrine Collaboration Agreement
Schedule of allocation of variable consideration	​ ​ ​ ​ ​ ​ Performance Obligation ​ Amount ​ ​ ​ (in thousands) ​ Variable Consideration ​ ​ ​ AADC Program ​ $ 170,209 ​ FA Program ​ ​ 114,023 ​ Discovery Program 1 ​ ​ 73,416 ​ Discovery Program 2 ​ ​ 73,416 ​ Total ​ $ 431,064 ​

Stock based compensation (Table

Stock based compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized for all stock-based compensation awards	Total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive income (loss) is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, June 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) Research and development ​ $ 3,256 ​ $ 1,332 ​ $ 4,671 ​ $ 2,530 ​ General and administrative ​ 1,998 ​ 6,782 ​ 4,100 ​ 7,906 ​ Total stock-based compensation expense ​ $ 5,254 ​ $ 8,114 ​ $ 8,771 ​ $ 10,436 ​
Summary of stock-based compensation expense by classification	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, ​ June 30, ​ ​ 2019 2018 ​ 2019 2018 ​ ​ ​ (in thousands) ​ Stock options ​ $ 4,528 ​ $ 7,902 ​ $ 7,304 ​ $ 9,868 ​ Restricted stock awards and units ​ ​ 668 ​ ​ 163 ​ ​ 1,351 ​ ​ 457 ​ Employee stock purchase plan awards ​ 58 ​ 49 ​ ​ 116 ​ ​ 111 ​ Total stock-based compensation expense ​ $ 5,254 ​ $ 8,114 ​ ​ 8,771 ​ ​ 10,436 ​
Summary of status and changes in unvested restricted stock	A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plans for the six months ended June 30, 2019 was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Grant Date ​ ​ ​ ​ ​ Fair Value ​ ​ Units Per Unit ​ Unvested restricted stock units as of December 31, 2018 — ​ ​ — ​ Granted 419,457 ​ $ 10.91 ​ Vested — ​ ​ — ​ Forfeited (16,675) ​ $ 9.09 ​ Unvested restricted stock units as of June 30, 2019 402,782 ​ $ 10.98 ​
Summary of status and changes in stock options	The following is a summary of stock option activity for the six months ended June 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Remaining Aggregate ​ ​ ​ ​ ​ Average ​ Contractual ​ Intrinsic ​ ​ ​ ​ ​ Exercise ​ Life ​ Value ​ ​ Shares Price (in years) (in thousands) ​ Outstanding at December 31, 2018 4,225,152 ​ $ 15.91 ​ ​ ​ ​ ​ ​ Granted 1,310,920 ​ $ 14.70 ​ ​ ​ ​ ​ ​ Exercised (88,821) ​ $ 11.24 ​ ​ ​ ​ ​ ​ Cancelled or forfeited (207,716) ​ $ 14.05 ​ ​ ​ ​ ​ ​ Outstanding at June 30, 2019 5,239,535 ​ $ 15.76 8.3 ​ $ 60,649 ​ Exercisable at June 30, 2019 1,972,340 ​ $ 13.87 7.2 ​ $ 26,574 ​ Vested and expected to vest at June 30, 2019 5,239,535 ​ $ 15.76 8.3 ​ $ 60,649 ​
Share-based Payment Arrangement, Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	The fair value of each option issued to employees and directors was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Six Months Ended ​ ​ ​ June 30, ​ June 30, ​ ​ ​ 2019 2018 ​ 2019 2018 ​ Risk-free interest rate 2.2 % 2.8 % 2.4 % 2.7 % ​ Expected dividend yield — % — % — % — % ​ Expected term (in years) 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ ​ Expected volatility 75.1 % 74.7 % 75.1 % 74.4 % ​

Net income (loss) per share (Ta

Net income (loss) per share (Tables)	6 Months Ended
	Jun. 30, 2019
Net income (loss) per share
Outstanding potentially dilutive securities excluded in the calculation of diluted net income (loss) per share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net income (loss) per share because to include them would be anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Unvested restricted common stock awards ​ 235,294 ​ ​ 347,724 ​ 235,294 ​ 347,724 ​ Unvested restricted common stock units ​ ​ 154,781 ​ ​ — ​ ​ 402,782 ​ ​ — ​ Outstanding stock options ​ 4,157,197 ​ ​ 6,700,282 ​ 5,239,535 ​ 6,700,282 ​ Total ​ 4,547,272 ​ ​ 7,048,006 ​ 5,877,611 ​ 7,048,006 ​
Schedule of weighted average number of shares, basic and diluted	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended June 30, ​ Six Months Ended June 30, ​ ​ ​ 2019 2018 ​ 2019 2018 ​ ​ ​ (in thousands, except share data) ​ Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ $ 11,153 ​ $ (25,541) ​ $ (16,017) ​ $ (45,468) ​ Denominator for basic net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 36,610,918 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​ Denominator for diluted net income (loss) per share: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average shares outstanding ​ ​ 36,610,918 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​ Common stock options and restricted stock units ​ ​ 1,330,339 ​ ​ — ​ ​ — ​ ​ — ​ Weighted average shares and conversions ​ ​ 37,941,257 ​ ​ 31,976,922 ​ ​ 34,990,989 ​ ​ 31,868,995 ​

Nature of business (Details)

Nature of business (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Nature of business
Accumulated deficit	$ (285,068)	$ (269,051)
Proceeds raised	640,000
Cash, cash equivalents, and marketable debt securities	$ 327,500

Summary of significant accoun_3

Summary of significant accounting policies and basis of presentation (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset	$ 30,041
Operating lease liability	$ 35,448
ASU 2016-02 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		$ 31,000
Operating lease liability		$ 36,700

Fair value measurements - Asset

Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Jun. 30, 2019USD ($)item	Dec. 31, 2018USD ($)item
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 79,885	$ 46,859
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input | item	0.017	0.025
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input | item	2.2	2.7
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input | item	0.888	1.127
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 248,857	$ 109,575
Total assets measured at fair value	328,379	155,982
Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	248,857	109,575
Total assets measured at fair value	327,965	155,748
Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	414	234
U.S. Treasury notes | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	247,577	108,947
U.S. Treasury notes | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	247,577	108,947
Equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	1,280	628
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	1,280	628
Warrants to purchase equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	414	234
Warrants to purchase equity securities | Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	414	234
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	79,108	46,173
Money market funds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	79,108	46,173
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 79,108	$ 46,173

Cash, cash equivalents, and a_3

Cash, cash equivalents, and available-for-sale marketable securities (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Cash and cash equivalents	$ 79,885,000	$ 46,859,000
Total marketable securities, amortized cost	248,568,000	110,171,000
Total money market funds and marketable securities	327,676,000	156,344,000
Fair Value
Total marketable securities	248,857,000	109,575,000
Total money market funds and marketable securities	327,965,000	155,748,000
Unrealized Gains and Losses
Total marketable securities, unrealized gains	290,000	1,000
Total money market funds and marketable securities	290,000	1,000
Total marketable securities, unrealized losses	1,000	597,000
Total money market funds and marketable securities	1,000	597,000
Other than temporary losses	$ 0
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 79,108,000	46,173,000
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	247,348,000	108,951,000
Fair Value
Marketable debt securities, fair value	247,577,000	108,947,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	230,000	1,000
Marketable securities, unrealized losses	1,000	5,000
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220,000	1,220,000
Fair Value
Marketable equity securities, fair value	1,280,000	628,000
Unrealized Gains and Losses
Equity securitiess, unrealized gains	$ 60,000
Equity securitiess, unrealized losses		$ 592,000

Cash, cash equivalents, and a_4

Cash, cash equivalents, and available-for-sale marketable securities - Restricted cash (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash, cash equivalents, and available for sale marketable securities
Cash and cash equivalents	$ 79,885	$ 46,859
Restricted cash included in deposits and other noncurrent assets	735	735
Total cash, cash equivalents, and restricted cash	$ 80,620	$ 47,594	$ 60,349	$ 32,265

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued expenses
Research and development costs	$ 6,908	$ 3,555
Employee compensation costs	2,319	3,780
Professional services	1,301	1,448
Accrued goods and services	976	784
Patent costs	162	120
Other	116	101
Total	$ 11,782	$ 9,788

Right-of-use asset and lease _3

Right-of-use asset and lease liabilities (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2014item	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Lease, Practical Expedients, Package [true false]				true
Lessee, Operating Lease, Existence of Option to Extend [true false]				true
Rent expense		$ 1,600	$ 1,000	$ 3,000	$ 1,800
Restricted cash included in deposits and other noncurrent assets		$ 735		$ 735		$ 735
Weighted average remaining lease term		7 years 4 months 24 days		7 years 4 months 24 days
Weighted average incremental borrowing rate		8.5		8.5
Total minimum lease payments
2019 (remainder)		$ 2,756		$ 2,756
2020		5,960		5,960
2021		6,138		6,138
2022		6,323		6,323
2023		6,512		6,512
2024+		20,897		20,897
Total		48,586		48,586
Less: imputed interest		(13,138)		(13,138)
Total lease liability		35,448		35,448
Current lease liabilities		$ 2,843		$ 2,843
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]		Other Liabilities, Current		Other Liabilities, Current
Non-current lease liabilities		$ 32,605		$ 32,605
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]		Other non-current liabilities		Other non-current liabilities
75 Sidney Street
Number of renewal options | item	2
Term of each renewal option	5 years
Leasehold improvements		$ 1,300		$ 1,300
Entitled leasehold improvements		300		300
64 Sidney Street
Leasehold improvements		$ 3,500		$ 3,500

Other liabilities (Details)

Other liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Other liabilities
Lease liability	$ 2,843
Total other current liabilities	2,843
Non-current lease liabilities	32,605
Deferred rent		$ 5,710
Other	1,001	1,001
Total other non-current liabilities	$ 33,606	$ 6,711

Commitments and contingencies -

Commitments and contingencies - Significant Agreements - (Details) - Sanofi Genzyme - USD ($) $ in Millions	Jun. 14, 2019	Oct. 31, 2017	Feb. 28, 2015
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received			$ 65
Premium over fair value on temporary equity issued			5
Per Milestone, maximum milestone payments to be received from collaborative partner		$ 540
Up-front fee paid on date of termination	$ 10
Milestone payable upon achievement	10
Collaborative Arrangement | Regulatory approval milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		220
Collaborative Arrangement | Regulatory approval milestone | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		40
Collaborative Arrangement | Regulatory approval milestone | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		50
Collaborative Arrangement | Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		320
Collaborative Arrangement | Commercial Milestone | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		50
Collaborative Arrangement | Commercial Milestone | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		100
Termination Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received	1
Huntington's Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 10
Parkinson's Program | Regulatory and commercial milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments no longer receivable		$ 105
Spinal Muscular Atrophy Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum potential in-kind services to be provided by collaborative partner			5
Option exercise payment required from collaborative partner for each program specified			30
Split Territory Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Option exercise payment required from collaborative partner for each program specified			$ 20
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Temporary equity issued (in shares)			10,000,000
Temporary equity issued			$ 30
Fair value of temporary equity issued			25
Premium over fair value on temporary equity issued			$ 5

Commitments and contingencies_2

Commitments and contingencies - Termination of Agreement (Details) - Termination Agreement - Sanofi Genzyme $ in Millions	Jun. 14, 2019USD ($)item
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of non central nervous system | item	2
Up-front payment made	$ 10
Milestone payment agreed to be paid	$ 10
Period in which milestone payment will be made of filing of investigational new drug	15 days
Percentage of threshold sublicensing income agreed to be paid	50.00%
Unused in-kind services waived	$ 0.4
Threshold number of capsids having exclusive options to evaluate for no consideration | item	4
Threshold additional number of capsids having options to evaluate for low six-figure payment per capsid | item	2
Threshold number of capsids that has right to obtain an exclusive license | item	2
Option exercise fee	$ 1
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
AAV capsids to license | item	1
Development and regulatory milestones
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 15
Sales milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 20

Commitments and contingencies_3

Commitments and contingencies - Accounting Analysis - (Details)	Jun. 14, 2019USD ($)	Feb. 28, 2019USD ($)	Feb. 28, 2015USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)item	Jun. 30, 2018USD ($)
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")			$ 133,000
Committee Obligations: Development Advisory Committee ("DAC")			207,000
PD Material Right			7,855,000
Total allocable arrangement consideration allocations.			74,639,000
Huntington's Program
Allocable arrangement consideration allocation
Research and development services			14,228,000
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services			6,040,000
Friedreich's ataxia Program
Allocable arrangement consideration allocation
Research and development services			14,821,000
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services			29,116,000
Future Program
Allocable arrangement consideration allocation
Research and development services			2,239,000
Sanofi Genzyme | Huntington's Program
Income Statement Related Disclosures [Abstract]
Deferred revenue	$ 10,000,000
Milestone payable upon achievement	10,000,000
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received			65,000,000
Additional consideration, premium over fair value on temporary equity issued			5,000,000
In-kind services and other funding to be provided by collaborative partner			4,600,000
Total allocable arrangement consideration			$ 74,600,000
Allocable arrangement consideration allocation
Additional option exercise payments			0
Income Statement Related Disclosures [Abstract]
Revenue recognized	28,700,000			$ 1,500,000	$ 700,000	$ 2,900,000	$ 1,200,000
Deferred revenue	48,700,000
Up-front fee paid on date of termination	10,000,000
Milestone payable upon achievement	10,000,000
Total payable amount	20,000,000
Sanofi Genzyme | Termination Agreement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 1,000,000
Income Statement Related Disclosures [Abstract]
Revenue recognized				30,200,000		31,600,000
Deferred revenue				10,000,000		10,000,000
AbbVie | Collaborative Arrangement
Income Statement Related Disclosures [Abstract]
Revenue recognized						4,900,000
Deferred revenue				57,100,000		57,100,000
AbbVie | AbbVie Alpha Synuclein Collaboration Agreement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		$ 65,000,000
Income Statement Related Disclosures [Abstract]
Revenue recognized				200,000		400,000
Deferred revenue				$ 64,600,000		$ 64,600,000
Number of development options | item						4
Redeemable Convertible Preferred Stock Series B | Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued			$ 5,000,000

Commitments and contingencies_4

Commitments and contingencies - AbbVie Tau Collaboration Agreement - (Details) - AbbVie Tau Collaboration Agreement - AbbVie $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Feb. 28, 2018USD ($)item	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)item	Jun. 30, 2018USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received	$ 69,000
First Research Compound payment	80,000
Two additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	2
One-time payment upon exercise of license option	$ 75,000
Milestone payments to be received for licensed compound in case Alzheimer disease indication	550,000
Milestone payments to be received for licensed compound in case first indication other than Alzheimer disease	230,000
Milestone payments to be received for licensed compound for subsequent non-Alzheimer disease indication	$ 115,000
Number of development options | item	3		3
Number of research antibodies | item	1
Revenue recognized		$ 1,900	$ 3,200	$ 2,300
Allocation of Transaction Price
Allocation of Transaction Price			69,000
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	2
The number of Licensed Products | item	1
Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	5
Antibodies to include in Research Period | item	3
Research Services
Allocation of Transaction Price
Allocation of Transaction Price			34,482
First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price			$ 34,518

Commitments and contingencies_5

Commitments and contingencies - AbbVie Alpha Synuclein Collaboration Agreement - (Details) - AbbVie Alpha Synuclein Collaboration Agreement - AbbVie $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Feb. 28, 2019USD ($)item	Jun. 30, 2019USD ($)	Jun. 30, 2019USD ($)item
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	1
Number of research antibodies | item	1
Non-refundable upfront payment received	$ 65,000
First Research Compound payment	80,000
Three additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	3
Milestone payments to be received for licensed compound in case Parkinson's disease indication	$ 450,000
Milestone payments to be received for licensed compound in case first indication other than Parkinson's disease	185,000
Milestone payments to be received for licensed compound for subsequent non-Parkinson's disease indication	$ 92,500
Binds to alpha-synuclein protein, after the termination date (in months)	18 months
Number of development options | item			4
Deferred revenue		$ 64,600	$ 64,600
Revenue recognized		$ 200	$ 400
Allocation of Transaction Price
Allocation of Transaction Price	$ 65,000
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	1
The number of Licensed Products | item	1
Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	4
Research Services
Allocation of Transaction Price
Allocation of Transaction Price	$ 23,768
First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price	41,232
Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Specified commercial milestone payments	$ 500,000

Commitments and contingencies_6

Commitments and contingencies - Neurocrine Collaboration Agreement - (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019USD ($)$ / shares	Mar. 31, 2019USD ($)itemshares	Jun. 30, 2019USD ($)$ / sharesshares
Neurocrine | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option after paying rate-shifting fee		60.00%
Neurocrine Collaboration Agreement | AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Percentage of profit or loss under co-co option after paying rate-shifting fee		45.00%
Neurocrine Collaboration Agreement | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option after paying rate-shifting fee		40.00%
Neurocrine Collaboration Agreement | Neurocrine
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of collaboration programs | item		4
Number of discovery programs | item		2
Number of targets | item		8
Upfront payment		$ 115,000	$ 115,000
Purchase of common stock, shares | shares		4,179,728	4,179,728
Purchase of common stock		$ 50,000	$ 5,000
Price per share | $ / shares	$ 11.9625		$ 11.9625
Estimated cost reimbursement			$ 431,100
Termination period		10 years
Period of advance notice for termination prior to first commercial sale		180 days
Period of advance notice for termination after first commercial sale		1 year
Discount related to equity investment			27,600
Equity investment	$ 50,000		50,000
Allocation of Transaction Price
Allocation of variable consideration	431,064		431,064
Allocation of fixed consideration	92,382		92,382
Revenue recognized	12,500		14,400
Deferred revenue	88,000		88,000
Reimbursement costs expected to be received	10,100		10,100
Costs to obtain collaboration agreement	800		800
Consideration received			5,000
Neurocrine Collaboration Agreement | Neurocrine | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of reduction in royalty payments		50.00%
Neurocrine Collaboration Agreement | Neurocrine | AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Rate-shifting fee		$ 35,000
Percentage of profit or loss under co-co option after paying rate-shifting fee		55.00%
Allocation of Transaction Price
Allocation of variable consideration	170,209		170,209
Allocation of fixed consideration	80,373		80,373
Neurocrine Collaboration Agreement | Neurocrine | FA Program
Allocation of Transaction Price
Allocation of variable consideration	114,023		114,023
Allocation of fixed consideration	6,005		6,005
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 1
Allocation of Transaction Price
Allocation of variable consideration	73,416		73,416
Allocation of fixed consideration	3,002		3,002
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 2
Allocation of Transaction Price
Allocation of variable consideration	73,416		73,416
Allocation of fixed consideration	$ 3,002		$ 3,002
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		$ 170,000
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		195,000
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | Discovery program 1
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		130,000
Neurocrine Collaboration Agreement | Neurocrine | Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		275,000
Aggregate maximum milestone payments to be received from collaborative partner		$ 1,100,000

Commitments and contingencies_7

Commitments and contingencies - Other Agreements - (Details) $ in Thousands	6 Months Ended	12 Months Ended	42 Months Ended
	Jun. 30, 2019USD ($)	Dec. 31, 2016USD ($)multiple	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2016USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-current liability	$ 33,606		$ 33,606	$ 6,711
Various contract research organizations and institutions
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 100
Various contract research organizations and institutions | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement			30 days
Various contract research organizations and institutions | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement			90 days
Non-profit organization agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum funding		$ 4,000
Funding multiple | multiple		2.6
Milestone payment liability					$ 1,000
Non-current liability					$ 1,000
MRIC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount of investment						$ 2,000

Preferred stock (Details)

Preferred stock (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Preferred stock
Preferred stock, authorized	5,000,000	5,000,000

Stock based compensation (Detai

Stock based compensation (Details)

Jan. 01, 2019shares

Jan. 01, 2018shares

Jan. 01, 2017shares

Jan. 01, 2016shares

Oct. 31, 2015shares

Jan. 31, 2014$ / sharesshares

Jun. 30, 2019USD ($)item$ / sharesshares

Jun. 30, 2018USD ($)shares

Jun. 30, 2019USD ($)item$ / sharesshares

Jun. 30, 2018USD ($)shares

Dec. 31, 2016item

Dec. 31, 2018$ / shares

Aug. 31, 2015shares

Apr. 30, 2015shares

Aug. 31, 2014shares

Apr. 30, 2014shares

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 5,254,000

$ 8,114,000

$ 8,771,000

$ 10,436,000

Stock options granted (in shares)

1,310,920

Exercise price | $ / shares

$ 15.76

$ 15.76

$ 15.91

Granted (in dollars per share) | $ / shares

$ 14.70

2014 Stock Option and Grant Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of shares authorized for stock-based compensation plan (in shares)

2,998,823

823,529

2,669,411

2,047,058

2,000,000

1,411,764

2015 Employee Stock Purchase Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock shares authorized for issuance

325,707

321,300

267,658

267,492

Number of shares authorized for stock-based compensation plan (in shares)

262,362

Period to purchase shares

6 months

Increase in ESSP shares (as a percent)

1.00%

2015 Employee Stock Purchase Plan | Minimum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Payroll deduction percentage

1.00%

2015 Employee Stock Purchase Plan | Maximum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Payroll deduction percentage

10.00%

2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Shares available for future issuance (in shares)

1,311,812

2,166,946

2,166,946

Percentage limit of issued and outstanding shares up to which additional shares may be issued

4.00%

Common stock shares authorized for issuance

1,302,830

1,285,200

1,070,635

1,069,971

Inducement Awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of executives issued awards | item

2

2

Stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 4,528,000

7,902,000

$ 7,304,000

9,868,000

Stock options | Inducement Awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

New stock options issued

58,125

Vesting period

4 years

Stock options | Inducement Awards | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

25.00%

Stock options | Inducement Awards | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

36 months

Vesting percentage

75.00%

Restricted stock units

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

668,000

$ 163,000

$ 1,351,000

$ 457,000

Unrecognized stock-based compensation expense | $

$ 3,600,000

$ 3,600,000

Granted (in shares)

0

419,457

0

Restricted stock granted, original issued price (in dollars per share) | $ / shares

$ 20.74

$ 10.91

Restricted stock units | 2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

65,052

419,457

Restricted stock units | Inducement Awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

3 years

Restricted stock units | Inducement Awards | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

33.00%

Restricted stock units | Inducement Awards | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

33.00%

Restricted stock units | Inducement Awards | Vesting period, tranche three

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

33.00%

Performance based milestone awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Probable performance milestones | item

1

Performance milestones | item

3

Stock-based compensation | $

$ 100,000

$ 0

$ 300,000

Compensation expense for remaining founders' awards | $

$ 0

Founders restricted stock

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

1,188,233

Restricted stock granted, original issued price (in dollars per share) | $ / shares

$ 0.0425

Number of awards | item

2

2

Founders restricted stock | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

835,292

Founders restricted stock | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Restricted stock granted (in shares)

352,941

Founders restricted stock | Minimum | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Founders restricted stock | Maximum | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Non-statutory stock options | Inducement Awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

New stock options issued

338,750

Share-based Payment Arrangement, Employee

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 4,500,000

$ 7,300,000

Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Vesting percentage

75.00%

Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan | Vesting period, tranche one

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Vesting percentage

25.00%

Share-based Payment Arrangement, Employee | 2014 Stock Option and Grant Plan | Vesting period, tranche two

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

3 years

Share-based Payment Arrangement, Employee | 2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

New stock options issued

557,580

1,310,920

Share-based Payment Arrangement, Employee | Stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

7,900,000

9,800,000

Share-based Payment Arrangement, Employee | Restricted stock units

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 700

$ 1,400,000

Share-based Payment Arrangement, Nonemployee

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options granted (in shares)

0

0

Share-based Payment Arrangement, Nonemployee | 2014 Stock Option and Grant Plan | Minimum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

1 year

Share-based Payment Arrangement, Nonemployee | 2014 Stock Option and Grant Plan | Maximum

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting period

4 years

Share-based Payment Arrangement, Nonemployee | 2015 Stock Option Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

New stock options issued

0

0

Share-based Payment Arrangement, Nonemployee | Stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 100,000

100,000

$ 100,000

200,000

Research and development

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

3,256,000

1,332,000

4,671,000

2,530,000

General and administrative

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation | $

$ 1,998,000

$ 6,782,000

$ 4,100,000

$ 7,906,000

Stock based compensation - Comp

Stock based compensation - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 5,254	$ 8,114	$ 8,771	$ 10,436
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	4,528	7,902	7,304	9,868
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	668	163	1,351	457
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	58	49	116	111
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	3,256	1,332	4,671	2,530
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 1,998	$ 6,782	4,100	$ 7,906
Sah
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 2,200
Consultant duration			3 years

Stock based compensation - Rest

Stock based compensation - Restricted Stock - (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restricted stock disclosures
Stock-based compensation	$ 5,254,000	$ 8,114,000	$ 8,771,000	$ 10,436,000
Restricted stock units
Shares
Granted (in shares)		0	419,457	0
Forfeited (in shares)			(16,675)
Balance, ending (in shares)	402,782		402,782
Weighted Average Grant Date Fair Value Per Share
Issued (in dollars per share)	$ 20.74		$ 10.91
Forfeited (in dollars per share)			9.09
Balance, ending (in dollars per share)	$ 10.98		$ 10.98
Restricted stock disclosures
Stock-based compensation	$ 668,000	$ 163,000	$ 1,351,000	$ 457,000
Unrecognized stock-based compensation expense	3,600,000		$ 3,600,000
Remaining weighted-average remaining vesting period			2 years 7 months 6 days
Share-based Payment Arrangement, Employee
Restricted stock disclosures
Stock-based compensation	4,500,000		$ 7,300,000
Share-based Payment Arrangement, Employee | Restricted stock units
Restricted stock disclosures
Stock-based compensation	$ 700		$ 1,400,000

Stock based compensation - Stoc

Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)			4,225,152
Granted (in shares)			1,310,920
Exercised (in shares)			(88,821)
Cancelled or forfeited (in shares)			(207,716)
Outstanding, ending balance (in shares)	5,239,535		5,239,535
Exercisable (in shares)	1,972,340		1,972,340
Vested and expected to vest (in shares)	5,239,535		5,239,535
Weighted Average Exercise Price
Outstanding (in dollars per share)			$ 15.91
Granted (in dollars per share)			14.70
Exercised (in dollars per share)			11.24
Cancelled or forfeited (in dollars per share)			14.05
Outstanding (in dollars per share)	$ 15.76		15.76
Exercisable (in dollars per share)	13.87		13.87
Vested and expected to vest (in dollars per share)	$ 15.76		$ 15.76
Remaining Contractual Life
Outstanding			8 years 3 months 18 days
Exercisable			7 years 2 months 12 days
Vested and expected to vest			8 years 3 months 18 days
Aggregate Intrinsic Value
Outstanding	$ 60,649		$ 60,649
Exercisable	26,574		26,574
Vested and expected to vest	60,649		60,649
Options disclosures
Stock-based compensation	5,254	$ 8,114	8,771	$ 10,436
Stock options
Options disclosures
Stock-based compensation	$ 4,528	$ 7,902	$ 7,304	$ 9,868
Share-based Payment Arrangement, Employee
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 14.61	$ 13.88	$ 9.79	$ 15.92
Stock-based compensation	$ 4,500		$ 7,300
Share-based Payment Arrangement, Employee | Stock options
Options disclosures
Stock-based compensation		$ 7,900		$ 9,800
Share-based Payment Arrangement, Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0		0
Share-based Payment Arrangement, Nonemployee | Stock options
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 100	$ 200

Stock based compensation - Blac

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Options disclosures
Stock-based compensation	$ 5,254	$ 8,114	$ 8,771	$ 10,436
Stock options granted (in shares)			1,310,920
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	30,500		$ 30,500
Remaining weighted-average remaining vesting period			2 years 9 months 18 days
Stock options
Options disclosures
Stock-based compensation	$ 4,528	$ 7,902	$ 7,304	$ 9,868
Share-based Payment Arrangement, Employee
Fair value assumptions
Risk-free interest rate	2.20%	2.80%	2.40%	2.70%
Expected term	6 years	6 years	6 years	6 years
Expected volatility	75.10%	74.70%	75.10%	74.40%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 14.61	$ 13.88	$ 9.79	$ 15.92
Stock-based compensation	$ 4,500		$ 7,300
Share-based Payment Arrangement, Employee | Stock options
Options disclosures
Stock-based compensation		$ 7,900		$ 9,800
Share-based Payment Arrangement, Nonemployee
Options disclosures
Stock options granted (in shares)	0		0
Share-based Payment Arrangement, Nonemployee | Stock options
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 100	$ 200

Net income (loss) per share (De

Net income (loss) per share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	4,547,272	7,048,006	5,877,611	7,048,006
Restricted common stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	235,294	347,724	235,294	347,724
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	154,781		402,782
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	4,157,197	6,700,282	5,239,535	6,700,282

Net income (loss) per share - B

Net income (loss) per share - Basic and diluted weighted average shares outstanding (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income (loss)	$ 11,153	$ (25,541)	$ (16,017)	$ (45,468)
Denominator for basic earnings per share:
Weighted average shares outstanding	36,610,918	31,976,922	34,990,989	31,868,995
Denominator for diluted earnings per share:
Weighted average shares outstanding	36,610,918	31,976,922	34,990,989	31,868,995
Common stock options and restricted stock units	1,330,339
Weighted average shares and conversions	37,941,257	31,976,922	34,990,989	31,868,995