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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	Apr. 26, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		46,515,858

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 557,555	$ 306,235
Marketable securities	525,958	212,613
Prepaid expenses and other current assets	11,575	6,227
Total current assets	1,095,088	525,075
Property and equipment, net	4,295	4,013
Restricted cash	1,269	849
Total assets	1,100,652	529,937
Current liabilities:
Accounts payable	8,350	9,350
Accrued expenses	29,522	42,601
Total current liabilities	37,872	51,951
Other liabilities	3,504	2,511
Total liabilities	41,376	54,462
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at March 31, 2018 and December 31, 2017; no shares issued or outstanding at March 31, 2018 and December 31, 2017
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at March 31, 2018 and December 31, 2017; 46,459,812 and 42,003,894 shares issued at March 31, 2018 and December 31, 2017, respectively; 46,458,298 and 42,002,934 shares outstanding at March 31, 2018 and December 31, 2017, respectively	5	5
Treasury stock, at cost, 1,514 and 960 shares at March 31, 2018 and December 31, 2017, respectively	(211)	(113)
Additional paid-in capital	1,724,712	1,066,059
Accumulated deficit	(665,045)	(590,447)
Accumulated other comprehensive loss	(185)	(29)
Total stockholders’ equity	1,059,276	475,475
Total liabilities and stockholders’ equity	$ 1,100,652	$ 529,937

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
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	46,459,812	42,003,894
Common stock, shares outstanding	46,458,298	42,002,934
Treasury stock, shares	1,514	960

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
Research and development	$ 49,270	$ 45,200
General and administrative	28,849	12,280
Total operating expenses	78,119	57,480
Loss from operations	(78,119)	(57,480)
Interest income, net	3,529	707
Other expense, net	(8)	(5)
Net loss	$ (74,598)	$ (56,778)
Net loss per share—basic and diluted	$ (1.68)	$ (1.52)
Weighted average number of common shares outstanding—basic and diluted	44,325,371	37,269,148
Comprehensive loss:
Net loss	$ (74,598)	$ (56,778)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	(156)	21
Total other comprehensive gain (loss)	(156)	21
Total comprehensive loss	$ (74,754)	$ (56,757)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (74,598)	$ (56,778)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,817	6,208
Premium on marketable securities	(4)
Amortization of premium (discount) on marketable securities	(1,348)	33
Depreciation	247	130
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(5,348)	(1,380)
Accounts payable	(877)	(4,643)
Accrued expenses and other liabilities	(12,460)	1,220
Net cash used in operating activities	(78,571)	(55,210)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	81,960	50,753
Purchases of marketable securities	(394,109)	(18,908)
Purchases of property and equipment	(652)	(245)
Net cash provided by (used in) investing activities	(312,801)	31,600
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	12,757	553
Payment of employee tax obligations related to vesting of restricted stock units	(904)
Payments of offering costs	(235)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	631,494
Net cash provided by financing activities	643,112	553
Net increase (decrease) in cash, cash equivalents and restricted cash	251,740	(23,057)
Cash, cash equivalents and restricted cash at beginning of period	307,084	169,081	$ 169,081
Cash, cash equivalents and restricted cash at end of period	558,824	146,024	$ 307,084
Supplemental disclosure of non-cash investing and financing activities
Purchases of property and equipment included in accounts payable	15	$ 64
Public offering costs included in accrued expenses	$ 105

Nature of the Business

3 Months Ended

Mar. 31, 2018

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 (“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 March 31, 2018, the Company had an accumulated deficit of $665.0 million. From its inception through March 31, 2018, the Company received net proceeds of $1.6 billion 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, November 2017 and February 2018. 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 unaudited interim condensed consolidated financial statements.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

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 unaudited condensed consolidated financial statements.

Basis of Presentation

The unaudited interim condensed consolidated financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017.

The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of the Company’s management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, consisting of only normal recurring adjustments, necessary for a fair statement of its financial position as of March 31, 2018, its results of operations and comprehensive loss for the three months ended March 31, 2018 and 2017, and its cash flows for the three months ended March 31, 2018 and 2017. The consolidated balance sheet at December 31, 2017 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three months ended March 31, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, or for any future period.

Principles of Consolidation

The unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K for the fiscal year ended December 31, 2017. Intercompany accounts and transactions have been eliminated.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the 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.

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.

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.

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 March 31, 2018 and December 31, 2017, cash equivalents were comprised of money market funds and overnight reverse repurchase agreements.

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. Marketable securities that have remaining contractual maturities of one year or less are classified as short term. No declines in value were deemed to be other than temporary during the three months ended March 31, 2018 and 2017.

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 March 31, 2018 and December 31, 2017 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements.

The carrying amounts reflected in the unaudited condensed consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at March 31, 2018 and December 31, 2017, respectively.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“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 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 statements of cash flows. The Company adopted the standard on the required effective date of January 1, 2018. This guidance did not have a significant impact on the Company’s 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 statements 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 statements of cash flows. The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s unaudited condensed consolidated statements of cash flows. Restricted cash balances are classified as non-current unless, under the terms of the applicable agreements, the funds will be released from restrictions within one year from the balance sheet date. The inclusion of restricted cash increased the beginning balances of the unaudited condensed consolidated statements of cash flows by $0.8 million and $0.6 million, respectively, and the ending balances by $1.3 million and $0.6 million, respectively, for the three months ended March 31, 2018 and 2017.

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 Company adopted the standard on the required effective date of January 1, 2018. This guidance did not have a significant impact on the Company’s 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

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 and December 31, 2017.

	March 31, 2018
	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	$	557,555	$	557,555	$	—	$	—
Total cash equivalents		557,555		557,555		—		—
Marketable securities:
U.S. government securities		89,531		—		89,531		—
U.S. corporate bonds		60,662		—		60,662		—
U.S. commercial paper		151,209		—		151,209		—
International commercial paper		224,556		—		224,556		—
Total marketable securities		525,958		—		525,958		—
Total cash equivalents and marketable securities	$	1,083,513	$	557,555	$	525,958	$	—

	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	$	—

During the three months ended March 31, 2018 and 2017, 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 March 31, 2018 and December 31, 2017:

	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	89,592	$	—	$	(61	)	$	89,531
U.S. corporate bonds		60,761		1		(100	)		60,662
U.S. commercial paper		151,223		—		(14	)		151,209
International commercial paper		224,567		—		(11	)		224,556
	$	526,143	$	1	$	(186	)	$	525,958

	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

As of March 31, 2018, all marketable securities held by the Company had remaining contractual maturities of one year or less.

There have been no impairments of the Company’s assets measured and carried at fair value during the three months ended March 31, 2018 and the year ended December 31, 2017.

Balance Sheet Components

3 Months Ended

Mar. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components

4.	Balance Sheet Components

Property and Equipment, net

Property and equipment, net consists of the following:

	March 31, 2018			December 31, 2017
	(in thousands)
Computer hardware and software	$	1,131		$	1,090
Furniture and equipment		890			1,029
Leasehold improvements		3,594			2,967
		5,615			5,086
Less: Accumulated depreciation		(1,320	)		(1,073	)
	$	4,295		$	4,013

Depreciation expense for the three months ended March 31, 2018 and 2017 was $0.2 million and $0.3 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:

	March 31, 2018		December 31, 2017
	(in thousands)
Development costs	$	19,142	$	23,473
Employee-related expenses		4,462		15,838
Professional services		5,781		3,166
Other accrued expenses		137		124
	$	29,522	$	42,601

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Operating Leases

The Company leases office space in two multi-tenant buildings in Cambridge, Massachusetts, consisting, as of March 31, 2018, of 54,943 square feet in one building under an operating lease that expires on August 15, 2024 and 19,805 square feet in the second building under an operating lease that will expire on February 28, 2022. Also see Note 9, “Subsequent Event”.

Future minimum lease payments under non-cancelable operating leases are as follows at March 31, 2018:

Years Ending December 31,	(in thousands)
2018	$	3,338
2019		4,537
2020		4,632
2021		4,728
2022		3,485
Thereafter		5,450
	$	26,170

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 brexanolone and the Company’s compound known as 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 of March 31, 2018, the Company paid to CyDex $1.0 million for licensing fees, which was recorded as research and development expense.

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. As of March 31, 2018, the Company recorded research and development expense and made cash payments of $1.5 million related to these clinical development and regulatory milestones.

For the three months ended March 31, 2018 and 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.

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. As of March 31, 2018, the Company recorded research and development expense and made cash payments of $0.1 million related to these regulatory and sales milestones.

For the three months ended March 31, 2018 and 2017, 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 As of March 31, 2018, the Company recorded research and development expense and made a cash payment of $50,000 related to these clinical and development milestones.

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 three months ended March 31, 2018 and 2017, 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. As of March 31, 2018, the Company recorded research and development expense of $1.8 million, comprised of $0.5 million in cash and $1.3 million related to the issuance of 39,681 shares of the Company’s common stock, related to the achievement of these milestones.

For the three months ended March 31, 2018 and 2017, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Sale of Equity Securities

3 Months Ended

Mar. 31, 2018

Other Liabilities Disclosure [Abstract]

Sale of Equity Securities

6.	Sale of Equity Securities

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

On February 13, 2018, the Company completed the sale of 4,032,012 shares of its common stock in an underwritten public offering at a price to the public of $164.00 per share, resulting in net proceeds of $631.2 million after deducting commissions and underwriting discounts and offering costs both paid and payable by the Company.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

7.	Stock-Based Compensation

Restricted Stock Units

During the three months ended March 31, 2017, the Company granted 32,500 restricted stock units to certain employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. 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 was in February 2018 and will be in February 2019, respectively.

During the three months ended March 31, 2018, the Company granted 33,600 performance restricted stock units to certain employees of the Company. These performance restricted stock units will vest upon the achievement of a certain regulatory milestone, in some cases upon meeting the milestone and, in other cases, on the first anniversary of meeting the milestone.

The fair value of restricted stock units that vested during the three months ended March 31, 2018 was $2.6 million. No restricted stock units vested during the three months ended March 31, 2017.

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2017		29,100
Granted		33,600
Vested		(14,550	)
Forfeited		(400	)
Outstanding as of March 31, 2018		47,750

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 no longer grants 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.

On December 15, 2016, the Board of Directors of the Company approved the 2016 Inducement Equity Plan (the “2016 Stock Option Plan”). 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.

As of March 31, 2018, the total number of shares reserved under all equity plans is 9,172,948, and 2,543,909 shares were available for future issuance under such plans.

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.

During the three months ended March 31, 2018 and 2017, the Company granted 323,753 and 449,208 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 three months ended March 31, 2018 and 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 three months ended March 31, 2018 and 2017.

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three months ended March 31, 2018 and 2017 was as follows:

	Three months ended March 31,
	2018		2017
	(in thousands)
Research and development	$	8,899	$	3,596
General and administrative		6,918		2,612
	$	15,817	$	6,208

Stock-based compensation expense by award type recognized during the three months ended March 31, 2018 and 2017 was as follows:

	Three months ended March 31,
	2018		2017
	(in thousands)
Stock options	$	15,381	$	6,033
Restricted stock units		168		100
Employee stock purchase plan		268		75
	$	15,817	$	6,208

The weighted average grant date fair value per share relating to outstanding stock options granted under the Company’s stock option plans during the three months ended March 31, 2018 and 2017 was $125.80 and $34.61, respectively.

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, 2017		5,586,593		$	43.58		8.09	$	676,717
Granted		1,376,044			187.51
Exercised		(347,725	)		32.65
Forfeited		(33,623	)		62.08
Outstanding as of March 31, 2018		6,581,289		$	74.16		8.29	$	608,335
Exercisable as of March 31, 2018		2,163,375		$	31.70		7.00	$	279,868

At March 31, 2018, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $172.0 million, which is expected to be recognized over the remaining weighted average vesting period of 3.23 years. The intrinsic value of stock options exercised during the three months ended March 31, 2018 and 2017 was $47.7 million and $4.8 million, respectively.

At March 31, 2018, 991,114 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $46.8 million.

Net Loss Per Share

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

8.	Net Loss Per Share

Basic and diluted net loss per share was calculated as follows for the three months ended March 31, 2018 and 2017:

	Three months ended March 31,
	2018			2017
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(74,598	)	$	(56,778	)
Denominator:
Weighted average common stock outstanding—basic		44,325,371			37,269,148
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—
Weighted average common stock outstanding—diluted		44,325,371			37,269,148
Net loss per share—basic and diluted	$	(1.68	)	$	(1.52	)

The following common stock equivalents outstanding as of March 31, 2018 and 2017 were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive:

	Three months ended March 31,
	2018		2017
Stock options		5,590,175		4,801,491
Restricted stock units		14,150		31,600
Employee stock purchase plan		3,918		4,676
		5,608,243		4,837,767

Stock options and restricted stock units 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.

Subsequent Event

3 Months Ended

Mar. 31, 2018

Subsequent Events [Abstract]

Subsequent Event

9.	Subsequent Event

In April 2018, the Company entered into the First Amendment to the lease for office space in a multi-tenant building. The Company increased the amount of square feet of office space from 19,805 square feet to 40,419 square feet, an increase of 20,614 square feet, consisting of (i) 13,481 rentable square feet beginning on August 1, 2018, and (ii) 7,133 rentable square feet beginning on October 1, 2018. The term for this additional space will expire on August 31, 2024. Additionally, the term of the existing lease will be extended from February 28, 2022 until August 31, 2024. The increase in future expected payments under the First Amendment will be approximately $14.7 million.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Principles of Consolidation

Use of Estimates

Research and Development

Stock-Based Compensation

Cash and Cash Equivalents

Marketable Securities

Marketable securities

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.

Recently Issued Accounting Pronouncements

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 and December 31, 2017.

	March 31, 2018
	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	$	557,555	$	557,555	$	—	$	—
Total cash equivalents		557,555		557,555		—		—
Marketable securities:
U.S. government securities		89,531		—		89,531		—
U.S. corporate bonds		60,662		—		60,662		—
U.S. commercial paper		151,209		—		151,209		—
International commercial paper		224,556		—		224,556		—
Total marketable securities		525,958		—		525,958		—
Total cash equivalents and marketable securities	$	1,083,513	$	557,555	$	525,958	$	—

	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	$	—

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 March 31, 2018 and December 31, 2017:

	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	89,592	$	—	$	(61	)	$	89,531
U.S. corporate bonds		60,761		1		(100	)		60,662
U.S. commercial paper		151,223		—		(14	)		151,209
International commercial paper		224,567		—		(11	)		224,556
	$	526,143	$	1	$	(186	)	$	525,958

	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

- 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)

3 Months Ended

Mar. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	March 31, 2018			December 31, 2017
	(in thousands)
Computer hardware and software	$	1,131		$	1,090
Furniture and equipment		890			1,029
Leasehold improvements		3,594			2,967
		5,615			5,086
Less: Accumulated depreciation		(1,320	)		(1,073	)
	$	4,295		$	4,013

Summary of Accrued Expenses

Accrued expenses consist of the following:

	March 31, 2018		December 31, 2017
	(in thousands)
Development costs	$	19,142	$	23,473
Employee-related expenses		4,462		15,838
Professional services		5,781		3,166
Other accrued expenses		137		124
	$	29,522	$	42,601

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018:

Years Ending December 31,	(in thousands)
2018	$	3,338
2019		4,537
2020		4,632
2021		4,728
2022		3,485
Thereafter		5,450
	$	26,170

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

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, 2017		29,100
Granted		33,600
Vested		(14,550	)
Forfeited		(400	)
Outstanding as of March 31, 2018		47,750

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 three months ended March 31, 2018 and 2017 was as follows:

	Three months ended March 31,
	2018		2017
	(in thousands)
Research and development	$	8,899	$	3,596
General and administrative		6,918		2,612
	$	15,817	$	6,208

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the three months ended March 31, 2018 and 2017 was as follows:

	Three months ended March 31,
	2018		2017
	(in thousands)
Stock options	$	15,381	$	6,033
Restricted stock units		168		100
Employee stock purchase plan		268		75
	$	15,817	$	6,208

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, 2017		5,586,593		$	43.58		8.09	$	676,717
Granted		1,376,044			187.51
Exercised		(347,725	)		32.65
Forfeited		(33,623	)		62.08
Outstanding as of March 31, 2018		6,581,289		$	74.16		8.29	$	608,335
Exercisable as of March 31, 2018		2,163,375		$	31.70		7.00	$	279,868

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2018

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 three months ended March 31, 2018 and 2017:

	Three months ended March 31,
	2018			2017
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(74,598	)	$	(56,778	)
Denominator:
Weighted average common stock outstanding—basic		44,325,371			37,269,148
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—
Weighted average common stock outstanding—diluted		44,325,371			37,269,148
Net loss per share—basic and diluted	$	(1.68	)	$	(1.52	)

Summary of Anti-Dilutive Common Stock Equivalents Outstanding

	Three months ended March 31,
	2018		2017
Stock options		5,590,175		4,801,491
Restricted stock units		14,150		31,600
Employee stock purchase plan		3,918		4,676
		5,608,243		4,837,767

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	96 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Nature Of Business [Line Items]
Accumulated deficit	$ (665,045)	$ (590,447)
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	$ 1,600,000

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents increased by amount due to inclusion of restricted cash	$ 251,740	$ (23,057)
Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents increased by amount due to inclusion of restricted cash	$ 1,300	$ 600	$ 800	$ 600

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 557,555	$ 306,235
Total marketable securities	525,958	212,613
Total cash equivalents and marketable securities	1,083,513	518,848
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	557,555	306,235
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	557,555	306,235
Total cash equivalents and marketable securities	557,555	306,235
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	557,555	306,235
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	525,958	212,613
Total cash equivalents and marketable securities	525,958	212,613
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	89,531	49,606
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	89,531	49,606
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	60,662	48,959
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	60,662	48,959
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	151,209	65,583
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	151,209	65,583
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	224,556	48,465
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	$ 224,556	$ 48,465

Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories	$ 0		$ 0
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	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 526,143	$ 212,642
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(186)	(31)
Fair Value	525,958	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,592	49,612
Gross Unrealized Losses	(61)	(6)
Fair Value	89,531	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,761	48,982
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(100)	(25)
Fair Value	60,662	48,959
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	151,223	65,583
Gross Unrealized Losses	(14)
Fair Value	151,209	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	224,567	48,465
Gross Unrealized Losses	(11)
Fair Value	$ 224,556	$ 48,465

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,615	$ 5,086
Less: Accumulated depreciation	(1,320)	(1,073)
Property and equipment, net	4,295	4,013
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,131	1,090
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	890	1,029
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,594	$ 2,967

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 0.2	$ 0.3
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	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Development costs	$ 19,142	$ 23,473
Employee-related expenses	4,462	15,838
Professional services	5,781	3,166
Other accrued expenses	137	124
Total accrued expenses	$ 29,522	$ 42,601

Commitments and Contingencies - Operating Leases - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 ft²
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease	54,943
Lease expire date	Aug. 15, 2024
Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease	19,805
Lease expire date	Feb. 28, 2022

Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 3,338
2019	4,537
2020	4,632
2021	4,728
2022	3,485
Thereafter	5,450
Total Operating leases, future minimum payments	$ 26,170

Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Commitments And Contingencies [Line Items]
Research and development expense	$ 49,270,000	$ 45,200,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense	1,000,000
Research and development expense related to milestone expense	0	0
Milestone payments	0	$ 0
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] \| 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
CyDex License Agreement [Member] \| Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense	1,500,000
Milestone payments	$ 1,500,000

Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended		12 Months Ended	31 Months Ended
	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2017
Regulatory and Sales Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 100,000
Milestone payments			100,000
University of California License Agreements [Member]
Commitments And Contingencies [Line Items]
Upfront payment		$ 50,000
Annual license maintenance fee					$ 15,000
Research and development expense related to milestone expense			0	$ 0
Milestone payments			$ 0	$ 0
University of California License Agreements [Member] \| After The Effective Date [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	27 years
University of California License Agreements [Member] \| After The First Sale [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	15 years
University of California License Agreements [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Milestone payments	$ 100,000
University of California License Agreements [Member] \| Clinical Development [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Percentage of net sales paid as royalties	1.00%
University of California License Agreements [Member] \| Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		700,000
Additional milestone payments						$ 0
University of California License Agreements [Member] \| 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	3 Months Ended		12 Months Ended
	Nov. 30, 2013	Mar. 31, 2018	Mar. 31, 2017	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
Clinical Development [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] - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Mar. 31, 2018	Mar. 31, 2017
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	126,984
Research and development expense related to milestone expense		1,800,000
Milestone payments		500,000
Payments for stock issued upon reaching a milestone		$ 1,300,000
Milestone based share compensation, shares issued		39,681
Clinical Development and Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 2,000,000

Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 13, 2018	Nov. 17, 2017
Equity [Abstract]
Issuance of common stock, shares	4,032,012	4,058,822
Common stock price per share	$ 164.00	$ 85.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and offering costs	$ 631.2	$ 325.8

Stock-Based Compensation - Additional Information (Detail) - USD ($)		3 Months Ended		12 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of options outstanding		9,172,948
Common stock available for issuance under stock option plan		2,543,909
Stock-based compensation expense		$ 15,817,000	$ 6,208,000
Weighted average grant date fair value per share		$ 125.80	$ 34.61
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 shares annual increase added to plan	1,680,117
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%
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		33,600	32,500
Share based compensation granted under plan vest period		2 years
Fair value of restricted stock units vested		$ 2,600,000
Restricted stock units vested		14,550	0
Stock-based compensation expense		$ 168,000	$ 100,000
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 Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		33,600
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		323,753	449,208
Total unrecognized stock-based compensation expense		$ 46,800,000
Number of shares outstanding and unvested stock options		991,114
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 0	$ 0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		1,376,044
Total number of options outstanding		6,581,289		5,586,593
Stock-based compensation expense		$ 15,381,000	6,033,000
Total unrecognized stock-based compensation expense		$ 172,000,000
Weighted average period of unrecognized compensation costs		3 years 2 months 23 days
Intrinsic value of options exercised		$ 47,700,000	$ 4,800,000

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (Detail) - Restricted Stock Units [Member] - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Shares at beginning balance-Restricted stock	29,100
Granted, Shares- Restricted stock	33,600	32,500
Vested, Shares-Restricted stock	(14,550)	0
Forfeited, Shares-Restricted stock	(400)
Outstanding, Shares at ending balance-Restricted stock	47,750

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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 15,817	$ 6,208
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	8,899	3,596
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 6,918	$ 2,612

Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 15,817	$ 6,208
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	15,381	6,033
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	168	100
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 268	$ 75

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares	9,172,948
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	5,586,593
Granted, Shares	1,376,044
Exercised, Shares	(347,725)
Forfeited, Shares	(33,623)
Ending balance, Outstanding Shares	6,581,289	5,586,593
Exercisable, Shares	2,163,375
Beginning balance, Outstanding Weighted Average Exercise Price	$ 43.58
Granted, Weighted Average Exercise Price	187.51
Exercised, Weighted Average Exercise Price	32.65
Forfeited, Weighted Average Exercise Price	62.08
Ending balance, Outstanding Weighted Average Exercise Price	74.16	$ 43.58
Exercisable, Weighted Average Exercise Price	$ 31.70
Outstanding, Weighted Average Remaining Life	8 years 3 months 14 days	8 years 1 month 2 days
Exercisable, Weighted Average Remaining Life	7 years
Outstanding, Aggregate Intrinsic Value	$ 608,335	$ 676,717
Exercisable, Aggregate Intrinsic Value	$ 279,868

Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (74,598)	$ (56,778)
Denominator:
Weighted average common stock outstanding—basic	44,325,371	37,269,148
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units	0	0
Weighted average common stock outstanding—diluted	44,325,371	37,269,148
Net loss per share—basic and diluted	$ (1.68)	$ (1.52)

Net Loss Per Share - Summary of Anti-Dilutive Common Stock Equivalents Outstanding (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	5,608,243	4,837,767
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	5,590,175	4,801,491
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	14,150	31,600
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	3,918	4,676

Subsequent Event - Additional Information (Detail) - Operating Lease Two [Member] $ in Millions			1 Months Ended	3 Months Ended
	Oct. 01, 2018 ft²	Aug. 01, 2018 ft²	Apr. 30, 2018 USD ($) ft²	Mar. 31, 2018 ft²
Subsequent Event [Line Items]
Office space rent under operating lease				19,805
Lease expire date				Feb. 28, 2022
Scenario Forecast [Member]
Subsequent Event [Line Items]
Increased office space rent	7,133	13,481
Subsequent Event [Member]
Subsequent Event [Line Items]
Office space rent under operating lease			40,419
Lease expire date			Aug. 31, 2024
Increased office space rent			20,614
Increase in future expected payments \| $			$ 14.7

- Definition

The increases or decreases in lease payments that result from changes occurring after the inception of the lease in the factors (other than the passage of time) on which lease payments are based, except that any escalation of minimum lease payments relating to increases in construction or acquisition cost of the leased property or for increases in some measure of cost or value during the construction or preconstruction period, are excluded from contingent rentals. Contingent rentals also may include amounts for which the triggering events have not yet occurred or the specified targets for which have not yet been achieved (such as sales based percentage rent), but which events are considered probable of occurring or which specified targets are considered probable of being achieved.

+ References + Details