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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	Apr. 25, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
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 Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		51,147,698

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 342,295	$ 190,943
Marketable securities	1,008,956	731,833
Prepaid expenses and other current assets	20,281	21,919
Total current assets	1,371,532	944,695
Property and equipment, net	6,513	5,643
Restricted cash	2,367	2,367
Right of use operating asset	38,124
Other long-term assets	4,378
Total assets	1,422,914	952,705
Current liabilities:
Accounts payable	13,556	34,036
Accrued expenses	47,651	51,994
Other current liabilities	7,169
Total current liabilities	68,376	86,030
Long-term lease operating liability, net of current portion	34,528
Other long-term liabilities	476	3,704
Total liabilities	103,380	89,734
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at March 31, 2019 and December 31, 2018; no shares issued or outstanding at March 31, 2019 and December 31, 2018
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at March 31, 2019 and December 31, 2018; 51,037,205 and 46,891,296 shares issued at March 31, 2019 and December 31, 2018; 51,034,172 and 46,888,263 shares outstanding at March 31, 2019 and December 31, 2018	5	5
Treasury stock, at cost, 3,033 shares at March 31, 2019 and December 31, 2018	(400)	(211)
Additional paid-in capital	2,446,770	1,827,021
Accumulated deficit	(1,126,735)	(963,329)
Accumulated other comprehensive loss	(106)	(515)
Total stockholders’ equity	1,319,534	862,971
Total liabilities and stockholders’ equity	$ 1,422,914	$ 952,705

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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	51,037,205	46,891,296
Common stock, shares outstanding	51,034,172	46,888,263
Treasury stock, shares	3,033	3,033

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 465
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 86,398	$ 49,270
General and administrative	83,919	28,849
Total operating expenses	170,317	78,119
Loss from operations	(169,852)	(78,119)
Interest income, net	6,442	3,529
Other income (expense), net	4	(8)
Net loss	$ (163,406)	$ (74,598)
Net loss per share—basic and diluted	$ (3.37)	$ (1.68)
Weighted average number of common shares outstanding—basic and diluted	48,491,834	44,325,371
Comprehensive loss:
Net loss	$ (163,406)	$ (74,598)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	409	(156)
Total other comprehensive gain (loss)	409	(156)
Total comprehensive loss	$ (162,997)	$ (74,754)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net loss	$ (163,406)	$ (74,598)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	44,116	15,817
Premium on marketable securities	(739)	(4)
Amortization of discount on marketable securities	(2,322)	(1,348)
Depreciation	415	247
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,637	(5,348)
Other long-term assets	(1,378)
Right of use operating asset	2,942
Operating lease liabilities	(2,500)
Accounts payable	(20,507)	(877)
Accrued expenses and other liabilities	(8,192)	(12,460)
Net cash used in operating activities	(149,934)	(78,571)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	307,263	81,960
Purchases of marketable securities	(580,916)	(394,109)
Purchases of property and equipment	(1,257)	(652)
Net cash used in investing activities	(274,910)	(312,801)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	15,681	12,757
Payment of employee tax obligations related to vesting of restricted stock units	(692)	(904)
Payments of offering costs	(70)	(235)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	561,277	631,494
Net cash provided by financing activities	576,196	643,112
Net increase in cash, cash equivalents and restricted cash	151,352	251,740
Cash, cash equivalents and restricted cash at beginning of period	193,310	307,084
Cash, cash equivalents and restricted cash at end of period	344,662	558,824
Supplemental disclosure of non-cash investing and financing activities
Purchases of property and equipment included in accounts payable	78	15
Public offering costs included in accrued expenses	$ 259	$ 105

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, 2017	$ 475,475	$ 5	$ (113)	$ 1,066,059	$ (29)	$ (590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960
Issuance of common stock from exercise of stock options, Amount	11,728			11,728
Issuance of common stock from exercise of stock options, Shares		402,227
Issuance of common stock under employee stock purchase plan	1,127			1,127
Issuance of common stock under employee stock purchase plan, Shares		11,683
Purchase of treasury stock, Amount	(98)		$ (98)
Purchase of treasury stock, Shares			554
Stock-based compensation expense	15,549			15,549
Public offering of common stock, net of offering costs, Amount	631,153			631,153
Public offering of common stock, net of offering costs, Shares		4,032,012
Unrealized loss on available-for-sale securities	(156)				(156)
Vesting of restricted stock units, net of employee tax obligations	(904)			(904)
Vesting of restricted stock units, net of employee tax obligations, Shares		9,442
Net loss	(74,598)					(74,598)
Balances at Mar. 31, 2018	1,059,276	$ 5	$ (211)	1,724,712	(185)	(665,045)
Balances, Shares at Mar. 31, 2018		46,458,298	1,514
Balances at Dec. 31, 2018	862,971	$ 5	$ (211)	1,827,021	(515)	(963,329)
Balances, Shares at Dec. 31, 2018		46,888,263	3,033
Issuance of common stock from exercise of stock options, Amount	14,072			14,072
Issuance of common stock from exercise of stock options, Shares		287,659
Issuance of common stock under employee stock purchase plan	1,799			1,799
Issuance of common stock under employee stock purchase plan, Shares		16,398
Purchase of treasury stock, Amount	(189)		$ (189)
Stock-based compensation expense	43,622			43,622
Public offering of common stock, net of offering costs, Amount	560,948			560,948
Public offering of common stock, net of offering costs, Shares		3,833,334
Unrealized loss on available-for-sale securities	409				409
Vesting of restricted stock units, net of employee tax obligations	(692)			(692)
Vesting of restricted stock units, net of employee tax obligations, Shares		8,518
Net loss	(163,406)					(163,406)
Balances at Mar. 31, 2019	$ 1,319,534	$ 5	$ (400)	$ 2,446,770	$ (106)	$ (1,126,735)
Balances, Shares at Mar. 31, 2019		51,034,172	3,033

Nature of the Business

3 Months Ended

Mar. 31, 2019

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 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’s lead product, ZULRESSO™ (brexanolone) injection, was approved by the U.S. Food and Drug Administration (“FDA”) in March 2019, for the treatment of postpartum depression (“PPD”) in adults. The Company expects to make ZULRESSO commercially available in the U.S. in late June 2019, after completion of controlled substance scheduling of brexanolone by the U.S. Drug Enforcement Administration (“DEA”) and incorporation of the scheduling into the Company’s FDA-approved label and other product information. The Company has a portfolio of other 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 and pharmaceutical industries, 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 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 (“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, 2019, the Company had an accumulated deficit of $1.1 billion. From its inception through March 31, 2019, the Company received net proceeds of $2.2 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, February 2018, and February 2019. 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. At some point after that time, the Company will require additional financing to fund its future operations.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

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, 2018, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018.

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, 2019, its results of operations and comprehensive loss for the three months ended March 31, 2019 and 2018, and its cash flows for the three months ended March 31, 2019 and 2018. The consolidated balance sheet at December 31, 2018 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three months ended March 31, 2019 are not necessarily indicative of the results for the year ending December 31, 2019, 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, 2018. Intercompany accounts and transactions have been eliminated.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the 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 U.S. 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 non-employee directors based on the estimated fair value on the date of grant, over the requisite service period.

Effective January 1, 2019, the Company recognizes compensation expense for stock-based awards, including grants of stock options and restricted stock, made to non-employee consultants based on the estimated fair value on the date of grant, over the requisite service period. Through December 31, 2018, the Company recognized compensation expense for stock-based awards granted to non-employee consultants based on the fair value of the awards on each date on which the awards vest. Compensation expense was recognized over the vesting period, provided that services were rendered by such non-employee consultants during that time. At the end of each financial reporting period, the fair value of unvested options was re-measured using the then-current fair value of the common stock of the Company and updated assumptions in the Black-Scholes option-pricing model.

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 options granted to employees and non-employee directors by the Company has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Through December 31, 2018, the expected term of its options granted to non-employee consultants has been determined based on the contractual term of the options, and effective January 1, 2019, the “simplified” method is used. 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 90 days or less at the date of purchase to be cash equivalents. As of March 31, 2019 and December 31, 2018, cash equivalents were comprised of cash equivalents and money market funds.

Marketable securities

Marketable securities consist of investments with original maturities greater than 90 days. The Company has classified its investments with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. 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 three months ended March 31, 2019 and 2018.

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, 2019 and December 31, 2018 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements herein.

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, 2019 and December 31, 2018, respectively.

Revenue Recognition

Effective January 1, 2017, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers (“Topic 606”). This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as collaboration arrangements and leases. Prior to the three months ended June 30, 2018, when the Company recorded its initial revenue under Topic 606, the Company did not have any revenue-generating arrangements and therefore there was no transition impact from the adoption of Topic 606.

Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer.

Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes.

The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct.

The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations.

If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment.

If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received.

In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangement in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangement. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied.

The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method.

Intangible assets

The Company had no intangible assets as of December 31, 2018. As a result of the approval by the FDA of the New Drug Application for ZULRESSO in March 2019, the Company was required to pay to CyDex Pharmaceuticals, Inc. (“CyDex”) and The Regents of the University of California milestone payments of $3.0 million and $0.5 million, respectively. At March 31, 2019, the amount of these milestones were capitalized as an intangible asset, and no amounts had been amortized.

Leases

The Company determines if an arrangement is a lease at contract inception. Operating lease assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. The Company uses the implicit interest rate when readily determinable and uses the Company’s incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments.

The lease payments used to determine the Company’s operating lease assets may include lease incentives, stated rent increases and escalation clauses linked to rates of inflation when determinable and are recognized in the Company’s operating lease assets in the Company’s condensed consolidated balance sheets. In addition, the Company’s contracts contain lease and non-lease components. The Company combines lease and non-lease components, which are accounted for together as lease components.

The Company’s operating leases are reflected in right of use operating asset, other current liabilities and long-term lease operating liability, net of current portion in the Company’s condensed consolidated balance sheets. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. Short-term leases, defined as leases that have a lease term of 12 months or less at the commencement date, are excluded from this treatment and are recognized on a straight-line basis over the term of the lease.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, Leases, which replaced the existing guidance in ASC 840, “Leases”, and in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements. The new leasing standards generally require lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. The Company adopted the standards on the required effective date of January 1, 2019 and did not restate comparative periods. Presentation of leases within the consolidated statements of operations and consolidated statements of cash flows is generally consistent with the former lease accounting guidance. The Company elected the package of practical expedients permitted under the transition guidance and as such, the adoption of this ASU did not change the classification of any of the Company’s leases. The Company elected to combine lease and non-lease components, elected not to record leases with an initial term of 12 months or less on the balance sheet and will recognize the associated lease payments in the consolidated statements of operations on a straight-line basis over the lease term. On the implementation date, $44.2 million was recognized as total lease liabilities and $41.1 million was recognized as total right-of-use assets on the Company’s consolidated balance sheet.

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 adopted the standard on the required effective date of January 1, 2019. This guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures.

In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718) – Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which aligns the accounting for share-based payment awards issued to employees and non-employees. Under the new guidance, the existing guidance regarding employees will apply to share-based transactions with non-employees, as long as the transaction is not effectively a form of financing, with the exception of specific guidance related to the attribution of compensation cost. The cost of non-employee awards will continue to be recorded as if the grantor had paid cash for the goods or services. In addition, the contractual term will be able to be used in lieu of an expected term in the option-pricing model for non-employee awards. The Company adopted the standard on the required effective date of January 1, 2019. 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, 2019

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, 2019 and December 31, 2018.

	March 31, 2019
	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	$	342,295	$	342,295	$	—	$	—
Total cash equivalents		342,295		342,295		—		—
Marketable securities:
U.S. government securities		248,155		—		248,155		—
U.S. corporate bonds		386,263		—		386,263		—
International corporate bonds		155,517		—		155,517		—
U.S. commercial paper		108,439		—		108,439		—
International commercial paper		110,582		—		110,582		—
Total marketable securities		1,008,956		—		1,008,956		—
	$	1,351,251	$	342,295	$	1,008,956	$	—

	December 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	$	190,943	$	190,943	$	—	$	—
Total cash equivalents		190,943		190,943		—		—
Marketable securities:
U.S. government securities		220,482		—		220,482		—
U.S. corporate bonds		258,566		—		258,566		—
International corporate bonds		78,468		—		78,468		—
U.S. commercial paper		77,611		—		77,611		—
International commercial paper		96,706		—		96,706		—
Total marketable securities		731,833		—		731,833		—
	$	922,776	$	190,943	$	731,833	$	—

During the three months ended March 31, 2019 and 2018, there were no transfers among the Level 1, Level 2 and Level 3 categories.

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of March 31, 2019 and December 31, 2018:

	March 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	248,082	$	86	$	(13	)	$	248,155
U.S. corporate bonds		386,333		119		(189	)		386,263
International corporate bonds		155,571		42		(96	)		155,517
U.S. commercial paper		108,487		5		(53	)		108,439
International commercial paper		110,589		—		(7	)		110,582
	$	1,009,062	$	252	$	(358	)	$	1,008,956

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	220,531	$	8	$	(57	)	$	220,482
U.S. corporate bonds		258,876		6		(316	)		258,566
International corporate bonds		78,600		—		(132	)		78,468
U.S. commercial paper		77,630		8		(27	)		77,611
International commercial paper		96,711		8		(13	)		96,706
	$	732,348	$	30	$	(545	)	$	731,833

As of March 31, 2019, all marketable securities, except for 58 bonds with a fair value of $166.2 million that have maturities of between one and two years, held by the Company had remaining contractual maturities of one year or less. As of December 31, 2018, all marketable securities, except for two U.S. corporate bonds, 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, 2019 and the year ended December 31, 2018.

Balance Sheet Components

3 Months Ended

Mar. 31, 2019

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,			December 31,
	2019			2018
	(in thousands)
Computer hardware and software	$	2,286		$	2,148
Furniture and equipment		1,125			1,002
Leasehold improvements		5,733			4,709
		9,144			7,859
Less: Accumulated depreciation		(2,631	)		(2,216	)
	$	6,513		$	5,643

Depreciation expense for the three months ended March 31, 2019 and 2018 was $0.4 million and $0.2 million, respectively.

The useful life for computer hardware and software is three years, furniture and equipment is five 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,
	March 31, 2019		2018
	(in thousands)
Development costs	$	23,932	$	21,216
Employee-related expenses		8,676		19,638
Professional services		11,746		10,903
Other accrued expenses		3,297		237
	$	47,651	$	51,994

Leases, Commitments and Contingencies

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Leases, Commitments and Contingencies

5.	Leases, Commitments and Contingencies

Operating Leases

The Company has leases for office space, vehicles, and certain equipment. All of the leases recorded on the balance sheet are operating leases. The Company’s leases have remaining lease terms ranging from less than one year to approximately six years. Some of the leases include options to extend the leases for up to five years and these options were not included for the purpose of determining the right-of-use assets and associated lease liabilities as the Company determined that the renewal of these leases is not reasonably certain so only the original lease term was taken into consideration. The leases do not include any restrictions or covenants that had to be accounted for under the lease guidance.

The Company leases office space in two multi-tenant buildings in Cambridge, Massachusetts, consisting, as of March 31, 2019, of 58,442 square feet in the first building under an operating lease that will expire on August 31, 2024 and 19,805 square feet in the second building under an operating lease that will expire on August 31, 2024.

In April 2018, the Company entered into the First Amendment to the lease for office space in the second multi-tenant building and thereby 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 square feet that began on August 1, 2018, and (ii) 7,133 square feet that began on October 1, 2018. The term for this additional space will expire on August 31, 2024. Additionally, the term of the existing lease was extended from February 28, 2022 until August 31, 2024.

In May 2018, the Company entered into a lease for office space in a multi-tenant building in Raleigh, North Carolina. The amount of square feet of office space is 15,525 square feet and the lease period began on September 1, 2018. The term for this space will expire on November 30, 2024.

In October 2018, the Company entered into the Seventh Amendment to the lease for office space in the first building and thereby increased the amount of square feet of office space from 54,943 square feet to 58,442 square feet. The increase of 3,499 square feet began on December 1, 2018. The term for this additional space will expire on August 31, 2024.

In December 2018, the Company entered into a lease in a third multi-tenant building in Cambridge, Massachusetts, for 15,975 square feet of office space which will begin on March 1, 2019. The term for this lease will expire on February 28, 2024.

The following table summarizes the presentation in the Company’s condensed consolidated balance sheets of operating leases:

(In thousands)	Balance sheet location	As of March 31, 2019
Assets
Right of use operating asset	Right of use operating asset	$	38,124

Liabilities
Current operating lease liabilities	Other current liabilities		7,149
Long-term operating lease liabilities	Long-term lease operating liability, net of current portion		34,528
Total operating lease liabilities		$	41,677

The following table summarizes the effect of lease costs in the Company’s condensed consolidated statements of operations:

	For the Three Months Ended
(In thousands)	March 31, 2019
Operating lease cost	$	2,444

The Company made an accounting policy election not to apply the recognition requirements to short-term leases. The Company recognizes the lease payments for short-term leases in profit or loss on a straight-line basis over the lease term, and variable lease payments in the period in which the obligation for those payments is incurred. For the three months ended March 31, 2019, the Company recorded $0.2 million of expense for its short-term leases.

The minimum lease payments are expected to be as follows:

Years Ending December 31,	(in thousands)
2019 (remaining nine months)	$	7,513
2020		10,101
2021		9,812
2022		8,694
2023		8,894
Thereafter		5,415
Total lease payments		50,429
Less imputed interest		(8,752	)
Present value of operating lease liabilities	$	41,677

Under the prior lease guidance, future minimum lease payments under non-cancelable operating leases were as follows at December 31, 2018:

Years Ending December 31,	(in thousands)
2019	$	7,918
2020		8,299
2021		8,463
2022		8,692
2023		8,894
Thereafter		5,415
	$	47,681

The weighted average remaining lease term and weighted average discount rate of the Company’s operating leases are as follows:

		As of March 31, 2019
Weighted average remaining lease term in years		5.30
Weighted average discount rate		7.5%

The interest rate implicit in lease contracts is typically not readily determinable and as such, the Company uses its incremental borrowing rate based on the information available at the lease commencement date, which represents an internally developed rate that would be incurred to borrow, on a collateralized basis, over a similar term, an amount equal to the lease payments in a similar economic environment.

Supplemental disclosure of cash flow information related to the Company’s operating leases included in cash flows used by operating activities in the condensed consolidated statements of cash flows is as follows:

For the Three Months Ended

(In thousands)

March 31, 2019

Cash paid for amounts included in the

measurement of lease liabilities

2,354

During the three months ended March 31, 2019, other than the initial adoption of the lease standard that required right of use assets and lease liabilities to be recorded, there were no right of use assets recorded arising from new lease liabilities.

License Agreements

CyDex License Agreement

In September 2015, the Company and 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, 2019, the Company has 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, 2019, the Company has recorded research and development expense and made cash payments of $2.3 million related to these clinical development and regulatory milestones; and has recorded an intangible asset of $3.0 million, which had not been paid as of March 31, 2019.

For the three months ended March 31, 2019, the Company recorded an intangible asset of $3.0 million related to a regulatory milestone for the brexanolone program under the license agreement with CyDex; this amount had not been paid as of March 31, 2019. For the three months ended March 31, 2018, the Company did not record any expense or intangibles, or make any milestone payments related to clinical development or regulatory 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 related to brexanolone 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. 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, 2019, the Company has recorded research and development expense and made cash payments of $0.3 million related to these regulatory and sales milestones; and has recorded an intangible asset and made a cash payment of $0.5 million related to these regulatory and sales milestones.

For the three months ended March 31, 2019, the Company recorded an intangible asset and made a cash payment of $0.5 million related to a regulatory milestone under the license agreements with The Regents of the University of California. For the three months ended March 31, 2018, 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, 2019, the Company has 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, 2019 and 2018, 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 then 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, 2019, 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, 2019 and 2018, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Collaborative Agreement

3 Months Ended

Mar. 31, 2019

Collaboration Agreement [Abstract]

Collaborative Agreement

6.	Collaboration Agreement

Effective June 12, 2018, the Company entered into a strategic collaboration with Shionogi & Co., Ltd., (“Shionogi”) for the clinical development and commercialization of SAGE-217 for the treatment of major depressive disorder (“MDD”) and other potential indications in Japan, Taiwan and South Korea.

Under the terms of the agreement, Shionogi will be responsible for all clinical development, regulatory filings and commercialization of SAGE-217 for MDD, and potentially other indications, in Japan, Taiwan and South Korea. Shionogi was required to make an upfront payment to the Company of $90.0 million, and the Company will be eligible to receive additional payments of up to $485.0 million if certain regulatory and commercial milestones are achieved by Shionogi. The potential future milestone payments include up to $70.0 million for the achievement of specified regulatory milestones, up to $30.0 million for the achievement of specified commercialization milestones, and up to $385.0 million for the achievement of specified net sales milestones. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or any royalty payments from Shionogi. The Company will receive tiered royalties on sales of SAGE-217 in Japan, Taiwan and South Korea, if development efforts are successful, with tiers averaging in the low to mid-twenty percent range, subject to other terms of the agreement. Shionogi has also granted to the Company certain rights to co-promote SAGE-217 in Japan. The Company maintains exclusive rights to develop and commercialize SAGE-217 outside of Japan, Taiwan and South Korea. The upfront cash payment and any payments for milestones and royalties are non-refundable, non-creditable and not subject to set-off.

The Company concluded that Shionogi meets the definition to be accounted for as a customer since the Company is delivering intellectual property and know-how rights for the SAGE-217 program in support of territories in which the parties are not jointly sharing the risks and rewards. In addition, the Company determined that the Shionogi collaboration met the requirements to be accounted for as a contract, including that it was probable that the Company will collect the consideration related to the up-front payment to which the Company was entitled in exchange for the goods or services that will be delivered to Shionogi.

In determining the appropriate amount of revenue to be recognized under ASC 606, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including the constraint on variable consideration; (iv) allocated the transaction price to the performance obligations; and (v) recognized revenue when (or as) the Company satisfies each performance obligation.

The Company determined that the performance obligations included the license to SAGE-217, supply of certain clinical materials and manufacturing supply of the active pharmaceutical ingredient (“API”). The performance obligation related to the license to SAGE-217 was determined to be distinct from other performance obligations and therefore was a standalone performance obligation for which control was transferred upon signing. The obligation to provide certain clinical materials was determined to be a separate performance obligation. The agreement related to supplying API was determined to be an option for Shionogi to purchase, rather than a firm obligation since no minimum amount or quantities are specified and, therefore, was not considered a performance obligation within the main agreement. Given this fact pattern, the Company has concluded the agreement has two performance obligations.

The Company completed the evaluation of the standalone selling prices of each of the performance obligations and determined that the standalone selling price of the license performance obligation was $90.0 million. The Company recognized the transaction price allocated to the license performance obligation of $90.0 million as revenue during the quarter upon delivery of the license to Shionogi and resulting ability of Shionogi to use and benefit from the license. The remaining transaction price related to the performance obligation for the supply of certain clinical material is not significant. The potential milestone payments that the Company is eligible to receive were excluded from the transaction price, as all milestone amounts were fully constrained based on the probability of achievement. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price.

Sale of Equity Securities

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Sale of Equity Securities

7.	Sale of Equity Securities

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 paid by the Company.

On February 27, 2019, the Company completed the sale of 3,833,334 shares of its common stock in an underwritten public offering at a price to the public of $150.00 per share, resulting in net proceeds of $560.9 million after deducting commissions and underwriting discounts and estimated offering costs paid by the Company.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8.	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 date, which was in February 2018 and 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. The milestones for these grants were not met, and accordingly, these grants were cancelled.

During the three months ended March 31, 2019, the Company granted 354,111 performance restricted stock units to employees of the Company. These performance restricted stock units will vest upon the achievement of a certain clinical and regulatory development milestones related to product candidates and a commercial milestone.

The fair value of restricted stock units that vested during the three months ended March 31, 2019 and 2018 was $2.0 million and $2.6 million, respectively.

The table below summarizes activity relating to restricted stock units:

	Shares
Outstanding as of December 31, 2018		82,700
Granted		354,111
Vested		(13,250	)
Forfeited		(34,091	)
Outstanding as of March 31, 2019		389,470

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 immediately prior to the completion of the Company’s 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 its 2011 Stock Option Plan, but any options or awards outstanding under the 2011 Stock Option Plan remain outstanding and effective.

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, 2019, 1,875,530 shares of common stock, representing 4% of the Company’s outstanding shares of common stock as of December 31, 2018, were added to the 2014 Stock Option Plan.

On December 15, 2016, the Board of Directors of the Company (the “Board”) 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. On September 20, 2018, the Board amended the 2016 Stock Option Plan to increase the total number of shares reserved for issuance under such plan by 1,200,000 shares.

As of March 31, 2019, the total number of shares reserved under all equity plans is 11,493,302, and 2,821,785 shares were available for future issuance under such plans.

During the three months ended March 31, 2018, the Company granted 323,753 options 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 and a commercial milestone. 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, 2019, the Company granted no options to employees to purchase shares of common stock that contain performance-based vesting criteria.

During the three months ended March 31, 2019, the achievement of the one commercial milestone that had not been met that is the criteria for vesting of performance-based stock options was considered probable, and therefore $14.2 million of stock-based compensation expense was recognized related to these awards for the three months ended March 31, 2019. Stock options with this milestone were granted during the years ended December 31, 2018 and 2017.

As of March 31, 2018, for grants that were outstanding, the achievement of the milestones that had not been met that are the criteria for vesting of performance-based stock options was considered not probable, and therefore no expense has been recognized related to these awards for the quarter ended March 31, 2018.

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Research and development	$	20,745	$	8,899
General and administrative		23,371		6,918
	$	44,116	$	15,817

Stock-based compensation expense by award type recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Stock options	$	43,564	$	15,381
Restricted stock units		58		168
Employee stock purchase plan		494		268
	$	44,116	$	15,817

The weighted average grant date fair value per share of stock options granted under the Company’s stock option plans during the three months ended March 31, 2019 and 2018 was $98.23 and $125.80, 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, 2018		7,530,767		$	93.22		8.03	$	227,447
Granted		1,179,861			155.30
Exercised		(298,102	)		48.32
Forfeited		(130,479	)		120.50
Outstanding as of March 31, 2019		8,282,047		$	103.25		8.10	$	499,599
Exercisable as of March 31, 2019		3,097,197		$	55.80		6.73	$	327,329

At March 31, 2019, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $342.8 million, which is expected to be recognized over the remaining weighted average vesting period of 2.91 years.

The intrinsic value of stock options exercised during the three months ended March 31, 2019 and 2018 was $26.7 million and $47.7 million, respectively.

At March 31, 2019, 745,454 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $49.5 million.

Net Loss Per Share

3 Months Ended

Mar. 31, 2019

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 three months ended March 31, 2019 and 2018:

	Three Months Ended March 31,
	2019			2018
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(163,406	)	$	(74,598	)
Denominator:
Weighted average common stock outstanding - basic		48,491,834			44,325,371
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—
Weighted average common stock outstanding - diluted		48,491,834			44,325,371
Net loss per share—basic and diluted	$	(3.37	)	$	(1.68	)

The following common stock equivalents outstanding as of March 31, 2019 and 2018 were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been anti-dilutive:

	Three Months Ended March 31,
	2019		2018
Stock options		7,536,593		5,590,175
Restricted stock units		—		14,150
Employee stock purchase plan		19,640		3,918
		7,556,233		5,608,243

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 calculation of common stock equivalents outstanding.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

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.

Revenue Recognition

Intangible Assets

Intangible assets

Leases

Recently Issued Accounting Pronouncements

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019 and December 31, 2018.

	March 31, 2019
	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	$	342,295	$	342,295	$	—	$	—
Total cash equivalents		342,295		342,295		—		—
Marketable securities:
U.S. government securities		248,155		—		248,155		—
U.S. corporate bonds		386,263		—		386,263		—
International corporate bonds		155,517		—		155,517		—
U.S. commercial paper		108,439		—		108,439		—
International commercial paper		110,582		—		110,582		—
Total marketable securities		1,008,956		—		1,008,956		—
	$	1,351,251	$	342,295	$	1,008,956	$	—

	December 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	$	190,943	$	190,943	$	—	$	—
Total cash equivalents		190,943		190,943		—		—
Marketable securities:
U.S. government securities		220,482		—		220,482		—
U.S. corporate bonds		258,566		—		258,566		—
International corporate bonds		78,468		—		78,468		—
U.S. commercial paper		77,611		—		77,611		—
International commercial paper		96,706		—		96,706		—
Total marketable securities		731,833		—		731,833		—
	$	922,776	$	190,943	$	731,833	$	—

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, 2019 and December 31, 2018:

	March 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	248,082	$	86	$	(13	)	$	248,155
U.S. corporate bonds		386,333		119		(189	)		386,263
International corporate bonds		155,571		42		(96	)		155,517
U.S. commercial paper		108,487		5		(53	)		108,439
International commercial paper		110,589		—		(7	)		110,582
	$	1,009,062	$	252	$	(358	)	$	1,008,956

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities	$	220,531	$	8	$	(57	)	$	220,482
U.S. corporate bonds		258,876		6		(316	)		258,566
International corporate bonds		78,600		—		(132	)		78,468
U.S. commercial paper		77,630		8		(27	)		77,611
International commercial paper		96,711		8		(13	)		96,706
	$	732,348	$	30	$	(545	)	$	731,833

- 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, 2019

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net, consists of the following:

	March 31,			December 31,
	2019			2018
	(in thousands)
Computer hardware and software	$	2,286		$	2,148
Furniture and equipment		1,125			1,002
Leasehold improvements		5,733			4,709
		9,144			7,859
Less: Accumulated depreciation		(2,631	)		(2,216	)
	$	6,513		$	5,643

Summary of Accrued Expenses

Accrued expenses consist of the following:

			December 31,
	March 31, 2019		2018
	(in thousands)
Development costs	$	23,932	$	21,216
Employee-related expenses		8,676		19,638
Professional services		11,746		10,903
Other accrued expenses		3,297		237
	$	47,651	$	51,994

Leases, Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Condensed Consolidated Balance Sheets of Operating Leases

The following table summarizes the presentation in the Company’s condensed consolidated balance sheets of operating leases:

(In thousands)	Balance sheet location	As of March 31, 2019
Assets
Right of use operating asset	Right of use operating asset	$	38,124

Liabilities
Current operating lease liabilities	Other current liabilities		7,149
Long-term operating lease liabilities	Long-term lease operating liability, net of current portion		34,528
Total operating lease liabilities		$	41,677

Schedule of Effect of Lease Cost In Condensed Consolidated Statements of Operations

The following table summarizes the effect of lease costs in the Company’s condensed consolidated statements of operations:

	For the Three Months Ended
(In thousands)	March 31, 2019
Operating lease cost	$	2,444

Schedule of Minimum Lease Payments

The minimum lease payments are expected to be as follows:

Years Ending December 31,	(in thousands)
2019 (remaining nine months)	$	7,513
2020		10,101
2021		9,812
2022		8,694
2023		8,894
Thereafter		5,415
Total lease payments		50,429
Less imputed interest		(8,752	)
Present value of operating lease liabilities	$	41,677

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

Under the prior lease guidance, future minimum lease payments under non-cancelable operating leases were as follows at December 31, 2018:

Years Ending December 31,	(in thousands)
2019	$	7,918
2020		8,299
2021		8,463
2022		8,692
2023		8,894
Thereafter		5,415
	$	47,681

Schedule of Weighted Average Remaining Lease Term and Weighted Average Discount Rate of Our Operating Leases

The weighted average remaining lease term and weighted average discount rate of the Company’s operating leases are as follows:

		As of March 31, 2019
Weighted average remaining lease term in years		5.30
Weighted average discount rate		7.5%

Schedule of Supplemental Disclosure of Cash Flow Information Related To Operating Leases

For the Three Months Ended

(In thousands)

March 31, 2019

Cash paid for amounts included in the

measurement of lease liabilities

2,354

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Relating to Restricted Stock Units

The table below summarizes activity relating to restricted stock units:

	Shares
Outstanding as of December 31, 2018		82,700
Granted		354,111
Vested		(13,250	)
Forfeited		(34,091	)
Outstanding as of March 31, 2019		389,470

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, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Research and development	$	20,745	$	8,899
General and administrative		23,371		6,918
	$	44,116	$	15,817

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, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Stock options	$	43,564	$	15,381
Restricted stock units		58		168
Employee stock purchase plan		494		268
	$	44,116	$	15,817

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, 2018		7,530,767		$	93.22		8.03	$	227,447
Granted		1,179,861			155.30
Exercised		(298,102	)		48.32
Forfeited		(130,479	)		120.50
Outstanding as of March 31, 2019		8,282,047		$	103.25		8.10	$	499,599
Exercisable as of March 31, 2019		3,097,197		$	55.80		6.73	$	327,329

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019 and 2018:

	Three Months Ended March 31,
	2019			2018
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(163,406	)	$	(74,598	)
Denominator:
Weighted average common stock outstanding - basic		48,491,834			44,325,371
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—
Weighted average common stock outstanding - diluted		48,491,834			44,325,371
Net loss per share—basic and diluted	$	(3.37	)	$	(1.68	)

Summary of Anti-Dilutive Common Stock Equivalents Outstanding

	Three Months Ended March 31,
	2019		2018
Stock options		7,536,593		5,590,175
Restricted stock units		—		14,150
Employee stock purchase plan		19,640		3,918
		7,556,233		5,608,243

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	108 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Nature Of Business [Line Items]
Accumulated deficit	$ (1,126,735)	$ (963,329)
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	$ 2,200,000

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Intangible assets			$ 0
Amortization of intangible assets	$ 0
Operating lease liabilities	41,677,000	$ 44,200,000
Operating lease right-of-use asset	38,124,000	$ 41,100,000
CyDex License Agreement [Member]
Summary Of Significant Accounting Policies [Line Items]
Milestone payments	3,000,000
The Regents of University of California [Member]
Summary Of Significant Accounting Policies [Line Items]
Milestone payments	$ 500,000

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 342,295	$ 190,943
Total marketable securities	1,008,956	731,833
Total cash equivalents and marketable securities	1,351,251	922,776
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	342,295	190,943
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	342,295	190,943
Total cash equivalents and marketable securities	342,295	190,943
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	342,295	190,943
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	1,008,956	731,833
Total cash equivalents and marketable securities	1,008,956	731,833
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	248,155	220,482
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	248,155	220,482
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	386,263	258,566
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	386,263	258,566
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	155,517	78,468
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	155,517	78,468
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	108,439	77,611
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	108,439	77,611
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	110,582	96,706
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	$ 110,582	$ 96,706

Fair Value Measurements - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) MarketableSecurity	Dec. 31, 2018 USD ($) MarketableSecurity	Mar. 31, 2018 USD ($)
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 not part of remaining contractual maturities of one year or less \| MarketableSecurity	58
Marketable securities fair value held to maturity	$ 166,200,000
Impairment of assets	$ 0	$ 0
U.S. Corporate Bonds [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Number of marketable securities not part of remaining contractual maturities of one year or less \| MarketableSecurity		2
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year	1 year
Marketable securities held, matuirity period	2 years
Minimum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities held, matuirity period	1 year

Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,009,062	$ 732,348
Gross Unrealized Gains	252	30
Gross Unrealized Losses	(358)	(545)
Fair Value	1,008,956	731,833
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	248,082	220,531
Gross Unrealized Gains	86	8
Gross Unrealized Losses	(13)	(57)
Fair Value	248,155	220,482
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	386,333	258,876
Gross Unrealized Gains	119	6
Gross Unrealized Losses	(189)	(316)
Fair Value	386,263	258,566
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	155,571	78,600
Gross Unrealized Gains	42
Gross Unrealized Losses	(96)	(132)
Fair Value	155,517	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	108,487	77,630
Gross Unrealized Gains	5	8
Gross Unrealized Losses	(53)	(27)
Fair Value	108,439	77,611
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,589	96,711
Gross Unrealized Gains		8
Gross Unrealized Losses	(7)	(13)
Fair Value	$ 110,582	$ 96,706

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 9,144	$ 7,859
Less: Accumulated depreciation	(2,631)	(2,216)
Property and equipment, net	6,513	5,643
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,286	2,148
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,125	1,002
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,733	$ 4,709

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 415	$ 247
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, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Development costs	$ 23,932	$ 21,216
Employee-related expenses	8,676	19,638
Professional services	11,746	10,903
Other accrued expenses	3,297	237
Total accrued expenses	$ 47,651	$ 51,994

Leases, Commitments and Contingencies - Operating Leases - Additional Information (Detail)				1 Months Ended				3 Months Ended
	Oct. 02, 2018 ft²	Sep. 30, 2018 ft²	Aug. 01, 2018 ft²	Dec. 31, 2018 ft²	Oct. 31, 2018 ft²	May 31, 2018 ft²	Apr. 30, 2018 ft²	Mar. 31, 2019 USD ($)	Mar. 31, 2018 ft²
Commitments And Contingencies [Line Items]
Operating lease option to extend								true
Operating lease renewal term								5 years
Expenses for short term lease \| $								$ 200,000
Right of use assets recorded arising from new lease liabilities \| $								$ 0
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease					58,442
Lease expire date					Aug. 31, 2024
Increased office space rent		54,943			3,499
Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease							40,419		19,805
Lease expire date							Aug. 31, 2024	Aug. 31, 2024	Feb. 28, 2022
Increased office space rent	7,133		13,481				20,614
Operating Lease Three [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease						15,525
Lease expire date						Nov. 30, 2024
Operating Lease Five [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease				15,975
Lease expire date				Feb. 28, 2024
Minimum [Member]
Commitments And Contingencies [Line Items]
Operating lease remaining lease terms								1 year
Maximum [Member]
Commitments And Contingencies [Line Items]
Operating lease remaining lease terms								6 years

Leases, Commitments and Contingencies - Schedule of Condensed Consolidated Balance Sheets of Operating Leases (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Assets
Right of use operating asset	$ 38,124	$ 41,100
Liabilities
Current operating lease liabilities	7,149
Long-term operating lease liabilities	34,528
Total operating lease liabilities	$ 41,677	$ 44,200

Leases, Commitments and Contingencies - Schedule of Effect of Lease Cost In Condensed Consolidated Statements of Operations (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating lease cost	$ 2,444

Leases, Commitments and Contingencies - Schedule of Minimum Lease Payments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Lessee Lease Description [Line Items]
Operating lease liabilities	$ 41,677	$ 44,200
ASU Topic (842) [Member]
Lessee Lease Description [Line Items]
2019 (remaining nine months)	7,513
2020	10,101
2021	9,812
2022	8,694
2023	8,894
Thereafter	5,415
Total lease payments	50,429
Less imputed interest	(8,752)
Operating lease liabilities	$ 41,677

Leases, Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 7,918
2020	8,299
2021	8,463
2022	8,692
2023	8,894
Thereafter	5,415
Total Operating leases, future minimum payments	$ 47,681

Leases, Commitments and Contingencies - Schedule of Weighted Average Remaining Lease Term and Weighted Average Discount Rate of Our Operating Leases (Detail)	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Weighted average remaining lease term in years	5 years 3 months 18 days
Weighted average discount rate	7.50%

Leases, Commitments and Contingencies - Schedule of Supplemental Disclosure of Cash Flow Information Related To Operating Leases (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Cash paid for amounts included in the measurement of lease liabilities	$ 2,354

Leases, Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Commitments And Contingencies [Line Items]
Research and development expense	$ 86,398,000	$ 49,270,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense	1,000,000
Milestone payments	3,000,000
Intangible asset related to milestone	3,000,000
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] \| Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		0
Milestone payments		0
Milestone payments related to intangible assets		$ 0
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] \| Brexanolone [Member]
Commitments And Contingencies [Line Items]
Intangible asset related to milestone	3,000,000
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	2,300,000
Milestone payments	$ 2,300,000

Leases, Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - University of California License Agreements [Member] - USD ($)		1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Upfront payment		$ 50,000
Annual license maintenance fee					$ 15,000
Research and development expense related to milestone expense				$ 0
Milestone payments				$ 0
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
Clinical Development [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Percentage of net sales paid as royalties	1.00%
Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Intangible asset related to milestone			$ 500,000
Milestone payments related to intangible assets			500,000
Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		700,000
Sales Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 2,000,000
Regulatory and Sales Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			300,000
Milestone payments			300,000
Intangible asset related to milestone			500,000
Milestone payments related to intangible assets			$ 500,000

Leases, 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, 2019	Mar. 31, 2018	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

Leases, Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Mar. 31, 2019	Mar. 31, 2018
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

Collaboration Agreement - Additional Information (Detail) - USD ($)		3 Months Ended
	Jun. 12, 2018	Mar. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 465,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement effective date	Jun. 12, 2018
Upfront payment	$ 90,000,000
Average percentage on tiered royalties	20.00%
Standalone selling price of license performance obligation		90,000,000
Revenue		$ 90,000,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payment receivable	$ 485,000,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Regulatory Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	70,000,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	30,000,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Sales Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 385,000,000

Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 27, 2019	Feb. 13, 2018
Equity [Abstract]
Issuance of common stock, shares	3,833,334	4,032,012
Common stock price per share	$ 150.00	$ 164.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering costs	$ 560.9	$ 631.2

Stock-Based Compensation - Additional Information (Detail)		3 Months Ended			12 Months Ended
	Jan. 01, 2019 shares	Mar. 31, 2019 USD ($) Milestone $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 shares	Dec. 31, 2018 shares	Sep. 20, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of options outstanding		11,493,302
Common stock available for issuance under stock option plan		2,821,785
Stock-based compensation expense \| $		$ 44,116,000	$ 15,817,000
Weighted average grant date fair value per share \| $ / shares		$ 98.23	$ 125.80
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	1,875,530
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%
2016 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares reserved for issuance						1,200,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		354,111		32,500
Share based compensation granted under plan vest period				2 years
Fair value of restricted stock units vested \| $		$ 2,000,000	$ 2,600,000
Stock-based compensation expense \| $		$ 58,000	$ 168,000
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		354,111	33,600
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		0	323,753
Total unrecognized stock-based compensation expense \| $		$ 49,500,000
Number of shares outstanding and unvested stock options		745,454
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Milestones not achieved \| Milestone		1
Stock-based compensation expense \| $		$ 14,200,000	$ 0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		1,179,861
Total number of options outstanding		8,282,047			7,530,767
Stock-based compensation expense \| $		$ 43,564,000	15,381,000
Total unrecognized stock-based compensation expense \| $		$ 342,800,000
Weighted average period of unrecognized compensation costs		2 years 10 months 28 days
Intrinsic value of options exercised \| $		$ 26,700,000	$ 47,700,000

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock Units (Detail) - Restricted Stock Units [Member] - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Shares at beginning balance	82,700
Granted, Shares	354,111	32,500
Vested, Shares	(13,250)
Forfeited, Shares	(34,091)
Outstanding, Shares at ending balance	389,470

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, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 44,116	$ 15,817
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	20,745	8,899
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 23,371	$ 6,918

Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 44,116	$ 15,817
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	43,564	15,381
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	58	168
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 494	$ 268

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, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares	11,493,302
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	7,530,767
Granted, Shares	1,179,861
Exercised, Shares	(298,102)
Forfeited, Shares	(130,479)
Ending balance, Outstanding Shares	8,282,047	7,530,767
Exercisable, Shares	3,097,197
Beginning balance, Outstanding Weighted Average Exercise Price	$ 93.22
Granted, Weighted Average Exercise Price	155.30
Exercised, Weighted Average Exercise Price	48.32
Forfeited, Weighted Average Exercise Price	120.50
Ending balance, Outstanding Weighted Average Exercise Price	103.25	$ 93.22
Exercisable, Weighted Average Exercise Price	$ 55.80
Outstanding, Weighted Average Remaining Life	8 years 1 month 6 days	8 years 10 days
Exercisable, Weighted Average Remaining Life	6 years 8 months 23 days
Outstanding, Aggregate Intrinsic Value	$ 499,599	$ 227,447
Exercisable, Aggregate Intrinsic Value	$ 327,329

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, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (163,406)	$ (74,598)
Denominator:
Weighted average common stock outstanding - basic	48,491,834	44,325,371
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	48,491,834	44,325,371
Net loss per share—basic and diluted	$ (3.37)	$ (1.68)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss Per Share - Summary of Anti-Dilutive Common Stock Equivalents Outstanding (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	7,556,233	5,608,243
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	7,536,593	5,590,175
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents		14,150
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	19,640	3,918