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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 15, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		45,777,940
Entity Public Float			$ 2,730,487,424

Consolidated Balance Sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 306,235,000	$ 168,517,000
Marketable securities	212,613,000	228,962,000
Prepaid expenses and other current assets	6,227,000	5,100,000
Total current assets	525,075,000	402,579,000
Property and equipment, net	4,013,000	1,388,000
Restricted cash	849,000	564,000
Total assets	529,937,000	404,531,000
Current liabilities:
Accounts payable	9,350,000	12,817,000
Accrued expenses	42,601,000	22,352,000
Total current liabilities	51,951,000	35,169,000
Other liabilities	2,511,000	845,000
Total liabilities	54,462,000	36,014,000
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at December 31, 2017 and 2016; no shares issued or outstanding at December 31, 2017 and 2016
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at December 31, 2017 and 2016; 42,003,894 and 37,222,518 shares issued at December 31, 2017 and 2016, respectively; 42,002,934 and 37,222,172 shares outstanding at December 31, 2017 and 2016, respectively	5,000	4,000
Treasury stock, at cost, 960 and 346 shares at December 31, 2017 and 2016, respectively	(113,000)	(17,000)
Additional paid-in capital	1,066,059,000	688,959,000
Accumulated deficit	(590,447,000)	(320,327,000)
Accumulated other comprehensive loss	(29,000)	(102,000)
Total stockholders’ equity	475,475,000	368,517,000
Total liabilities and stockholders’ equity	$ 529,937,000	$ 404,531,000

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	42,003,894	37,222,518
Common stock, shares outstanding	42,002,934	37,222,172
Treasury stock, shares	960	346

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 210,277	$ 120,756	$ 69,357
General and administrative	62,878	39,407	25,293
Total operating expenses	273,155	160,163	94,650
Loss from operations	(273,155)	(160,163)	(94,650)
Interest income, net	3,099	1,211	178
Other expense, net	(64)	(35)	(23)
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Net loss per share—basic and diluted	$ (7.09)	$ (4.75)	$ (3.40)
Weighted average number of common shares outstanding—basic and diluted	38,113,678	33,492,795	27,778,288
Comprehensive loss:
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	73	(102)
Total other comprehensive gain (loss)	73	(102)
Total comprehensive loss	$ (270,047)	$ (159,089)	$ (94,495)

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated other comprehensive income (loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2014	$ 121,885	$ 3		$ 188,727		$ (66,845)
Balances, Shares at Dec. 31, 2014		25,621,791
Issuance of common stock from exercise of stock options, Amount	603			603
Issuance of common stock from exercise of stock options, Shares		417,475
Vesting of restricted stock, Amount	17			17
Vesting of restricted stock, Shares		128,051
Issuance of common stock under employee stock purchase plan	127			127
Issuance of common stock under employee stock purchase plan, Shares		3,852
Issuance of common stock in payment of consultant fees, Amount	1,211			1,211
Issuance of common stock in payment of consultant fees, Shares		23,809
Stock-based compensation expense	15,176			15,176
Public offering of common stock, net of offering costs, Amount	129,171			129,171
Public offering of common stock, net of offering costs, Shares		2,628,571
Net loss	(94,495)					(94,495)
Balances at Dec. 31, 2015	173,695	$ 3		335,032		(161,340)
Balances, Shares at Dec. 31, 2015		28,823,549
Issuance of common stock from exercise of stock options, Amount	1,011			1,011
Issuance of common stock from exercise of stock options, Shares		124,557
Vesting of restricted stock, Amount	11			11
Vesting of restricted stock, Shares		42,781
Issuance of common stock under employee stock purchase plan	510			510
Issuance of common stock under employee stock purchase plan, Shares		10,499
Purchase of treasury stock, Amount	$ (17)		$ (17)
Purchase of treasury stock, Shares	346		346
Stock-based compensation expense	$ 22,855			22,855
Public offering of common stock, net of offering costs, Amount	329,541	$ 1		329,540
Public offering of common stock, net of offering costs, Shares		8,220,786
Unrealized gain (loss) on available-for-sale securities	(102)				$ (102)
Net loss	(158,987)					(158,987)
Balances at Dec. 31, 2016	368,517	$ 4	$ (17)	688,959	(102)	(320,327)
Balances, Shares at Dec. 31, 2016		37,222,172	346
Issuance of common stock from exercise of stock options, Amount	15,437	$ 1		15,436
Issuance of common stock from exercise of stock options, Shares		706,222
Issuance of common stock under employee stock purchase plan	771			771
Issuance of common stock under employee stock purchase plan, Shares		15,718
Purchase of treasury stock, Amount	$ (96)		$ (96)
Purchase of treasury stock, Shares	614		614
Stock-based compensation expense	$ 35,142			35,142
Public offering of common stock, net of offering costs, Amount	325,751			325,751
Public offering of common stock, net of offering costs, Shares		4,058,822
Unrealized gain (loss) on available-for-sale securities	73				73
Net loss	(270,120)					(270,120)
Balances at Dec. 31, 2017	$ 475,475	$ 5	$ (113)	$ 1,066,059	$ (29)	$ (590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	35,534	23,020	15,240
Non-cash licensing and consulting fees			1,211
Premium on marketable securities	(105)	(756)
Amortization of premium (discount) on marketable securities	(293)	286
Depreciation	531	281	115
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,127)	(3,362)	(681)
Accounts payable	(3,693)	7,796	2,590
Accrued expenses and other liabilities	19,996	13,044	5,339
Net cash used in operating activities	(219,277)	(118,678)	(70,681)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	261,225	30,499
Purchases of marketable securities	(244,403)	(259,093)
Purchases of property and equipment	(1,360)	(1,421)	(198)
Increase in restricted cash	(285)	(525)
Net cash provided by (used in) investing activities	15,177	(230,540)	(198)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	15,972	1,406	730
Payments of offering costs	(179)	(599)	(584)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	326,025	330,175	129,720
Net cash provided by financing activities	341,818	330,982	129,866
Net increase (decrease) in cash and cash equivalents	137,718	(18,236)	58,987
Cash and cash equivalents at beginning of period	168,517	186,753	127,766
Cash and cash equivalents at end of period	306,235	168,517	186,753
Supplemental disclosure of non-cash financing activities
Purchases of property and equipment financed with landlord tenant incentive	1,665
Purchases of property and equipment included in accounts payable	138	$ 8
Public offering costs included in accounts payable or accrued expenses	$ 95		$ 165

Nature of the Business

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business

1.	Nature of the Business

Sage Therapeutics, Inc. (“Sage” or the “Company”) is a clinical-stage biopharmaceutical company committed to developing and commercializing novel medicines to treat life-altering central nervous system, or CNS, disorders, where there are no approved therapies or existing therapies are inadequate. The Company has a portfolio of product candidates with a current focus on modulating two critical CNS receptor systems, GABA and NMDA. The GABA receptor family, which is recognized as the major inhibitory neurotransmitter in the CNS, mediates downstream neurologic and bodily function via activation of GABA_A receptors. The NMDA-type receptors of the glutamate receptor system are a major excitatory receptor system in the CNS. Dysfunction in these systems is implicated in a broad range of CNS disorders. The Company is targeting CNS indications where patient populations are easily identified, clinical endpoints are well-defined, and development pathways are feasible.

The Company was incorporated under the laws of the State of Delaware on April 16, 2010, and commenced operations on January 19, 2011 as Sterogen Biopharma, Inc. On September 13, 2011, the Company changed its name to Sage Therapeutics, Inc.

The Company is subject to risks and uncertainties common to companies in the biotech industry, including, but not limited to, the risks associated with developing product candidates at each stage of non-clinical and clinical development; the challenges associated with gaining regulatory approval of such product candidates; the risks associated with commercializing pharmaceutical products, if approved for marketing and sale; the potential for development by third parties of new technological innovations that may compete with the Company’s products; the dependence on key personnel; the challenges of protecting proprietary technology; the need to comply with government regulations; the high costs of drug development; and the uncertainty of being able to secure additional capital when needed to fund operations.

Under Accounting Standards Update, or ASU, 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40), or ASC 205-40, the Company has the responsibility to evaluate whether conditions and/or events raise substantial doubt about its ability to meet its future financial obligations as they become due within one year after the date that the financial statements are issued.

The Company has incurred losses and negative cash flows from operations since its inception. As of December 31, 2017, the Company had an accumulated deficit of $590.4 million. From its inception through December 31, 2017, the Company received net proceeds of $969.0 million from the sales of redeemable convertible preferred stock, the issuance of convertible notes, and the sales of common stock in its initial public offering (“IPO”) in July 2014 and follow-on public offerings in April 2015, January 2016, September 2016 and November 2017. In February 2018, the Company received net proceeds of $631.1 million from the sale of common stock (see Note 13). Until such time, if ever, as the Company can generate substantial product revenue and achieve profitability, the Company expects to finance its cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances, licensing arrangements and other sources of funding. If the Company is unable to raise additional funds through equity or debt financings when needed, the Company may be required to delay, limit, reduce or terminate product development or future commercialization efforts or grant rights to develop and market products or product candidates that the Company would otherwise prefer to develop and market itself.

The Company expects that, based on its current operating plans, the Company’s existing cash, cash equivalents and marketable securities will be sufficient to fund its current planned operations for at least the next twelve months from the issuance of these consolidated financial statements.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

The following is a summary of significant accounting policies followed in the preparation of these financial statements.

Basis of Presentation

The accompanying consolidated financial statements include those of the Company and its subsidiaries after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. As of December 31, 2017, cash equivalents were comprised of money market funds and overnight reverse repurchase agreements. As of December 31, 2016, cash equivalents were comprised of money market funds.

Marketable securities

Marketable securities consist of investments with original maturities greater than 90 days. The Company considers its investment portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive items in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other expense, net, based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers several factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. No declines in value were deemed to be other than temporary during the year ended December 31, 2017.

Restricted Cash

A deposit of $39,000 was restricted from withdrawal as of December 31, 2017 and 2016. The restriction is related to securing the lease for the initial facility of the Company and the restriction expires in 2024, in accordance with the most recent amendment to the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheets. For this same facility, a deposit of $0.3 million was restricted from withdrawal as of December 31, 2017. The restriction is related to securing an amendment to the lease of the initial facility of the Company and the restriction expires in 2024, in accordance with the most recent amendment to the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheets.

A deposit of $0.5 million was restricted from withdrawal as of December 31, 2017 and 2016. The restriction is related to securing the separate facility lease in May 2016 that will expire on February 28, 2022, under which, beginning on September 1, 2016, the Company rents 19,805 square feet of additional office space in a separate multi-tenant building. The restriction expires in 2022, in accordance with the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheets.

Property and Equipment

Property and equipment are recorded at cost and depreciated over their estimated useful lives using the straight-line method. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Repairs and maintenance costs are expensed as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.

Research and Development

Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, overhead costs, depreciation, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.

The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.

Patent Costs

The Company expenses patent costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.

Stock-Based Compensation

The Company recognizes compensation expense for stock-based awards, including grants of stock options and restricted stock, made to employees and nonemployee directors based on the estimated fair value on the date of grant, over the requisite service period.

The Company recognizes compensation expense for stock-based awards granted to non-employee consultants based on the fair value of the award on each date on which the awards vest. Compensation expense is recognized over the vesting period, provided that services are rendered by such non-employee consultants during that time. At the end of each financial reporting period, the fair value of unvested options is re-measured using the then-current fair value of our common stock and updated assumptions in the Black-Scholes option-pricing model; and the fair value of restricted stock awards is re-measured using the then-current fair value of our common stock.

For awards that vest upon achievement of a performance condition, the Company recognizes compensation expense when achievement of the performance condition is met or during the period from which meeting the condition is deemed probable until the expected date of meeting the performance condition.

The fair value of each option grant is estimated using the Black-Scholes option-pricing model. Through December 31, 2015, the Company lacked sufficient Company-specific historical and implied volatility information, and as a result, the Company used the volatility of a group of publicly-traded peer companies in the Black-Scholes calculations. Beginning in 2016, the Company estimated its expected volatility using a weighted average of the historical volatility of publicly-traded peer companies and the volatility of its common stock, and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its traded stock price for the duration of the expected term. The expected term of the Company’s options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options, while the expected term of its options granted to consultants and nonemployees has been determined based on the contractual term of the options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The Company also applies a forfeiture rate in order to calculate stock-based compensation expense. Expected forfeitures are based on the historical experience of the Company and management’s expectations of future forfeitures. To the extent actual forfeitures differ from the estimates, the difference is recorded as a cumulative adjustment in the period in which the estimates are revised. The Company recognizes stock-based compensation expense for only the portion of awards that are expected to vest.

Treasury Stock

The Company records treasury stock at cost. Treasury stock consists of shares received from an employee as consideration for exercises of stock options.

Basic and Diluted Net Loss Per Share

Basic net loss per share is computed by dividing the net loss by the weighted average number of common shares outstanding for the period. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of common shares outstanding for the period, including potentially-dilutive common shares, by assuming the dilutive effect of outstanding stock options and unvested restricted stock, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, because dilutive common shares are not assumed to have been issued if their effect is antidilutive.

The Company reported a net loss for the years ended December 31, 2017, 2016 and 2015.

Risks and Uncertainties

The product candidates developed by the Company require approvals from the U.S. Food and Drug Administration or foreign regulatory agencies prior to commercial sales. There can be no assurance that the current and future product candidates of the Company will receive the necessary approvals. If the Company fails to successfully complete clinical development and generate results sufficient to file for regulatory approval or is denied approval or approval is delayed, it may have a material adverse impact on the Company’s business and its financial statements.

Concentration of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains accounts for all cash and cash equivalents at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. The Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.

Income Taxes

The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

Fair Value Measurements

Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are classified and disclosed in one of the following three categories:

Level 1	—	Quoted market prices in active markets for identical assets or liabilities.

Level 2	—	Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3	—	Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The Company’s cash equivalents and marketable securities at December 31, 2017 and 2016 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements.

The carrying amounts reflected in the consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at December 31, 2017 and 2016, respectively.

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The singular focus of the Company is on advancing medicines to treat central nervous system disorders, where there are inadequate or no approved existing therapies. All tangible assets are held within the United States.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2017 and 2016, the difference between net loss and comprehensive loss was the unrealized gain (loss) on marketable securities. For the year ended December 31, 2015, there was no difference between net loss and comprehensive loss.

Public Offerings

On July 23, 2014, the Company completed the sale of 5,750,000 shares of its common stock in its IPO at a price to the public of $18.00 per share, resulting in net proceeds to the Company of $94.0 million after deducting underwriting discounts and commissions and offering costs paid by the Company. The shares began trading on the Nasdaq Global Market on July 18, 2014.

On April 20, 2015, the Company completed the sale of 2,628,571 shares of its common stock at a price to the public of $52.50 per share, resulting in net proceeds to the Company of $129.1 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

On January 12, 2016, the Company completed the sale of 3,157,894 shares of its common stock at a price to the public of $47.50 per share, resulting in net proceeds to the Company of $140.4 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

On September 14, 2016, the Company completed the sale of 5,062,892 shares of its common stock at a price to the public of $39.75 per share, resulting in net proceeds to the Company of $189.2 million after deducting underwriting discounts and commissions paid by the Company.

On November 17, 2017, the Company completed the sale of 4,058,822 shares of its common stock at a price to the public of $85.00 per share, resulting in net proceeds to the Company of $325.8 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

Also see Note 13, “Subsequent Event”.

Recently Issued Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The FASB has continued to issue accounting standards updates to clarify and provide implementation guidance related to Revenue from Contracts with Customers, including ASU 2016-08, Revenue from Contract with Customers: Principal versus Agent Considerations, ASU 2016-10, Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing, and ASU 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. These amendments address a number of areas, including the entity’s identification of its performance obligations in a contract, collectability, non-cash consideration, presentation of sales tax and an entity’s evaluation of the nature of its promise to grant a license of intellectual property and whether or not that revenue is recognized over time or at a point in time. These new standards will be effective for the Company beginning January 1, 2018. The Company had the option to early adopt the standard for the year ending December 31, 2017. The Company early adopted the standard as of January 1, 2017, although there is no impact of this new guidance on its consolidated financial statements as it does not currently have any revenue generating arrangements.

In February 2016, the FASB issued ASU No. 2016-02, Leases, which will replace the existing guidance in ASC 840, “Leases.” The updated standard aims to increase transparency and comparability among organizations by requiring lessees to recognize leased assets and leased liabilities on the consolidated balance sheets and requiring disclosure of key information about leasing arrangements. The standard will be effective on January 1, 2019, with early adoption permitted. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting, which intends to simplify several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, as well as certain classifications on the statement of cash flows. The standard is effective for annual periods that began after December 15, 2016 and for interim periods within those fiscal years. The Company adopted ASU 2016-09 on the required effective date of January 1, 2017. The Company elected to maintain its existing policy to estimate forfeitures when determining periodic stock-based compensation expense. The adoption of the other provisions of ASU 2016-09 had no impact on the Company’s financial position, results of operations or cash flows.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new "expected loss model" that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. Early adoption is permitted for annual periods beginning after December 15, 2018, and interim periods therein. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The standard reduces the diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The standard will be effective on January 1, 2018. The Company currently does not expect this guidance to have a significant impact on its consolidated financial statements and related disclosures.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash that changes the presentation of restricted cash and cash equivalents in the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This standard is effective for the Company in the fiscal year beginning January 1, 2018, but early adoption is permissible. After adopting the standard, the amounts of restricted cash shown on the consolidated balance sheets of the Company would be included in cash and cash equivalents in the statement of cash flows. For the years ended December 31, 2017 and 2016, our restricted cash and restricted cash equivalents were not significant. The Company currently does not expect this guidance to have a significant impact on its consolidated financial statements and related disclosures.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting, which applies to entities that change the terms or conditions of a share-based payment award. The amendments in this standard include guidance on determining whether changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under Topic 718 unless all of the following conditions are met: (1) the fair value of the modified award is the same as the fair value of the original award immediately before the original award is modified, and if the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, then the entity is not required to estimate the value immediately before and after the modification; (2) the vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) the classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. The amendments are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017 and should be applied prospectively to an award modified on or after the adoption date. The Company currently does not expect this guidance to have a significant impact on its consolidated financial statements and related disclosures.

Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy. The Company’s investments in marketable securities are classified within Level 2 of the fair value hierarchy.

The fair values of the Company’s marketable securities are based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are reflected within Level 2, as they are primarily based on observable pricing for similar assets or other market observable inputs. Typical inputs used by these pricing services include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers or estimates of cash flow, prepayment spreads and default rates.

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2017 and 2016:

	December 31, 2017
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	306,235	$	306,235	$	—	$	—
Total cash equivalents		306,235		306,235		—		—
Marketable securities:
U.S. government securities		49,606		—		49,606		—
U.S. corporate bonds		48,959		—		48,959		—
U.S. commercial paper		65,583		—		65,583		—
International commercial paper		48,465		—		48,465		—
Total marketable securities		212,613		—		212,613		—
Total cash equivalents and marketable securities	$	518,848	$	306,235	$	212,613	$	—

	December 31, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	168,517	$	168,517	$	—	$	—
Total cash equivalents		168,517		168,517		—		—
Marketable securities:
U.S. government securities		100,031		—		100,031		—
U.S. corporate bonds		58,452		—		58,452		—
International corporate bonds		24,190		—		24,190		—
U.S. commercial paper		30,351		—		30,351		—
International commercial paper		15,938		—		15,938		—
Total marketable securities		228,962		—		228,962		—
Total cash equivalents and marketable securities	$	397,479	$	168,517	$	228,962	$	—

During the years ended December 31, 2017 and 2016, there were no transfers among the Level 1, Level 2 and Level 3 categories.

Marketable Securities

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2017 and 2016:

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	49,612	$	—	$	(6	)	$	49,606
U.S. corporate bonds		48,982		2		(25	)		48,959
U.S. commercial paper		65,583		—		—			65,583
International commercial paper		48,465		—		—			48,465
	$	212,642	$	2	$	(31	)	$	212,613

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	100,055	$	2	$	(26	)	$	100,031
U.S. corporate bonds		58,508		—		(56	)		58,452
International corporate bonds		24,212		—		(22	)		24,190
U.S. commercial paper		30,351		—		—			30,351
International commercial paper		15,938		—		—			15,938
	$	229,064	$	2	$	(104	)	$	228,962

As of December 31, 2017, all marketable securities held by the Company had remaining contractual maturities of one year or less.

As of December 31, 2017 and 2016, the Company held 11 and 27 marketable securities, respectively, that were in a loss position for less than one year due to fluctuations in interest rates.

There have been no impairments of the Company’s assets measured and carried at fair value during the years ended December 31, 2017 and 2016.

Balance Sheet Components

12 Months Ended

Dec. 31, 2017

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components

4.	Balance Sheet Components

Property and Equipment, net

Property and equipment, net consists of the following:

	December 31,
	2017			2016
	(in thousands)
Computer hardware and software	$	1,090		$	708
Furniture and equipment		1,029			695
Leasehold improvements		2,967			527
		5,086			1,930
Less: Accumulated depreciation		(1,073	)		(542	)
	$	4,013		$	1,388

Depreciation expense for the years ended December 31, 2017, 2016 and 2015 was $0.5 million, $0.3 million and $0.1 million, respectively.

The useful life for computer hardware and software is 3 years, furniture and equipment is 5 years and leasehold improvements is the lesser of the useful life or the term of the respective lease.

Accrued Expenses

Accrued expenses consist of the following:

	December 31,
	2017		2016
	(in thousands)
Development costs	$	23,473	$	14,541
Employee-related expenses		15,838		5,948
Professional services		3,166		1,751
Other accrued expenses		124		112
	$	42,601	$	22,352

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Operating Leases

In May 2017, the Company entered into the Sixth Amendment to the Lease (“the Sixth Amendment”) to increase the amount of square feet of office space that the Company leases in a multi-tenant building. Prior to entering the Sixth Amendment, the Company rented 22,067 square feet of office space in this multi-tenant building under an operating lease that was scheduled to expire on February 28, 2022. The Sixth Amendment increased the amount of leased space at this location by 32,876 square feet, with the additional space consisting of (i) 8,200 square feet beginning on August 15, 2017, and (ii) 24,676 square feet beginning on January 1, 2018. The lease for this additional space will expire no later than August 15, 2024. Additionally, the term of the existing lease has been extended from March 1, 2022 until the expiration date of the Sixth Amendment on August 15, 2024.

In May 2016, the Company entered into a separate lease that will expire on February 28, 2022, under which, beginning on September 1, 2016, the Company rents 19,805 square feet of additional office space in a separate multi-tenant building.

Rent expense, net of sublease income, for the years ended December 31, 2017, 2016 and 2015, was $3.7 million, $2.0 million, and $0.4 million, respectively.

Future minimum lease payments under non-cancelable operating leases are as follows at December 31, 2017:

Years Ending December 31,	(in thousands)
2018	$	4,444
2019		4,537
2020		4,632
2021		4,728
2022		3,485
Thereafter		5,450
	$	27,276

License Agreements

CyDex License Agreement

In September 2015, the Company and CyDex Pharmaceuticals, Inc. (“CyDex”) amended and restated their existing commercial license agreement. Under the terms of the commercial license agreement as amended and restated, CyDex has granted to the Company an exclusive license to CyDex’s Captisol drug formulation technology and related intellectual property for the manufacture of pharmaceutical products incorporating the Company’s compounds known as brexanolone and SAGE-689, and the development and commercialization of the resulting products in the treatment, prevention or diagnosis of any disease or symptom in humans or animals other than (i) the ocular treatment of any disease or condition with a formulation, including a hormone; (ii) topical ocular treatment of inflammatory conditions; (iii) treatment and prophylaxis of fungal infections in humans; and (iv) any ocular treatment for retinal degeneration. As consideration for the inclusion of SAGE-689 in the license granted by CyDex, the Company paid to CyDex $0.1 million, which was recorded as research and development expense for the year ended December 31, 2015 in connection with the execution of the amended and restated license agreement.

The Company is obligated to make milestone payments under the amended and restated license agreement with CyDex based on the achievement of clinical development and regulatory milestones in the amount of up to $0.8 million in clinical milestones and up to $3.8 million in regulatory milestones for each of the first two fields with respect to brexanolone; up to $1.3 million in clinical milestones and up to $8.5 million in regulatory milestones for each of the third and fourth fields with respect to brexanolone; and up to $0.8 million in clinical milestones and up to $1.8 million in regulatory milestones for one field with respect to SAGE-689.

For the year ended December 31, 2015, additional clinical development milestones were met for the brexanolone program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense and made cash payments totaling $0.8 million.

For the year ended December 31, 2016, additional clinical development milestones were met for the brexanolone program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense and made cash payments totaling $0.8 million.

For the year ended December 31, 2017, the Company did not record any expense or make any milestone payments related to clinical development milestones for the brexanolone program under the license agreement with CyDex.

University of California License Agreements

In October 2013, the Company entered into a non-exclusive license agreement with The Regents of the University of California under which the Company was granted a non-exclusive license to certain clinical data and clinical material for use in the development and commercialization of biopharmaceutical products in the licensed field, including status epilepticus and postpartum depression. In May 2014, the license agreement was amended to add the treatment of essential tremor to the licensed field of use, materials and milestone fee provisions of the agreement. As of December 31, 2015, the Company paid to The Regents of the University of California clinical development milestones of $0.1 million and will be required to pay royalties of less than 1% on net sales for a period of fifteen years following the sale of the first product developed using the data and materials. The license will terminate on the earlier to occur of (i) 27 years after the effective date or (ii) 15 years after the last-derived product is first commercially sold.

For the years ended December 31, 2013 and 2014, the Company did not record any expense or make any milestone or royalty payments under the license agreement with the University of California.

In June 2015, the Company entered into an exclusive license agreement with The Regents of the University of California whereby the Company was granted an exclusive license to certain patent rights related to the use of allopregnanolone to treat various diseases. In exchange for such license, the Company paid an upfront payment of $50,000 and will make payments of $15,000 for annual maintenance fees until the calendar year following the first sale, if any, of a licensed product. The Company is obligated to make milestone payments following the achievement of specified regulatory and sales milestones of up to $0.7 million and $2.0 million in the aggregate, respectively, of which none have been paid to date. Following the first sale, if any, of a licensed product, the Company is obligated to pay royalties at a low single digit percentage of net sales, if any, of licensed products, subject to specified minimum annual royalty amounts. Unless terminated by operation of law or by acts of the parties under the terms of the agreement, the license agreement will terminate when the last-to-expire patents or last-to-be abandoned patent applications expire, whichever is later.

For the year ended December 31, 2015, three clinical development milestones were met, and accordingly, the Company recorded research and development expenses and made cash payments totaling $0.1 million.

For the years ended December 31, 2017 and 2016, the Company did not record any expense or make any milestone or royalty payments under either license agreement with The Regents of the University of California.

Washington University License Agreement

In November 2013, the Company entered into a license agreement with Washington University whereby the Company was granted exclusive, worldwide rights to develop and commercialize a novel set of neuroactive steroids developed by Washington University. In exchange for development and commercialization rights, the Company paid an upfront, non-refundable payment of $50,000 and is required to pay an annual license maintenance fee of $15,000 on each subsequent anniversary date, until the first Phase 2 clinical trial for a licensed product is initiated. The Company is obligated to make milestone payments to Washington University based on achievement of clinical development and regulatory milestones of up to $0.7 million and $0.5 million, respectively. Additionally, the Company fulfilled its obligation to issue to Washington University 47,619 shares of common stock on December 13, 2013. The fair value of these shares of $0.1 million was recorded as research and development expense in 2013.

The Company is obligated to pay royalties to Washington University at rates in the low single digits on net sales of licensed products covered under patent rights and royalties at rates in the low single digits on net sales of licensed products not covered under patent rights. Additionally, the Company has the right to sublicense and is required to make payments at varying percentages of sublicensing revenue received, initially in the mid-teens and descending to the mid-single digits over time.

For the year ended December 31, 2015, a regulatory milestone was met for one of the programs under the license agreement with Washington University, and accordingly, the Company recorded research and development expenses and made a cash payment of $50,000.

For the years ended December 31, 2017 and 2016, the Company did not record any expense or make any milestone payments under the license agreement with Washington University.

Consulting Agreement

In January 2014, the Company entered into a consulting agreement with a non-employee advisor whereby the Company is obligated to make cash payments of up to $2.0 million and to issue up to 126,984 shares of common stock upon attainment of certain clinical development and regulatory milestones.

During the year ended December 31, 2014, the first clinical development milestones for each of two programs included in the consulting agreement were met. Accordingly, the Company recorded research and development expense for the year ended December 31, 2014 of $0.2 million, comprised of $50,000 in cash and $0.1 million related to the issuance of 15,872 shares of the Company’s common stock, related to the achievement of these milestones.

During the year ended December 31, 2015, the second and third clinical development milestones for one of the programs included in the consulting agreement were met. Accordingly, the Company recorded research and development expense for the year ended December 31, 2015 of $1.7 million, comprised of $0.5 million in cash and $1.2 million related to the issuance of 23,809 shares of the Company’s common stock, related to the achievement of these milestones.

For the years ended December 31, 2017 and 2016, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Preferred Stock

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Preferred Stock

6.	Preferred Stock

The Board of Directors of the Company is authorized, without action by the stockholders, to designate and issue up to an aggregate of 5,000,000 shares of preferred stock in one or more series. The Board of Directors of the Company can designate the rights, preferences and privileges of the shares of each series and any of its qualifications, limitations or restrictions. The Board of Directors of the Company may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of common stock. As of December 31, 2017 and 2016, the Company had no shares of preferred stock issued or outstanding. The preferred stock was classified under stockholders’ equity as of December 31, 2017 and 2016.

Common Stock

12 Months Ended

Dec. 31, 2017

Text Block [Abstract]

Common Stock

7.	Common Stock

As of December 31, 2017 and 2016, the Company authorized 120,000,000 shares of common stock with a par value of $0.0001 per share.

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the Board of Directors, if any. As of December 31, 2017 and 2016, no dividends have been declared.

During the year ended December 31, 2017, the Company received 614 shares of the Company’s common stock from an employee as proceeds for an exercise of stock options. The total cost of shares held in treasury at December 31, 2017 was $113,000.

During the year ended December 31, 2016, the Company received 346 shares of the Company’s common stock from an employee as proceeds for an exercise of stock options. The total cost of shares held in treasury at December 31, 2016 was $17,000.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8.	Stock-Based Compensation

Restricted Stock Units

During the year ended December 31, 2017, the Company granted 32,500 restricted stock units to employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. During the year ended December 31, 2017, the Company recorded $0.6 million of stock-based compensation expense related to its restricted stock units. These restricted stock units vest ratably over two years, with cliff vesting of 50% at both the one-year and two-year anniversary of the grant, which will be in February 2018 and February 2019, respectively.

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2016		—
Granted		32,500
Vested		—
Forfeited		(3,400	)
Outstanding as of December 31, 2017		29,100

Stock Option Plans

On July 2, 2014, the stockholders of the Company approved the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”), which became effective upon the completion of the IPO. The 2014 Stock Option Plan provides for the grant of restricted stock awards, restricted stock units, incentive stock options and non-statutory stock options. The 2014 Stock Option Plan replaced the Company’s 2011 Stock Option and Grant Plan (the “2011 Stock Option Plan”). The Company will no longer grant stock options or other awards under the 2011 Stock Option Plan. Any options or awards outstanding under the 2011 Stock Option Plan remained outstanding and effective. As of December 31, 2017, the total number of shares reserved under all equity plans is 7,866,789, and the Company had 2,251,096 shares available for future issuance under such plans.

On December 15, 2016, the Board of Directors of the Company approved the 2016 Inducement Equity Plan (the “2016 Stock Option Plan”), for which the initial grants were during the year ended December 31, 2017. The 2016 Stock Option Plan provides for the grant of equity awards to individuals who have not previously been an employee or a non-employee director of the Company to induce them to accept employment and to provide them with a proprietary interest in the Company.

The 2014 Stock Option Plan provides for an annual increase, to be added on the first day of each fiscal year, by up to 4% of the Company’s outstanding shares of common stock as of the last day of the prior year. On January 1, 2018, 1,680,117 shares of common stock, representing 4% of the Company’s outstanding shares of common stock as of December 31, 2017, were added to the 2014 Stock Option Plan.

Terms of restricted stock awards, restricted stock units, and stock options, including vesting requirements, are determined by the Board of Directors or the Compensation Committee of the Board of Directors, subject to the provisions of the applicable stock option plan. Options granted by the Company, that are not performance-based, generally vest based on the continued service of the grantee with the Company during a specified period following grant. These awards, when granted to employees, generally vest ratably over four years, with a 25% cliff vesting at the one year anniversary. All option awards expire in 10 years.

During the years ended December 31, 2017, 2016 and 2015, the Company granted 449,208, 74,039 and 497,100 options, respectively, to employees to purchase shares of common stock that contain performance-based vesting criteria, primarily related to the achievement of certain clinical and regulatory development milestones related to product candidates. Recognition of stock-based compensation expense associated with these performance-based stock options commences when the performance condition is considered probable of achievement, using management’s best estimates, which consider the inherent risk and uncertainty regarding the future outcomes of the milestones.

During the year ended December 31, 2015, one milestone was achieved. This milestone represents 35% of the performance-based option grants that were made during the year ended December 31, 2015. During the year ended December 31, 2015, the Company recognized stock-based compensation expense related to this milestone of $4.8 million.

During the year ended December 31, 2016, one milestone was achieved. This milestone represents 50% and 30% of the performance-based option grants that were made during the years ended December 31, 2016 and 2015, respectively. During the year ended December 31, 2016, the Company recognized stock-based compensation expense related to this milestone of $5.0 million.

During the year ended December 31, 2017, the achievement of the remaining milestones that are the criteria for vesting of performance-based stock options was considered not probable, nor met, and therefore no expense has been recognized related to these awards for the year ended December 31, 2017.

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the years ended December 31, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	19,893	$	11,197	$	5,924
General and administrative		15,641		11,823		9,316
	$	35,534	$	23,020	$	15,240

Stock-based compensation expense by award type recognized during the years ended December 31, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Stock options	$	34,525	$	22,820	$	14,796
Restricted stock units		617		—		—
Employee stock purchase plan		392		166		117
Restricted stock awards		—		34		327
	$	35,534	$	23,020	$	15,240

For stock option awards, the fair value is estimated at the grant date using the Black-Scholes option-pricing model, taking into account the terms and conditions upon which options are granted. The fair value of the options is amortized on a straight-line basis for awards to employees and on a graded basis for awards to non-employees over the requisite service period of the awards. The weighted average grant date fair value per share relating to outstanding stock options granted under the Company’s stock option plans during the years ended December 31, 2017, 2016 and 2015 was $41.94, $24.97 and $34.08, respectively.

The fair value of each option granted to employees and nonemployee directors during the years ended December 31, 2017, 2016 and 2015 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:

	Year Ended December 31,
	2017			2016			2015
Expected dividend yield		0	%		0	%		0	%
Expected volatility		79.89	%		80.15	%		90.54	%
Risk-free interest rate		2.03	%		1.47	%		1.59	%
Expected life of option	6.03 years			6.05 years			6.03 years

Expected dividend yield: the Company has not paid, and does not anticipate paying, any dividends in the foreseeable future.

Risk-free interest rate: the Company determined the risk-free interest rate by using a weighted average equivalent to the expected term based on the U.S. Treasury yield curve in effect as of the date of grant.

Expected volatility: the Company does not have sufficient history to support a calculation of volatility using only its historical data. Starting in 2016, the Company uses a weighted-average volatility considering the Company’s own volatility since the IPO in July 2014 and the volatilities of a peer group of comparable companies for time periods prior to the IPO. Prior to 2016, the Company used volatilities based on an analysis of reported data for a peer group of comparable companies.

Expected term (in years): the expected term represents the period that the Company’s stock option grants are expected to be outstanding. The Company has been publicly-traded since July 2014, and there is not sufficient historical term data to calculate the expected term of the options. Therefore, the Company elected to utilize the “simplified” method to estimate the expected term of options granted to employees. Under this approach, the weighted average expected life is presumed to be the average of the vesting term and the contractual term of the option.

Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company estimates forfeitures based on historical termination behavior. For the years ended December 31, 2017, 2016 and 2015, forfeiture rates of 13.3%, 9.6% and 10.0%, respectively, were applied.

For options granted to non-employees, the expected term is 10 years, which is the contractual term of each option. All other assumptions used to calculate the grant date fair value are generally consistent with the assumptions used for options granted to employees.

The table below summarizes activity related to stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Granted		2,360,933			60.59
Exercised		(765,950	)		20.94
Forfeited		(240,197	)		43.61
Outstanding as of December 31, 2017		5,586,593		$	43.58		8.09	$	676,717
Vested and expected to vest as of December 31, 2017		4,504,257		$	42.22		7.98	$	551,729
Exercisable as of December 31, 2017		2,045,229		$	27.95		6.99	$	279,699

At December 31, 2017, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $94.4 million, which is expected to be recognized over the remaining weighted average vesting period of 2.58 years. The total fair value of options vested for the years ended December 31, 2017, 2016 and 2015 was $24.2 million, $24.1 million, and $9.2 million, respectively.

At December 31, 2017, 670,637 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $16.4 million.

The intrinsic value of stock options exercised during the years ended December 31, 2017, 2016 and 2015 was $77.7 million, $4.6 million and $28.4 million, respectively.

2014 Employee Stock Purchase Plan

On July 2, 2014, the Company’s stockholders approved the 2014 Employee Stock Purchase Plan, which had been previously approved by the Board of Directors. A total of 282,000 shares of common stock were initially authorized for issuance under this plan. The 2014 Employee Stock Purchase Plan became effective upon the completion of the IPO. As of December 31, 2017, 30,069 shares have been issued under this plan. At December 31, 2017, accrued expenses includes $0.3 million of stock-based compensation expense related to an enrollment period for which the related shares had not been issued as of December 31, 2017.

Net Loss Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

9.	Net Loss Per Share

Basic and diluted net loss per share was calculated as follows for the years ended December 31, 2017, 2016 and 2015:

	Year Ended December 31,
	2017			2016			2015
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(270,120	)	$	(158,987	)	$	(94,495	)
Denominator:
Weighted average common stock outstanding—basic		38,113,678			33,492,795			27,778,288
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—			—
Weighted average common stock outstanding—diluted		38,113,678			33,492,795			27,778,288
Net loss per share—basic and diluted	$	(7.09	)	$	(4.75	)	$	(3.40	)

The following common stock equivalents outstanding as of December 31, 2017 and 2016 were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive:

	Year Ended December 31,
	2017		2016
Stock options		4,915,956		3,985,935
Restricted stock units		29,100		—
Employee stock purchase plan		11,683		6,784
		4,956,739		3,992,719

Stock options that are outstanding and contain performance-based vesting criteria for which the performance conditions have not been met are excluded from the common stock equivalents outstanding because meeting the performance conditions is not considered probable.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

10.	Income Taxes

There is no provision for income taxes because the Company has historically incurred operating losses and maintains a full valuation allowance against its net deferred tax assets. The reported amount of income tax expense for the years differs from the amount that would result from applying domestic federal statutory tax rates to pretax losses primarily because of changes in valuation allowance.

A reconciliation of the U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2017			2016			2015
Tax due at statutory rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal		5.1			4.2			4.0
Stock-based compensation		6.4			(1.0	)		(0.9	)
Foreign rate differential		(3.2	)		(3.1	)		(3.1	)
Federal and state credits		7.2			10.3			13.2
Change in valuation allowance		(22.5	)		(41.5	)		(43.1	)
Other		—			—			0.1
Federal and state rate change		(24.6	)		—			—
Research and orphan drug credit addback		(2.4	)		(2.9	)		(4.2	)
		0.0	%		0.0	%		0.0	%

Significant components of the Company’s net deferred tax asset at December 31, 2017 and 2016 are as follows:

	December 31,
	2017			2016
	(in thousands)
Net operating losses	$	131,151		$	84,374
Capitalized start-up costs		1,362			2,155
Accounting method change		—			(673	)
Tax credit carryforwards		54,523			33,477
Accrued expenses		3,057			2,287
Depreciation and amortization		685			999
Stock options		13,573			12,103
Others		1,090			350
Total net deferred tax asset before valuation allowance		205,441			135,072
Valuation allowance		(205,441	)		(135,072	)
Net deferred tax asset	$	—		$	—

As of December 31, 2017, the Company had federal and state net operating loss carryforwards of $482.9 million and $479.9 million, respectively, which begin to expire in 2031. As of December 31, 2017, the Company had federal and state research and development tax credits carryforwards of $12.7 million and $2.3 million, respectively, which begin to expire in 2031 and 2027, respectively. As of December 31, 2017, the Company had federal orphan drug tax credit carry forwards of $40.0 million, which begin to expire in 2034.

On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“The TCJA). This legislation reduced the U.S. corporate tax rate from the existing rate of 34% to 21% for tax years beginning after December 31, 2017. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities existing as of December 31, 2017 from the 34% federal rate in effect through the end of 2017, to the new 21% rate. This revaluation resulted in a reduction to the Company’s deferred tax asset of $66.4 million. This amount was offset by a corresponding reduction to the Company’s valuation allowance. The other provisions of the TCJA did not have a material impact on the December 31, 2017 consolidated financial statements. Estimates used to prepare our income tax expense are based on the Company’s initial analysis of the TCJA. Given the complexity of the TCJA, anticipated guidance from the U. S. Treasury regarding implementation of the TCJA, and the potential for additional guidance from the Securities and Exchange Commission and the FASB related to the TCJA, these estimates may be adjusted during fiscal 2018 to reflect any such guidance provided.

The Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, for the quarter ended March 31, 2017. As a result of adoption, the deferred tax assets as of December 31, 2016 associated with net operating losses have increased by $8.0 million and the deferred tax assets associated with federal and state research credits increased by $1.4 million. These amounts will be offset by a corresponding increase in the valuation allowance. The adoption of ASU 2016-09 has no impact to the Company’s operations, financial position or cash flows.

As of December 31, 2017, net deferred tax assets increased approximately $70.4 million, primarily due to the operating loss and tax credits incurred during the year. This increase in net deferred tax assets was offset by a corresponding increase in the valuation allowance.

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and tax credit carryforwards. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, a full valuation allowance of $205.4 million and $135.1 million has been established at December 31, 2017 and 2016, respectively.

Pursuant to Section 382 of the Internal Revenue Code, and similar state tax law, certain substantial changes in the Company’s ownership may result in a limitation on the amount of net operating loss carryforwards and tax carryforwards that may be used in future years. Utilization of the net operating loss (“NOL”) and tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of NOL and tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company completed a detailed Section 382 study during 2017 on its Net Operating Losses and Credits incurred from the year it began operations in 2011 through December 31, 2016. Based on the study, the Company underwent two ownership changes for Section 382 purposes which occurred on March 11, 2014 and December 31, 2015. As a result of the ownership changes, all of the Company’s NOL and Tax Credit carryforwards as of the ownership change dates are subject to limitation under Section 382. Any NOLs or Tax Credits generated after the December 2015 change are not subject to this annual limitation. However, subsequent ownership changes, as defined by Section 382, may potentially further limit the amount of net operating loss carryforwards that could be utilized to offset future taxable income.

The Company applies the authoritative guidance on accounting for and disclosure of uncertainty in tax positions, which requires the Company to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority.

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	—
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		—
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense when in a taxable income position. As of December 31, 2017 and 2016, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations, and the Company’s tax returns are open under statute from 2014 to the present. The tax attributes prior to 2014 may still be adjusted upon examination. The Company’s policy is to record interest and penalties related to income taxes as part of the tax provision.

During November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which simplifies the presentation of deferred income taxes. ASU 2015-17 can be early adopted for any period that has not been issued on a prospective or retrospective basis. The Company elected to early adopt this guidance effective December 31, 2015 on a prospective basis. Adoption of this ASU resulted in a reclassification of the current deferred tax liability to a non-current deferred tax liability, in the amount of $0.6 million, which is netted with the long-term deferred tax asset in its consolidated balance sheet as of December 31, 2015. No prior periods were retrospectively adjusted.

Employee Benefit Plan

12 Months Ended

Dec. 31, 2017

Postemployment Benefits [Abstract]

Employee Benefit Plan

11.	Employee Benefit Plan

The Company maintains a 401(k) profit sharing plan (the “Plan”) for its employees. Each employee may elect to contribute a portion of his or her annual compensation to the Plan, subject to annual limits established by the Internal Revenue Service. For the years ended December 31, 2017, 2016 and 2015, the Company matched 50% of eligible contributions up to 6% of employee contributions. For the years ended December 31, 2017, 2016 and 2015 the Company contributed $0.9 million, $0.4 million and $0.2 million, respectively.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

12.	Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	57,480		$	70,854		$	74,373		$	70,448		$	273,155
Loss from operations		(57,480	)		(70,854	)		(74,373	)		(70,448	)		(273,155	)
Net loss		(56,778	)		(70,202	)		(73,719	)	$	(69,421	)		(270,120	)
Net loss per share—basic and diluted	$	(1.52	)	$	(1.88	)	$	(1.97	)	$	(1.75	)	$	(7.09	)

	2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	30,714		$	35,006		$	38,064		$	56,379		$	160,163
Loss from operations		(30,714	)		(35,006	)		(38,064	)		(56,379	)		(160,163	)
Net loss		(30,543	)		(34,747	)		(37,796	)		(55,901	)		(158,987	)
Net loss per share—basic and diluted	$	(0.97	)	$	(1.08	)	$	(1.15	)	$	(1.50	)	$	(4.75	)

Subsequent Event

12 Months Ended

Dec. 31, 2017

Subsequent Events [Abstract]

Subsequent Event

13.	Subsequent Event

On February 13, 2018, the Company completed the sale of 4,032,012 shares of its common stock at a price to the public of $164.00 per share, resulting in net proceeds to the Company of $631.1 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Cash and Cash Equivalents

Marketable Securities

Marketable securities

Restricted Cash

Property and Equipment

Impairment of Long-Lived Assets

Research and Development

Patent Costs

The Company expenses patent costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.

Stock-Based Compensation

Treasury Stock

The Company records treasury stock at cost. Treasury stock consists of shares received from an employee as consideration for exercises of stock options.

Basic and Diluted Net Loss Per Share

The Company reported a net loss for the years ended December 31, 2017, 2016 and 2015.

Risks and Uncertainties

Concentration of Credit Risk and of Significant Suppliers

Income Taxes

Fair Value Measurements

Level 1	—	Quoted market prices in active markets for identical assets or liabilities.

Level 2	—	Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3	—	Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Segment Data

Comprehensive Loss

Public Offerings

Also see Note 13, “Subsequent Event”.

Recently Issued Accounting Pronouncements

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Summary of Company's Money Market Funds and Marketable Securities

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2017 and 2016:

	December 31, 2017
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	306,235	$	306,235	$	—	$	—
Total cash equivalents		306,235		306,235		—		—
Marketable securities:
U.S. government securities		49,606		—		49,606		—
U.S. corporate bonds		48,959		—		48,959		—
U.S. commercial paper		65,583		—		65,583		—
International commercial paper		48,465		—		48,465		—
Total marketable securities		212,613		—		212,613		—
Total cash equivalents and marketable securities	$	518,848	$	306,235	$	212,613	$	—

	December 31, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Cash equivalents	$	168,517	$	168,517	$	—	$	—
Total cash equivalents		168,517		168,517		—		—
Marketable securities:
U.S. government securities		100,031		—		100,031		—
U.S. corporate bonds		58,452		—		58,452		—
International corporate bonds		24,190		—		24,190		—
U.S. commercial paper		30,351		—		30,351		—
International commercial paper		15,938		—		15,938		—
Total marketable securities		228,962		—		228,962		—
Total cash equivalents and marketable securities	$	397,479	$	168,517	$	228,962	$	—

Summary of Gross Unrealized Gains and Losses of Marketable Securities

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2017 and 2016:

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	49,612	$	—	$	(6	)	$	49,606
U.S. corporate bonds		48,982		2		(25	)		48,959
U.S. commercial paper		65,583		—		—			65,583
International commercial paper		48,465		—		—			48,465
	$	212,642	$	2	$	(31	)	$	212,613

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	100,055	$	2	$	(26	)	$	100,031
U.S. corporate bonds		58,508		—		(56	)		58,452
International corporate bonds		24,212		—		(22	)		24,190
U.S. commercial paper		30,351		—		—			30,351
International commercial paper		15,938		—		—			15,938
	$	229,064	$	2	$	(104	)	$	228,962

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2017

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2017			2016
	(in thousands)
Computer hardware and software	$	1,090		$	708
Furniture and equipment		1,029			695
Leasehold improvements		2,967			527
		5,086			1,930
Less: Accumulated depreciation		(1,073	)		(542	)
	$	4,013		$	1,388

Summary of Accrued Expenses

Accrued expenses consist of the following:

	December 31,
	2017		2016
	(in thousands)
Development costs	$	23,473	$	14,541
Employee-related expenses		15,838		5,948
Professional services		3,166		1,751
Other accrued expenses		124		112
	$	42,601	$	22,352

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases

Future minimum lease payments under non-cancelable operating leases are as follows at December 31, 2017:

Years Ending December 31,	(in thousands)
2018	$	4,444
2019		4,537
2020		4,632
2021		4,728
2022		3,485
Thereafter		5,450
	$	27,276

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Relating to Restricted Stock

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2016		—
Granted		32,500
Vested		—
Forfeited		(3,400	)
Outstanding as of December 31, 2017		29,100

Summary of Stock-Based Compensation Expense for Stock Options, Restricted Stock Units and Employee Stock Purchase Plan Recognized

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the years ended December 31, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	19,893	$	11,197	$	5,924
General and administrative		15,641		11,823		9,316
	$	35,534	$	23,020	$	15,240

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the years ended December 31, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Stock options	$	34,525	$	22,820	$	14,796
Restricted stock units		617		—		—
Employee stock purchase plan		392		166		117
Restricted stock awards		—		34		327
	$	35,534	$	23,020	$	15,240

Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted

	Year Ended December 31,
	2017			2016			2015
Expected dividend yield		0	%		0	%		0	%
Expected volatility		79.89	%		80.15	%		90.54	%
Risk-free interest rate		2.03	%		1.47	%		1.59	%
Expected life of option	6.03 years			6.05 years			6.03 years

Summary of Activity Relating to Stock Options

The table below summarizes activity related to stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Granted		2,360,933			60.59
Exercised		(765,950	)		20.94
Forfeited		(240,197	)		43.61
Outstanding as of December 31, 2017		5,586,593		$	43.58		8.09	$	676,717
Vested and expected to vest as of December 31, 2017		4,504,257		$	42.22		7.98	$	551,729
Exercisable as of December 31, 2017		2,045,229		$	27.95		6.99	$	279,699

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss Per Share

Basic and diluted net loss per share was calculated as follows for the years ended December 31, 2017, 2016 and 2015:

	Year Ended December 31,
	2017			2016			2015
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(270,120	)	$	(158,987	)	$	(94,495	)
Denominator:
Weighted average common stock outstanding—basic		38,113,678			33,492,795			27,778,288
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—			—
Weighted average common stock outstanding—diluted		38,113,678			33,492,795			27,778,288
Net loss per share—basic and diluted	$	(7.09	)	$	(4.75	)	$	(3.40	)

Summary of Anti-Dilutive Common Stock Equivalents Outstanding

	Year Ended December 31,
	2017		2016
Stock options		4,915,956		3,985,935
Restricted stock units		29,100		—
Employee stock purchase plan		11,683		6,784
		4,956,739		3,992,719

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate

A reconciliation of the U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2017			2016			2015
Tax due at statutory rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal		5.1			4.2			4.0
Stock-based compensation		6.4			(1.0	)		(0.9	)
Foreign rate differential		(3.2	)		(3.1	)		(3.1	)
Federal and state credits		7.2			10.3			13.2
Change in valuation allowance		(22.5	)		(41.5	)		(43.1	)
Other		—			—			0.1
Federal and state rate change		(24.6	)		—			—
Research and orphan drug credit addback		(2.4	)		(2.9	)		(4.2	)
		0.0	%		0.0	%		0.0	%

Summary of Significant Components of Company's Net Deferred Tax Asset

Significant components of the Company’s net deferred tax asset at December 31, 2017 and 2016 are as follows:

	December 31,
	2017			2016
	(in thousands)
Net operating losses	$	131,151		$	84,374
Capitalized start-up costs		1,362			2,155
Accounting method change		—			(673	)
Tax credit carryforwards		54,523			33,477
Accrued expenses		3,057			2,287
Depreciation and amortization		685			999
Stock options		13,573			12,103
Others		1,090			350
Total net deferred tax asset before valuation allowance		205,441			135,072
Valuation allowance		(205,441	)		(135,072	)
Net deferred tax asset	$	—		$	—

Summary of Rollforward of Company's Unrecognized Tax Benefits

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	—
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		—
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Financial Information

	2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	57,480		$	70,854		$	74,373		$	70,448		$	273,155
Loss from operations		(57,480	)		(70,854	)		(74,373	)		(70,448	)		(273,155	)
Net loss		(56,778	)		(70,202	)		(73,719	)	$	(69,421	)		(270,120	)
Net loss per share—basic and diluted	$	(1.52	)	$	(1.88	)	$	(1.97	)	$	(1.75	)	$	(7.09	)

	2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	30,714		$	35,006		$	38,064		$	56,379		$	160,163
Loss from operations		(30,714	)		(35,006	)		(38,064	)		(56,379	)		(160,163	)
Net loss		(30,543	)		(34,747	)		(37,796	)		(55,901	)		(158,987	)
Net loss per share—basic and diluted	$	(0.97	)	$	(1.08	)	$	(1.15	)	$	(1.50	)	$	(4.75	)

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands						93 Months Ended
	Feb. 13, 2018	Nov. 17, 2017	Sep. 14, 2016	Jan. 12, 2016	Apr. 20, 2015	Dec. 31, 2017	Dec. 31, 2016
Nature Of Business [Line Items]
Accumulated deficit						$ (590,447)	$ (320,327)
Net proceeds from sale of common stock		$ 325,800	$ 189,200	$ 140,400	$ 129,100
Subsequent Event [Member]
Nature Of Business [Line Items]
Net proceeds from sale of common stock	$ 631,100
Redeemable Convertible Preferred Stock [Member] \| Convertible Notes [Member] \| Initial Public Offering [Member]
Nature Of Business [Line Items]
Net proceeds from sale of equity and notes						$ 969,000

Summary of Significant Accounting Policies - Additional Information (Detail)						1 Months Ended
	Nov. 17, 2017 USD ($) $ / shares shares	Sep. 14, 2016 USD ($) $ / shares shares	Jan. 12, 2016 USD ($) $ / shares shares	Apr. 20, 2015 USD ($) $ / shares shares	Jul. 23, 2014 USD ($) $ / shares shares	May 31, 2017 ft²	May 31, 2016 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								$ 849,000	$ 564,000
Issuance of common stock, shares \| shares	4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share \| $ / shares	$ 85.00	$ 39.75	$ 47.50	$ 52.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering costs	$ 325,800,000	$ 189,200,000	$ 140,400,000	$ 129,100,000
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares \| shares					5,750,000
Common stock price per share \| $ / shares					$ 18.00
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts					$ 94,000,000
Operating Lease Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								39,000	39,000
Operating Lease Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								300,000
Operating Lease One [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								$ 500,000	$ 500,000
Additional Office space rented under operating lease \| ft²						22,067	19,805
Lease expire date						Feb. 28, 2022	Feb. 28, 2022

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 306,235	$ 168,517
Total marketable securities	212,613	228,962
Total cash equivalents and marketable securities	518,848	397,479
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Total cash equivalents and marketable securities	306,235	168,517
Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	212,613	228,962
Total cash equivalents and marketable securities	212,613	228,962
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	49,606	100,031
U.S. Government Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	49,606	100,031
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,959	58,452
U.S. Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,959	58,452
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities		24,190
International Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities		24,190
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	65,583	30,351
U.S. Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	65,583	30,351
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,465	15,938
International Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 48,465	$ 15,938

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) MarketableSecurity	Dec. 31, 2016 USD ($) MarketableSecurity
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories	$ 0	$ 0
Number of marketable securities loss position due to fluctuations in interest rates \| MarketableSecurity	11	27
Impairment of assets	$ 0	$ 0
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year

Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 212,642	$ 229,064
Gross Unrealized Gains	2	2
Gross Unrealized Losses	(31)	(104)
Fair Value	212,613	228,962
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,612	100,055
Gross Unrealized Gains		2
Gross Unrealized Losses	(6)	(26)
Fair Value	49,606	100,031
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,982	58,508
Gross Unrealized Gains	2
Gross Unrealized Losses	(25)	(56)
Fair Value	48,959	58,452
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		24,212
Gross Unrealized Losses		(22)
Fair Value		24,190
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,583	30,351
Fair Value	65,583	30,351
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,465	15,938
Fair Value	$ 48,465	$ 15,938

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,086	$ 1,930
Less: Accumulated depreciation	(1,073)	(542)
Property and equipment, net	4,013	1,388
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,090	708
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,029	695
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,967	$ 527

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 0.5	$ 0.3	$ 0.1
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 23,473	$ 14,541
Employee-related expenses	15,838	5,948
Professional services	3,166	1,751
Other accrued expenses	124	112
Total accrued expenses	$ 42,601	$ 22,352

Commitments and Contingencies - Operating Leases - Additional Information (Detail) $ in Millions				1 Months Ended		12 Months Ended
	Jan. 01, 2018 ft²	Aug. 15, 2017 ft²	May 31, 2017 ft²	May 31, 2017 ft²	May 31, 2016 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items]
Rent expense, net of sublease income \| $						$ 3.7	$ 2.0	$ 0.4
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease				22,067	19,805
Lease expire date				Feb. 28, 2022	Feb. 28, 2022
Lease existence of option to extend			true
Commencement date of lease					Sep. 01, 2016
Sixth Amendment to Lease [Member]
Commitments And Contingencies [Line Items]
Lease expire date			Aug. 15, 2024
Increased office space rent		8,200	32,876
Sixth Amendment to Lease [Member] \| Subsequent Event [Member]
Commitments And Contingencies [Line Items]
Increased office space rent	24,676

Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 4,444
2019	4,537
2020	4,632
2021	4,728
2022	3,485
Thereafter	5,450
Total Operating leases, future minimum payments	$ 27,276

Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Research and development expense	$ 210,277,000	$ 120,756,000	$ 69,357,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense			100,000
CyDex License Agreement [Member] \| First and Second Clinical Development Milestones [Member] \| Brexanolone [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] \| First and Second Regulatory Milestones [Member] \| Brexanolone [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	3,800,000
CyDex License Agreement [Member] \| Third and Fourth Clinical Development Milestones [Member] \| Brexanolone [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	1,300,000
CyDex License Agreement [Member] \| Third and Fourth Regulatory Milestones [Member] \| Brexanolone [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	8,500,000
CyDex License Agreement [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense	0	800,000	800,000
Milestone payments	0	$ 800,000	$ 800,000
CyDex License Agreement [Member] \| Clinical Development [Member] \| SAGE-689 [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] \| Regulatory Milestones [Member] \| SAGE-689 [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 1,800,000

Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - University of California License Agreements [Member]		1 Months Ended	12 Months Ended					31 Months Ended
	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Milestone	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 0	$ 0		$ 0	$ 0
Milestone payments			$ 0	$ 0		$ 0	$ 0
Upfront payment		$ 50,000
Annual license maintenance fee					$ 15,000
After The Effective Date [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	27 years
After The First Sale [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	15 years
Clinical Development [Member]
Commitments And Contingencies [Line Items]
Milestone payments	$ 100,000
Research and development expense related to milestone expense					100,000
Milestone payments					$ 100,000
Milestones achieved \| Milestone					3
Clinical Development [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Percentage of net sales paid as royalties	1.00%
Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		700,000
Additional milestone payments								$ 0
Sales Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 2,000,000
Additional milestone payments								$ 0

Commitments and Contingencies - Washington University License Agreement - Additional Information (Detail) - Washington University License Agreement [Member] - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Commitments And Contingencies [Line Items]
Upfront non-refundable payment	$ 50,000
Annual license maintenance fee					$ 15,000
Issuance of common stock, shares					47,619
Research and development expense related to issue of common stock obligation					$ 100,000
Research and development expense related to milestone expense		$ 0	$ 0
Milestone payments		$ 0	$ 0
Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense				$ 50,000
Milestone payments				$ 50,000
Maximum [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	700,000
Maximum [Member] \| Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 500,000

Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member]	1 Months Ended	12 Months Ended
	Jan. 31, 2014 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Milestone shares	Dec. 31, 2014 USD ($) Milestone shares
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		$ 0	$ 0
Milestone payments		$ 0	$ 0
Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Shares of common stock for attaining milestones \| shares	126,984
Clinical Development and Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 2,000,000
First Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone					2
Research and development expense related to milestone expense					$ 200,000
Milestone payments					50,000
Payments for stock issued upon reaching a milestone					$ 100,000
Milestone based share compensation, shares issued \| shares					15,872
Second Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone				1
Research and development expense related to milestone expense				$ 1,700,000
Milestone payments				500,000
Payments for stock issued upon reaching a milestone				$ 1,200,000
Milestone based share compensation, shares issued \| shares				23,809
Third Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone				1
Research and development expense related to milestone expense				$ 1,700,000
Milestone payments				500,000
Payments for stock issued upon reaching a milestone				$ 1,200,000
Milestone based share compensation, shares issued \| shares				23,809

Preferred Stock - Additional Information (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Common Stock - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Common stock, shares authorized	120,000,000	120,000,000
Common stock, par value	$ 0.0001	$ 0.0001
Dividends declared	$ 0	$ 0
Voting rights	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders.
Purchase of treasury stock, Shares	614	346
Total cost of shares held in treasury	$ 113,000	$ 17,000

Stock-Based Compensation - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2018 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) Milestone $ / shares shares	Dec. 31, 2015 USD ($) Milestone $ / shares shares	Jul. 02, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 35,534,000	$ 23,020,000	$ 15,240,000
Share based compensation granted under plan vest period		4 years
Total number of options outstanding		7,866,789
Share based compensation, vest period		1 year
Share based compensation, term of plan		10 years
Weighted average grant date fair value per share \| $ / shares		$ 41.94	$ 24.97	$ 34.08
Share option, forfeiture rates		13.30%	9.60%	10.00%
Non-employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, term of plan		10 years
2011 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		0
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance under stock option plan		2,251,096
Milestones achieved \| Milestone			1	1
2014 Stock Option Plan [Member] \| Milestone One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved				35.00%
2014 Stock Option Plan [Member] \| Milestone Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved			50.00%	30.00%
2014 Stock Option Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of increase on outstanding shares of Common stock		4.00%
2014 Stock Option Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares annual increase added to plan	1,680,117
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock initially authorized for issuance under this plan					282,000
Number of shares issued under the plan		30,069
Accrued expenses \| $		$ 300,000
Restricted Stock Units Vest One Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		25.00%
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		32,500
Stock-based compensation expense \| $		$ 617,000
Share based compensation granted under plan vest period		2 years
Restricted Stock Units [Member] \| Restricted Stock Units Vest One Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		50.00%
Restricted stock units vesting period month and year		2018-02
Restricted Stock Units [Member] \| Restricted Stock Units Vest Two Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		50.00%
Restricted stock units vesting period month and year		2019-02
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		449,208	74,039	497,100
Total unrecognized stock-based compensation expense \| $		$ 16,400,000
Number of shares outstanding and unvested stock options		670,637
Performance Milestone [Member] \| 2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $			$ 5,000,000	$ 4,800,000
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 34,525,000	$ 22,820,000	14,796,000
Options granted		2,360,933
Total number of options outstanding		5,586,593	4,231,807
Total unrecognized stock-based compensation expense \| $		$ 94,400,000
Weighted average period of unrecognized compensation costs		2 years 6 months 29 days
Total fair value of options vested \| $		$ 24,200,000	$ 24,100,000	9,200,000
Intrinsic value of options exercised \| $		$ 77,700,000	$ 4,600,000	$ 28,400,000

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares- Restricted stock	32,500
Forfeited, Shares-Restricted stock	(3,400)
Outstanding, Shares at ending balance-Restricted stock	29,100

Stock-Based Compensation - Summary of Stock-Based Compensation Expense for Stock Options, Restricted Stock Units and Employee Stock Purchase Plan Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 35,534	$ 23,020	$ 15,240
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	19,893	11,197	5,924
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 15,641	$ 11,823	$ 9,316

Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 35,534	$ 23,020	$ 15,240
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	34,525	22,820	14,796
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	617
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 392	166	117
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense		$ 34	$ 327

Stock-Based Compensation - Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	79.89%	80.15%	90.54%
Risk-free interest rate	2.03%	1.47%	1.59%
Expected life of option	6 years 10 days	6 years 18 days	6 years 10 days

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares	7,866,789
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	4,231,807
Granted, Shares	2,360,933
Exercised, Shares	(765,950)
Forfeited, Shares	(240,197)
Ending balance, Outstanding Shares	5,586,593	4,231,807
Vested and expected to vest, Shares	4,504,257
Exercisable, Shares	2,045,229
Beginning balance, Outstanding Weighted Average Exercise Price	$ 29.99
Granted, Weighted Average Exercise Price	60.59
Exercised, Weighted Average Exercise Price	20.94
Forfeited, Weighted Average Exercise Price	43.61
Ending balance, Outstanding Weighted Average Exercise Price	43.58	$ 29.99
Vested and expected to vest, Weighted Average Exercise Price	42.22
Exercisable, Weighted Average Exercise Price	$ 27.95
Outstanding, Weighted Average Remaining Life	8 years 1 month 2 days	8 years 2 months 26 days
Vested and expected to vest, Weighted Average Remaining Life	7 years 11 months 23 days
Exercisable, Weighted Average Remaining Life	6 years 11 months 26 days
Outstanding, Aggregate Intrinsic Value	$ 676,717	$ 92,843
Vested and expected to vest, Aggregate Intrinsic Value	551,729
Exercisable, Aggregate Intrinsic Value	$ 279,699

Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (69,421)	$ (73,719)	$ (70,202)	$ (56,778)	$ (55,901)	$ (37,796)	$ (34,747)	$ (30,543)	$ (270,120)	$ (158,987)	$ (94,495)
Denominator:
Weighted average common stock outstanding—basic									38,113,678	33,492,795	27,778,288
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units									0	0	0
Weighted average common stock outstanding—diluted									38,113,678	33,492,795	27,778,288
Net loss per share—basic and diluted	$ (1.75)	$ (1.97)	$ (1.88)	$ (1.52)	$ (1.50)	$ (1.15)	$ (1.08)	$ (0.97)	$ (7.09)	$ (4.75)	$ (3.40)

Net Loss Per Share - Summary of Anti-Dilutive Common Stock Equivalents Outstanding (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	4,956,739	3,992,719
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	4,915,956	3,985,935
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	29,100
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	11,683	6,784

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Provision for income taxes		$ 0
Federal operating loss carryforwards		482,900,000
State operating loss carryforwards		$ 479,900,000
Federal and state net operating loss carryforwards expiration year		2031
Federal orphan drug tax credit carry forwards		$ 40,000,000
Tax credit carry forwards expiration period		2034
U.S. corporate tax rate		34.00%	34.00%	34.00%
Reduction to deferred tax asset		$ 66,400,000
Change in amount of deferred tax assets		70,400,000
Valuation allowance		205,441,000	$ 135,072,000
Accrued interest or penalties related to uncertain tax positions		0	0
Uncertain tax positions		0
ASU 2016-09 [Member]
Income Tax Disclosure [Line Items]
Increase in deferred tax assets associated with net operating losses			8,000,000
Increase in deferred tax assets associated with federal and state research credits			$ 1,400,000
ASU 2015-17 [Member] \| ASU Early Adoption [Member]
Income Tax Disclosure [Line Items]
Decrease in deferred tax liability current				$ (600,000)
Increase in deferred tax liability non-current				$ 600,000
Scenario Forecast [Member]
Income Tax Disclosure [Line Items]
U.S. corporate tax rate	21.00%
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 12,700,000
Research and development tax credits carryforwards expiration year		2031
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 2,300,000
Research and development tax credits carryforwards expiration year		2027

Income Taxes - Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax due at statutory rate	34.00%	34.00%	34.00%
State taxes, net of federal	5.10%	4.20%	4.00%
Stock-based compensation	6.40%	(1.00%)	(0.90%)
Foreign rate differential	(3.20%)	(3.10%)	(3.10%)
Federal and state credits	7.20%	10.30%	13.20%
Change in valuation allowance	(22.50%)	(41.50%)	(43.10%)
Other			0.10%
Federal and state rate change	(24.60%)
Research and orphan drug credit addback	(2.40%)	(2.90%)	(4.20%)
Total effective tax rate	0.00%	0.00%	0.00%

Income Taxes - Summary of Significant Components of Company's Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating losses	$ 131,151	$ 84,374
Capitalized start-up costs	1,362	2,155
Accounting method change		(673)
Tax credit carryforwards	54,523	33,477
Accrued expenses	3,057	2,287
Depreciation and amortization	685	999
Stock options	13,573	12,103
Others	1,090	350
Total net deferred tax asset before valuation allowance	205,441	135,072
Valuation allowance	$ (205,441)	$ (135,072)

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Employer matching contribution, percent of match	50.00%	50.00%	50.00%
Maximum annual contributions per employee, percent	6.00%	6.00%	6.00%
Employer contribution	$ 0.9	$ 0.4	$ 0.2

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 70,448	$ 74,373	$ 70,854	$ 57,480	$ 56,379	$ 38,064	$ 35,006	$ 30,714	$ 273,155	$ 160,163	$ 94,650
Loss from operations	(70,448)	(74,373)	(70,854)	(57,480)	(56,379)	(38,064)	(35,006)	(30,714)	(273,155)	(160,163)	(94,650)
Net loss	$ (69,421)	$ (73,719)	$ (70,202)	$ (56,778)	$ (55,901)	$ (37,796)	$ (34,747)	$ (30,543)	$ (270,120)	$ (158,987)	$ (94,495)
Net loss per share—basic and diluted	$ (1.75)	$ (1.97)	$ (1.88)	$ (1.52)	$ (1.50)	$ (1.15)	$ (1.08)	$ (0.97)	$ (7.09)	$ (4.75)	$ (3.40)

Subsequent Event - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 13, 2018	Nov. 17, 2017	Sep. 14, 2016	Jan. 12, 2016	Apr. 20, 2015
Subsequent Event [Line Items]
Issuance of common stock, shares		4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share		$ 85.00	$ 39.75	$ 47.50	$ 52.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering expenses payable		$ 325.8	$ 189.2	$ 140.4	$ 129.1
Subsequent Event [Member]
Subsequent Event [Line Items]
Issuance of common stock, shares	4,032,012
Common stock price per share	$ 164.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering expenses payable	$ 631.1