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Document and Entity Information - USD ($)	12 Months Ended
Dec. 31, 2018	Feb. 12, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		47,092,657
Entity Public Float			$ 7,110,731,043

Consolidated Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 190,943,000	$ 306,235,000
Marketable securities	731,833,000	212,613,000
Prepaid expenses and other current assets	21,919,000	6,227,000
Total current assets	944,695,000	525,075,000
Property and equipment, net	5,643,000	4,013,000
Restricted cash	2,367,000	849,000
Total assets	952,705,000	529,937,000
Current liabilities:
Accounts payable	34,036,000	9,350,000
Accrued expenses	51,994,000	42,601,000
Total current liabilities	86,030,000	51,951,000
Other liabilities	3,704,000	2,511,000
Total liabilities	89,734,000	54,462,000
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at December 31, 2018 and 2017; no shares issued or outstanding at December 31, 2018 and 2017
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at December 31, 2018 and 2017; 46,891,296 and 42,003,894 shares issued at December 31, 2018 and 2017, respectively; 46,888,263 and 42,002,934 shares outstanding at December 31, 2018 and 2017, respectively	5,000	5,000
Treasury stock, at cost, 3,033 and 960 shares at December 31, 2018 and December 31, 2017, respectively	(211,000)	(113,000)
Additional paid-in capital	1,827,021,000	1,066,059,000
Accumulated deficit	(963,329,000)	(590,447,000)
Accumulated other comprehensive loss	(515,000)	(29,000)
Total stockholders’ equity	862,971,000	475,475,000
Total liabilities and stockholders’ equity	$ 952,705,000	$ 529,937,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 90,273
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 282,107	$ 210,277	$ 120,756
General and administrative	201,404	62,878	39,407
Total operating expenses	483,511	273,155	160,163
Loss from operations	(393,238)	(273,155)	(160,163)
Interest income, net	20,334	3,099	1,211
Other income (expense), net	22	(64)	(35)
Net loss	$ (372,882)	$ (270,120)	$ (158,987)
Net loss per share—basic and diluted	$ (8.08)	$ (7.09)	$ (4.75)
Weighted average number of common shares outstanding—basic and diluted	46,121,194	38,113,678	33,492,795
Comprehensive loss:
Net loss	$ (372,882)	$ (270,120)	$ (158,987)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	(486)	73	(102)
Total other comprehensive gain (loss)	(486)	73	(102)
Total comprehensive loss	$ (373,368)	$ (270,047)	$ (159,089)

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, 2015	$ 173,695	$ 3		$ 335,032		$ (161,340)
Balances, Shares at Dec. 31, 2015		28,823,549
Issuance of common stock from exercise of stock options, Amount	1,011			1,011
Issuance of common stock from exercise of stock options, Shares		124,557
Vesting of restricted stock units, net of employee tax obligations	11			11
Vesting of restricted stock units, net of employee tax obligations, Shares		42,781
Issuance of common stock under employee stock purchase plan	510			510
Issuance of common stock under employee stock purchase plan, Shares		10,499
Purchase of treasury stock, Amount	$ (17)		$ (17)
Purchase of treasury stock, Shares	(346)		346
Stock-based compensation expense	$ 22,855			22,855
Public offering of common stock, net of offering costs, Amount	329,541	$ 1		329,540
Public offering of common stock, net of offering costs, Shares		8,220,786
Unrealized gain (loss) on available-for-sale securities	(102)				$ (102)
Net loss	(158,987)					(158,987)
Balances at Dec. 31, 2016	368,517	$ 4	$ (17)	688,959	(102)	(320,327)
Balances, Shares at Dec. 31, 2016		37,222,172	346
Issuance of common stock from exercise of stock options, Amount	15,437	$ 1		15,436
Issuance of common stock from exercise of stock options, Shares		706,222
Issuance of common stock under employee stock purchase plan	771			771
Issuance of common stock under employee stock purchase plan, Shares		15,718
Purchase of treasury stock, Amount	$ (96)		$ (96)
Purchase of treasury stock, Shares	(614)		614
Stock-based compensation expense	$ 35,142			35,142
Public offering of common stock, net of offering costs, Amount	325,751			325,751
Public offering of common stock, net of offering costs, Shares		4,058,822
Unrealized gain (loss) on available-for-sale securities	73				73
Net loss	(270,120)					(270,120)
Balances at Dec. 31, 2017	475,475	$ 5	$ (113)	1,066,059	(29)	(590,447)
Balances, Shares at Dec. 31, 2017		42,002,934	960
Issuance of common stock from exercise of stock options, Amount	27,014			27,014
Issuance of common stock from exercise of stock options, Shares		824,188
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
Issuance of common stock under employee stock purchase plan	2,705			2,705
Issuance of common stock under employee stock purchase plan, Shares		19,687
Purchase of treasury stock, Amount	$ (98)		$ (98)
Purchase of treasury stock, Shares	(2,073)		2,073
Stock-based compensation expense	$ 100,993			100,993
Public offering of common stock, net of offering costs, Amount	631,154			631,154
Public offering of common stock, net of offering costs, Shares		4,032,012
Unrealized gain (loss) on available-for-sale securities	(486)				(486)
Net loss	(372,882)					(372,882)
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

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (372,882)	$ (270,120)	$ (158,987)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	101,963	35,534	23,020
Premium on marketable securities	(215)	(105)	(756)
Amortization of premium (discount) on marketable securities	(9,892)	(293)	286
Depreciation	1,143	531	281
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(15,462)	(1,127)	(3,362)
Accounts payable	24,544	(3,693)	7,796
Accrued expenses and other liabilities	10,130	19,996	13,044
Net cash used in operating activities	(260,671)	(219,277)	(118,678)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	974,757	261,225	30,499
Purchases of marketable securities	(1,484,358)	(244,403)	(259,093)
Purchases of property and equipment	(2,860)	(1,360)	(1,421)
Net cash provided by (used in) investing activities	(512,461)	15,462	(230,015)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	29,108	15,972	1,406
Payment of employee tax obligations related to vesting of restricted stock units	(904)
Payments of offering costs	(340)	(179)	(599)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	631,494	326,025	330,175
Net cash provided by financing activities	659,358	341,818	330,982
Net increase (decrease) in cash, cash equivalents and restricted cash	(113,774)	138,003	(17,711)
Cash, cash equivalents and restricted cash at beginning of period	307,084	169,081	186,792
Cash, cash equivalents and restricted cash at end of period	193,310	307,084	169,081
Supplemental disclosure of non-cash operating, investing and financing activities
Landlord tenant incentive included in other current assets	229
Purchases of property and equipment financed with landlord tenant incentive		1,665
Purchases of property and equipment included in accounts payable	$ 51	138	$ 8
Public offering costs included in accounts payable		$ 95

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

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

Basis of Presentation

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

Use of Estimates

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

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents. As of December 31, 2018, cash equivalents were comprised of cash equivalents and money market funds. As of December 31, 2017, cash equivalents were comprised of cash equivalents, money market funds and overnight reverse repurchase agreements.

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 years ended December 31, 2018 and December 31, 2017.

Restricted Cash

For the initial facility of the Company, a deposit of $39,000 was restricted from withdrawal as of December 31, 2018 and 2017. The restriction is related to securing the initial lease and the restriction will expire in 2024, in accordance with the most recent amendment to the operating lease agreement. A deposit of $0.3 million was restricted from withdrawal as of December 31, 2018 and 2017. The restriction is related to securing an amendment to the lease of the initial facility of the Company and the restriction will expire in 2024, in accordance with the most recent amendment to the operating lease agreement. These balances are included in restricted cash on the accompanying consolidated balance sheets.

A deposit of $0.5 million was restricted from withdrawal as of December 31, 2018 and 2017. The restriction is related to securing the separate facility lease on the second multi-tenant building in May 2016 and the restriction will expire in 2024, in accordance with the most recent amendment to the operating lease agreement. A deposit of $0.4 million was restricted from withdrawal as of December 31, 2018. The restriction is related to securing an amendment to the lease of the separate facility of the Company and the restriction will expire in 2024, in accordance with the most recent amendment to the operating lease agreement. These balances are included in restricted cash on the accompanying consolidated balance sheets.

A deposit of $0.4 million was restricted from withdrawal as of December 31, 2018. The restriction is related to securing a separate facility lease on the third multi-tenant building in December 2018 that will expire on February 28, 2024, under which, beginning on March 1, 2019, the Company will rent 15,975 square feet of additional office space. The restriction will expire in 2024, in accordance with the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheet.

A deposit of $0.7 million was restricted from withdrawal as of December 31, 2018. The restriction is related to securing leases on automobiles for employees who are based in the field. The restriction will expire in 2021, in accordance with the lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheet.

Property and Equipment

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

Impairment of Long-Lived Assets

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

Research and Development

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

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

Patent Costs

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

Stock-Based Compensation

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

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

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 by the Company has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options, while the expected term of its options granted to consultants and non-employee directors has been determined based on the contractual term of the options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

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

Treasury Stock

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

Basic and Diluted Net Loss Per Share

Basic net loss per share is computed by dividing the net loss by the weighted average number of common shares outstanding for the period. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, because dilutive common shares are not assumed to have been issued if their effect is antidilutive.

The Company reported a net loss for the years ended December 31, 2018, 2017 and 2016.

Risks and Uncertainties

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

Concentration of Credit Risk and of Significant Suppliers

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

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

Income Taxes

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

Fair Value Measurements

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

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

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

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

The Company’s cash equivalents and marketable securities at December 31, 2018 and 2017 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 consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at December 31, 2018 and 2017, respectively.

Segment Data

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

Comprehensive Loss

Comprehensive loss includes net loss and other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders.

Public Offerings

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

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

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

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

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

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

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.

Recently Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, Leases, which will replace the existing guidance in ASC 840, “Leases”, and in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements. The new leasing standards generally requires 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. We will adopt the new standard effective January 1, 2019 and will not restate comparative periods. Presentation of leases within the consolidated statements of operations and consolidated statements of cash flows will be generally consistent with the current lease accounting guidance. We will elect the package of practical expedients permitted under the transition guidance and as such, the adoption of this ASU will not change the classification of any of our leases. We will elect to combine lease and non-lease components, elect not to record leases with an initial term of 12 months or less on the balance sheet and recognize the associated lease payments in the consolidated statements of operations on a straight-line basis over the lease term. We estimate that approximately $45.0 million will be recognized as total lease liabilities and approximately $41.0 million will be recognized as total right-of-use assets on our consolidated balance sheet as of January 1, 2019. Otherwise, we do not expect the new standard to have a material impact on our consolidated financial statements.

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

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The standard reduces the diversity in practice in how certain cash receipts and cash payments are presented and classified in the statements of cash flows. The Company adopted the standard on the required effective date of January 1, 2018. This guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash that changes the presentation of restricted cash and cash equivalents in the statements of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statements of cash flows. The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s consolidated statements of cash flows. Restricted cash balances are classified as non-current unless, under the terms of the applicable agreements, the funds will be released from restrictions within one year from the balance sheet date. The inclusion of restricted cash increased the beginning balances of the consolidated statements of cash flows by $0.8 million, $0.6 million and $39,000, respectively, and the ending balances by $2.4 million, $0.8 million and $0.6 million, respectively, for the years ended December 31, 2018, 2017 and 2016.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting, which applies to entities that change the terms or conditions of a share-based payment award. The amendments in this standard include guidance on determining whether changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under Topic 718 unless all of the following conditions are met: (1) the fair value of the modified award is the same as the fair value of the original award immediately before the original award is modified, and if the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, then the entity is not required to estimate the value immediately before and after the modification; (2) the vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) the classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. The Company adopted the standard on the required effective date of January 1, 2018. This guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures.

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 amendments in the new guidance are effective for public entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, including in interim periods, but no earlier than an entity’s adoption of Accounting Standards Codification 606. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

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

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

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

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

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2018 and 2017:

	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	$	—

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

During the years ended December 31, 2018 and 2017, there were no transfers among the Level 1, Level 2 and Level 3 categories.

Marketable Securities

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2018 and 2017:

	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

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

As of 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.

As of December 31, 2018 and 2017, the Company held 145 and 11 marketable securities, respectively, that were in a loss position for less than one year due to fluctuations in interest rates.

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

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Operating Leases

The Company leases office space in two multi-tenant buildings in Cambridge, Massachusetts, consisting, as of December 31, 2018, 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.

Rent expense for the years ended December 31, 2018, 2017 and 2016, was $6.5 million, $3.7 million, and $2.0 million, respectively.

Future minimum lease payments under non-cancelable operating leases are 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

License Agreements

CyDex License Agreement

In September 2015, the Company and CyDex Pharmaceuticals, Inc. (“CyDex”) amended and restated their existing commercial license agreement. Under the terms of the commercial license agreement as amended and restated, CyDex has granted to the Company an exclusive license to CyDex’s Captisol drug formulation technology and related intellectual property for the manufacture of pharmaceutical products incorporating brexanolone and the Company’s compound known as SAGE-689, and the development and commercialization of the resulting products in the treatment, prevention or diagnosis of any disease or symptom in humans or animals other than (i) the ocular treatment of any disease or condition with a formulation, including a hormone; (ii) topical ocular treatment of inflammatory conditions; (iii) treatment and prophylaxis of fungal infections in humans; and (iv) any ocular treatment for retinal degeneration. As of December 31, 2018, 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 December 31, 2018, the Company has recorded research and development expense and made cash payments of $2.3 million related to these clinical development and regulatory milestones.

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

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

For the year ended December 31, 2018, additional clinical development milestones were met for the brexanolone program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense and made cash payments totaling $0.8 million.

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 December 31, 2018, the Company has recorded research and development expense and made cash payments of $0.3 million related to these regulatory and sales milestones.

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

For the year ended December 31, 2018, the Company recorded research and development expense and made cash payments of $0.2 million related to regulatory milestones under the license agreements 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 December 31, 2018, 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 years ended December 31, 2018, 2017 and 2016, the Company did not record any expense or make any milestone payments under the license agreement with Washington University.

Consulting Agreement

In January 2014, the Company entered into a consulting agreement with a 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 December 31, 2018, the Company recorded research and development expense of $1.8 million, comprised of $0.5 million in cash and $1.3 million related to the issuance of 39,681 shares of the Company’s common stock, related to the achievement of these milestones.

For the years ended December 31, 2018, 2017 and 2016, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

9.	Stock-Based Compensation

Restricted Stock Units

During the year ended December 31, 2017, the Company granted 32,500 restricted stock units to employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. 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 will be in February 2019, respectively.

During the year ended December 31, 2018, the Company granted 71,400 performance restricted stock units to employees of the Company. 33,600 performance restricted stock units will vest upon the achievement of a certain regulatory milestone; in one case, upon meeting the milestone and, in the other case, upon the first anniversary of achievement of the milestone. 37,800 performance restricted stock units will vest upon the achievement of a certain commercial milestone.

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

The table below summarizes activity relating to restricted stock units:

	Shares
Outstanding as of December 31, 2017		29,100
Granted		71,400
Vested		(14,550	)
Forfeited		(3,250	)
Outstanding as of December 31, 2018		82,700

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 the 2011 Stock Option Plan. Any options or awards outstanding under the 2011 Stock Option Plan remained outstanding and effective.

The 2014 Stock Option Plan provides for an annual increase, to be added on the first day of each fiscal year, of 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.

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

During the years ended December 31, 2018, 2017, 2016 and 2015, the Company granted 524,003, 449,208, 74,039 and 497,100 options, respectively, to employees to purchase shares of common stock that contain performance-based vesting criteria, primarily related to the achievement of certain clinical and regulatory development milestones related to product candidates and commercial milestones. 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 activities described by the milestones.

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

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

During the year ended December 31, 2018, a performance milestone was achieved under a stock option granted to a consultant. The milestone was related to the consummation of a licensing or corporate partnering arrangement. During the year ended December 31, 2018, the Company recognized stock-based compensation expense related to this milestone of $6.9 million.

During the year ended December 31, 2018, one milestone was achieved. This milestone represents 33% of the performance-based option grants that were made during the year ended December 31, 2017. During the year ended December 31, 2018, the Company recognized stock-based compensation expense related to this milestone of $4.4 million.

During the year ended December 31, 2018, the remaining milestone for the performance-based option grants that were made during the years ended December 31, 2016 and December 31, 2015 was not met, and accordingly, those options were cancelled. They represented 50% and 35% of the performance-based option grants that were made during the years ended December 31, 2016 and December 31, 2015, respectively. The Company recognized no stock-based compensation expense related to this milestone.

As of December 31, 2018, for grants that are 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 year ended December 31, 2018.

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Research and development	$	50,871	$	19,893	$	11,197
General and administrative		51,092		15,641		11,823
	$	101,963	$	35,534	$	23,020

Stock-based compensation expense by award type recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Stock options	$	100,342	$	34,525	$	22,820
Restricted stock units		651		617		—
Employee stock purchase plan		970		392		166
Restricted stock awards		—		—		34
	$	101,963	$	35,534	$	23,020

For stock option awards, the fair value is estimated at the grant date using the Black-Scholes option-pricing model, taking into account the terms and conditions upon which options are granted. The fair value of the options is amortized on a straight-line basis for awards to employees and on a graded basis for awards to non-employees over the requisite service period of the awards. The weighted average grant date fair value per share of stock options granted under the Company’s stock option plans during the years ended December 31, 2018, 2017 and 2016 was $109.92, $41.94 and $24.97, respectively.

The fair value of each option granted to employees and non-employee directors during the years ended December 31, 2018, 2017 and 2016 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:

	Year Ended December 31,
	2018			2017			2016
Expected dividend yield		0	%		0	%		0	%
Expected volatility		74.45	%		79.89	%		80.15	%
Risk-free interest rate		2.68	%		2.03	%		1.47	%
Expected life of option	6.04 years			6.03 years			6.05 years

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

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

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

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

Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company estimates forfeitures based on historical termination behavior. For the years ended December 31, 2018, 2017 and 2016, the weighted-average forfeiture rates were 10.7%, 13.3% and 9.6%, respectively.

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

The table below summarizes activity related to stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2017		5,586,593		$	43.58		8.09	$	676,717
Granted		3,103,494			164.50
Exercised		(767,147	)		34.67
Forfeited		(392,173	)		64.74
Outstanding as of December 31, 2018		7,530,767		$	93.22		8.03	$	227,447
Vested and expected to vest as of December 31, 2018		6,254,429		$	85.55		7.86	$	209,356
Exercisable as of December 31, 2018		2,846,158		$	40.69		6.71	$	159,799

At December 31, 2018, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $282.6 million, which is expected to be recognized over the remaining weighted average vesting period of 2.88 years.

At December 31, 2018, 772,347 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $50.1 million.

The intrinsic value of stock options exercised during the years ended December 31, 2018, 2017 and 2016 was $98.3 million, $77.7 million and $4.6 million, respectively.

2014 Employee Stock Purchase Plan

On July 2, 2014, the Company’s stockholders approved the 2014 Employee Stock Purchase Plan, which had been previously approved by the Board of Directors. A total of 282,000 shares of common stock were initially authorized for issuance under this plan. The 2014 Employee Stock Purchase Plan became effective upon the completion of the IPO. As of December 31, 2018, 49,756 shares have been issued under this plan and 232,244 shares are available for issuance under this plan. At December 31, 2018, accrued expenses includes $0.5 million of stock-based compensation expense related to an enrollment period for which the related shares had not been issued as of December 31, 2018.

Shares Reserved and Available for Future Issuance

As of December 31, 2018, the total number of shares reserved under all equity plans, consisting of the 2011 Stock Option Plan, the 2014 Stock Option Plan, the 2014 Employee Stock Purchase Plan and the 2016 Stock Option Plan, is 9,945,522, and 2,332,055 shares were available for issuance under such plans.

Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

10.	Net Loss Per Share

Basic and diluted net loss per share was calculated as follows for the years ended December 31, 2018, 2017 and 2016:

	Year ended December 31,
	2018			2017			2016
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(372,882	)	$	(270,120	)	$	(158,987	)
Denominator:
Weighted average common stock outstanding - basic		46,121,194			38,113,678			33,492,795
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—			—
Weighted average common stock outstanding - diluted		46,121,194			38,113,678			33,492,795
Net loss per share—basic and diluted	$	(8.08	)	$	(7.09	)	$	(4.75	)

The following common stock equivalents outstanding as of December 31, 2018, 2017 and 2016 were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been anti-dilutive:

	Year ended December 31,
	2018		2017		2016
Stock options		6,758,420		4,915,956		3,985,935
Restricted stock units		13,500		29,100		—
Employee stock purchase plan		16,398		11,683		6,784
		6,788,318		4,956,739		3,992,719

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.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

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

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

	Year Ended December 31,
	2018			2017			2016
Tax due at statutory rate		21.0	%		34.0	%		34.0	%
State taxes, net of federal		6.0			5.1			4.2
Stock-based compensation		2.5			6.4			(1.0	)
Foreign rate differential		(4.3	)		(3.2	)		(3.1	)
Federal and state credits		2.5			7.2			10.3
Change in valuation allowance		(27.6	)		(22.5	)		(41.5	)
Other		(0.1	)		—			—
Federal and state rate change		—			(24.6	)		—
Research and orphan drug credit addback		—			(2.4	)		(2.9	)
		0.0	%		0.0	%		0.0	%

Significant components of the Company’s net deferred tax asset at December 31, 2018 and 2017 are as follows:

	December 31,
	2018			2017
	(in thousands)
Net operating losses	$	206,460		$	131,151
Capitalized start-up costs		1,242			1,362
Tax credit carryforwards		63,653			54,523
Accrued expenses		4,498			3,057
Depreciation and amortization		1,001			685
Stock options		30,925			13,573
Others		568			1,090
Total net deferred tax asset before valuation allowance		308,347			205,441
Valuation allowance		(308,347	)		(205,441	)
Net deferred tax asset	$	—		$	—

As of December 31, 2018, the Company had federal and state net operating loss carryforwards of $755.0 million and $760.7 million, respectively, which begin to expire in 2031 and 2030, respectively. As of December 31, 2018, the Company had federal and state research and development tax credits carryforwards of $20.3 million and $4.1 million, respectively, which begin to expire in 2031 and 2027, respectively. As of December 31, 2018, the Company had federal orphan drug tax credit carry forwards of $40.0 million, which begin to expire in 2034.

On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“The TCJA”). This legislation reduced the U.S. corporate tax rate from the existing rate of 34% to 21% for tax years beginning after December 31, 2017. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities existing as of December 31, 2017 from the 34% federal rate in effect through the end of 2017, to the new 21% rate. This revaluation resulted in a reduction to the Company’s deferred tax asset of $66.4 million as of December 31, 2017. This amount was offset by a corresponding reduction to the Company’s valuation allowance. The other provisions of the TCJA did not have a material impact on the December 31, 2017 consolidated financial statements. Our final determination of the TCJA impact and the remeasurement of our deferred assets and liabilities was completed prior to the deadline of one year from the enactment of the TCJA. For the year ended December 31, 2018, there were no material changes to our analysis originally performed as of December 31, 2017.

As of December 31, 2018, net deferred tax assets increased approximately $102.9 million, primarily due to the operating loss, timing differences related to stock-based compensation and tax credits incurred during the year. This increase in net deferred tax assets was offset by a corresponding increase in the valuation allowance.

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and tax credit carryforwards. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, a full valuation allowance of $308.3 million and $205.4 million has been established at December 31, 2018 and 2017, respectively.

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

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

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

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	—
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		—
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense when in a taxable income position. As of December 31, 2018 and 2017, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations, and the Company’s tax returns are open under statute from 2015 to the present. The tax attributes prior to 2015 may still be adjusted upon examination. The Company’s policy is to record interest and penalties related to income taxes as part of the tax provision.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

13.	Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total revenue	$	—		$	90,000		$	—		$	273		$	90,273
Total operating expenses		78,119			112,147			128,745			164,500			483,511
Loss from operations		(78,119	)		(22,147	)		(128,745	)		(164,227	)		(393,238	)
Net loss		(74,598	)		(16,978	)		(122,918	)		(158,388	)		(372,882	)
Net loss per share—basic and diluted	$	(1.68	)	$	(0.36	)	$	(2.63	)	$	(3.38	)	$	(8.08	)

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

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Cash and Cash Equivalents

Marketable Securities

Marketable securities

Restricted Cash

Property and Equipment

Impairment of Long-Lived Assets

Research and Development

Patent Costs

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

Stock-Based Compensation

Treasury Stock

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

Basic and Diluted Net Loss Per Share

The Company reported a net loss for the years ended December 31, 2018, 2017 and 2016.

Risks and Uncertainties

Concentration of Credit Risk and of Significant Suppliers

Income Taxes

Fair Value Measurements

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

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

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

Segment Data

Comprehensive Loss

Comprehensive loss includes net loss and other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders.

Public Offerings

Revenue Recognition

Recently Issued Accounting Pronouncements

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Company's Money Market Funds and Marketable Securities

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2018 and 2017:

	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	$	—

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

Summary of Gross Unrealized Gains and Losses of Marketable Securities

The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2018 and 2017:

	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

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

- Definition

Tabular disclosure of 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

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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, 2017		29,100
Granted		71,400
Vested		(14,550	)
Forfeited		(3,250	)
Outstanding as of December 31, 2018		82,700

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

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Research and development	$	50,871	$	19,893	$	11,197
General and administrative		51,092		15,641		11,823
	$	101,963	$	35,534	$	23,020

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Stock options	$	100,342	$	34,525	$	22,820
Restricted stock units		651		617		—
Employee stock purchase plan		970		392		166
Restricted stock awards		—		—		34
	$	101,963	$	35,534	$	23,020

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

	Year Ended December 31,
	2018			2017			2016
Expected dividend yield		0	%		0	%		0	%
Expected volatility		74.45	%		79.89	%		80.15	%
Risk-free interest rate		2.68	%		2.03	%		1.47	%
Expected life of option	6.04 years			6.03 years			6.05 years

Summary of Activity Relating to Stock Options

The table below summarizes activity related to stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2017		5,586,593		$	43.58		8.09	$	676,717
Granted		3,103,494			164.50
Exercised		(767,147	)		34.67
Forfeited		(392,173	)		64.74
Outstanding as of December 31, 2018		7,530,767		$	93.22		8.03	$	227,447
Vested and expected to vest as of December 31, 2018		6,254,429		$	85.55		7.86	$	209,356
Exercisable as of December 31, 2018		2,846,158		$	40.69		6.71	$	159,799

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss Per Share

Basic and diluted net loss per share was calculated as follows for the years ended December 31, 2018, 2017 and 2016:

	Year ended December 31,
	2018			2017			2016
Basic net loss per share:
Numerator:
Net loss (in thousands)	$	(372,882	)	$	(270,120	)	$	(158,987	)
Denominator:
Weighted average common stock outstanding - basic		46,121,194			38,113,678			33,492,795
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units		—			—			—
Weighted average common stock outstanding - diluted		46,121,194			38,113,678			33,492,795
Net loss per share—basic and diluted	$	(8.08	)	$	(7.09	)	$	(4.75	)

Summary of Anti-Dilutive Common Stock Equivalents Outstanding

	Year ended December 31,
	2018		2017		2016
Stock options		6,758,420		4,915,956		3,985,935
Restricted stock units		13,500		29,100		—
Employee stock purchase plan		16,398		11,683		6,784
		6,788,318		4,956,739		3,992,719

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

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

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

	Year Ended December 31,
	2018			2017			2016
Tax due at statutory rate		21.0	%		34.0	%		34.0	%
State taxes, net of federal		6.0			5.1			4.2
Stock-based compensation		2.5			6.4			(1.0	)
Foreign rate differential		(4.3	)		(3.2	)		(3.1	)
Federal and state credits		2.5			7.2			10.3
Change in valuation allowance		(27.6	)		(22.5	)		(41.5	)
Other		(0.1	)		—			—
Federal and state rate change		—			(24.6	)		—
Research and orphan drug credit addback		—			(2.4	)		(2.9	)
		0.0	%		0.0	%		0.0	%

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

Significant components of the Company’s net deferred tax asset at December 31, 2018 and 2017 are as follows:

	December 31,
	2018			2017
	(in thousands)
Net operating losses	$	206,460		$	131,151
Capitalized start-up costs		1,242			1,362
Tax credit carryforwards		63,653			54,523
Accrued expenses		4,498			3,057
Depreciation and amortization		1,001			685
Stock options		30,925			13,573
Others		568			1,090
Total net deferred tax asset before valuation allowance		308,347			205,441
Valuation allowance		(308,347	)		(205,441	)
Net deferred tax asset	$	—		$	—

Summary of Rollforward of Company's Unrecognized Tax Benefits

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

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	—
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		—
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Financial Information

	2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total revenue	$	—		$	90,000		$	—		$	273		$	90,273
Total operating expenses		78,119			112,147			128,745			164,500			483,511
Loss from operations		(78,119	)		(22,147	)		(128,745	)		(164,227	)		(393,238	)
Net loss		(74,598	)		(16,978	)		(122,918	)		(158,388	)		(372,882	)
Net loss per share—basic and diluted	$	(1.68	)	$	(0.36	)	$	(2.63	)	$	(3.38	)	$	(8.08	)

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

Summary of Significant Accounting Policies - Additional Information (Detail)							1 Months Ended				3 Months Ended	12 Months Ended
	Feb. 13, 2018 USD ($) $ / shares shares	Nov. 17, 2017 USD ($) $ / shares shares	Sep. 14, 2016 USD ($) $ / shares shares	Jan. 12, 2016 USD ($) $ / shares shares	Apr. 20, 2015 USD ($) $ / shares shares	Jul. 23, 2014 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) ft²	Oct. 31, 2018 ft²	May 31, 2018 ft²	Apr. 30, 2018 ft²	Mar. 31, 2018 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							$ 2,367,000					$ 2,367,000	$ 849,000
Issuance of common stock, shares \| shares	4,032,012	4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share \| $ / shares	$ 164.00	$ 85.00	$ 39.75	$ 47.50	$ 52.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering costs	$ 631,200,000	$ 325,800,000	$ 189,200,000	$ 140,400,000	$ 129,100,000
Accounting Standards Update 2016-02 [Member] \| Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease liabilities																$ 45,000,000
Right-of-use assets																$ 41,000,000
Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents increased by amount due to inclusion of restricted cash												2,400,000	800,000	$ 600,000	$ 39,000
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares \| shares						5,750,000
Common stock price per share \| $ / shares						$ 18.00
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts						$ 94,000,000
Operating Lease Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							39,000					$ 39,000	39,000
Additional Office space rented under operating lease \| ft²										40,419	19,805
Lease expire date										Aug. 31, 2024	Feb. 28, 2022	Aug. 31, 2024
Operating Lease Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							300,000					$ 300,000	300,000
Additional Office space rented under operating lease \| ft²									15,525
Lease expire date									Nov. 30, 2024
Operating Lease One [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							500,000					$ 500,000	$ 500,000
Additional Office space rented under operating lease \| ft²								58,442				58,442
Lease expire date								Aug. 31, 2024				Aug. 31, 2024
Operating Lease Six [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							400,000					$ 400,000
Operating Lease Four [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							400,000					$ 400,000
Additional Office space rented under operating lease \| ft²												15,975
Lease expire date												Feb. 28, 2024
Operating Lease Five [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							$ 700,000					$ 700,000
Additional Office space rented under operating lease \| ft²							15,975
Lease expire date							Feb. 28, 2024

Stock-Based Compensation - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2019 shares	Dec. 31, 2018 USD ($) Milestone $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) Milestone $ / shares shares	Dec. 31, 2015 USD ($) Milestone shares	Sep. 20, 2018 shares	Jul. 02, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation granted under plan vest period		4 years
Share based compensation, vest period		1 year
Share based compensation, term of plan		10 years
Stock-based compensation expense \| $		$ 101,963,000	$ 35,534,000	$ 23,020,000
Weighted average grant date fair value per share \| $ / shares		$ 109.92	$ 41.94	$ 24.97
Share option, weighted-average forfeiture rates		10.70%	13.30%	9.60%
Total number of options outstanding		232,244
Non-employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, term of plan		10 years
2011 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		0
Total number of options outstanding		9,945,522
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Milestones achieved \| Milestone		1		1	1
Total number of options outstanding		9,945,522
2014 Stock Option Plan [Member] \| Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $				$ 0
2014 Stock Option Plan [Member] \| Milestone One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved					35.00%
2014 Stock Option Plan [Member] \| Milestone Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved				50.00%	30.00%
2014 Stock Option Plan [Member] \| Milestone Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved		33.00%		50.00%	35.00%
2014 Stock Option Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares annual increase added to plan	1,875,530
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
Shares of common stock initially authorized for issuance under this plan						1,200,000
Common stock available for issuance under stock option plan		2,332,055
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares reserved for issuance							282,000
Shares of common stock initially authorized for issuance under this plan							282,000
Number of shares issued under the plan		49,756
Accrued expenses \| $		$ 500,000
Total number of options outstanding		9,945,522
Restricted Stock Units Vest One Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		25.00%
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		71,400	32,500
Share based compensation granted under plan vest period			2 years
Fair value of restricted stock units vested \| $		$ 2,600,000	$ 0
Stock-based compensation expense \| $		$ 651,000	$ 617,000
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		71,400
Performance Restricted Stock Units [Member] \| Regulatory Milestones [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		33,600
Performance Restricted Stock Units [Member] \| Commercial Milestones [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted		37,800
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		524,003	449,208	74,039	497,100
Total unrecognized stock-based compensation expense \| $		$ 50,100,000
Number of shares outstanding and unvested stock options		772,347
Performance Milestone [Member] \| 2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 4,400,000		$ 5,000,000	$ 4,800,000
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		0
Performance-Based Stock Options [Member] \| Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 6,900
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		3,103,494
Stock-based compensation expense \| $		$ 100,342,000	$ 34,525,000	22,820,000
Total unrecognized stock-based compensation expense \| $		$ 282,600,000
Weighted average period of unrecognized compensation costs		2 years 10 months 17 days
Intrinsic value of options exercised \| $		$ 98,300,000	$ 77,700,000	$ 4,600,000
Total number of options outstanding		7,530,767	5,586,593

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares	232,244
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	5,586,593
Granted, Shares	3,103,494
Exercised, Shares	(767,147)
Forfeited, Shares	(392,173)
Ending balance, Outstanding Shares	7,530,767	5,586,593
Vested and expected to vest, Shares	6,254,429
Exercisable, Shares	2,846,158
Beginning balance, Outstanding Weighted Average Exercise Price	$ 43.58
Granted, Weighted Average Exercise Price	164.50
Exercised, Weighted Average Exercise Price	34.67
Forfeited, Weighted Average Exercise Price	64.74
Ending balance, Outstanding Weighted Average Exercise Price	93.22	$ 43.58
Vested and expected to vest, Weighted Average Exercise Price	85.55
Exercisable, Weighted Average Exercise Price	$ 40.69
Outstanding, Weighted Average Remaining Life	8 years 10 days	8 years 2 months 26 days
Vested and expected to vest, Weighted Average Remaining Life	7 years 10 months 9 days
Exercisable, Weighted Average Remaining Life	6 years 8 months 15 days
Outstanding, Aggregate Intrinsic Value	$ 227,447	$ 676,717
Vested and expected to vest, Aggregate Intrinsic Value	209,356
Exercisable, Aggregate Intrinsic Value	$ 159,799

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Current tax expense	$ 0
Deferred tax expense	0
Federal operating loss carryforwards	755,000,000
State operating loss carryforwards	$ 760,700,000
Federal net operating loss carryforwards expiration year	2031
State net operating loss carryforwards expiration year	2030
Federal orphan drug tax credit carry forwards	$ 40,000,000
Tax credit carry forwards expiration period	2034
U.S. corporate tax rate	21.00%	34.00%	34.00%
Reduction to deferred tax asset		$ 66,400,000
Change in amount of deferred tax assets	$ 102,900,000
Valuation allowance	308,347,000	205,441,000
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Uncertain tax positions	0
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards	$ 20,300,000
Research and development tax credits carryforwards expiration year	2031
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards	$ 4,100,000
Research and development tax credits carryforwards expiration year	2027

Income Taxes - Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax due at statutory rate	21.00%	34.00%	34.00%
State taxes, net of federal	6.00%	5.10%	4.20%
Stock-based compensation	2.50%	6.40%	(1.00%)
Foreign rate differential	(4.30%)	(3.20%)	(3.10%)
Federal and state credits	2.50%	7.20%	10.30%
Change in valuation allowance	(27.60%)	(22.50%)	(41.50%)
Other	(0.10%)
Federal and state rate change		(24.60%)
Research and orphan drug credit addback		(2.40%)	(2.90%)
Total effective tax rate	0.00%	0.00%	0.00%

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	46,891,296	42,003,894
Common stock, shares outstanding	46,888,263	42,002,934
Treasury stock, shares	3,033	960

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	105 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Nature Of Business [Line Items]
Accumulated deficit	$ (963,329)	$ (590,447)
Redeemable Convertible Preferred Stock [Member] \| Convertible Notes [Member] \| Initial Public Offering [Member]
Nature Of Business [Line Items]
Net proceeds from sale of equity and notes	$ 1,600

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) MarketableSecurity	Dec. 31, 2017 USD ($) MarketableSecurity
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories \| $	$ 0	$ 0
Number of marketable securities loss position due to fluctuations in interest rates \| MarketableSecurity	145	11
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

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,859	$ 5,086
Less: Accumulated depreciation	(2,216)	(1,073)
Property and equipment, net	5,643	4,013
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,148	1,090
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,002	1,029
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 4,709	$ 2,967

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 1.1	$ 0.5	$ 0.3
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 190,943	$ 306,235
Total marketable securities	731,833	212,613
Total cash equivalents and marketable securities	922,776	518,848
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	190,943	306,235
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	190,943	306,235
Total cash equivalents and marketable securities	190,943	306,235
Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	190,943	306,235
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	731,833	212,613
Total cash equivalents and marketable securities	731,833	212,613
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	220,482	49,606
U.S. Government Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	220,482	49,606
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	258,566	48,959
U.S. Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	258,566	48,959
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	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	78,468
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	77,611	65,583
U.S. Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	77,611	65,583
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	96,706	48,465
International Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 96,706	$ 48,465

Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 732,348	$ 212,642
Gross Unrealized Gains	30	2
Gross Unrealized Losses	(545)	(31)
Fair Value	731,833	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	220,531	49,612
Gross Unrealized Gains	8
Gross Unrealized Losses	(57)	(6)
Fair Value	220,482	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	258,876	48,982
Gross Unrealized Gains	6	2
Gross Unrealized Losses	(316)	(25)
Fair Value	258,566	48,959
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,600
Gross Unrealized Losses	(132)
Fair Value	78,468
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,630	65,583
Gross Unrealized Gains	8
Gross Unrealized Losses	(27)
Fair Value	77,611	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96,711	48,465
Gross Unrealized Gains	8
Gross Unrealized Losses	(13)
Fair Value	$ 96,706	$ 48,465

Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Development costs	$ 21,216	$ 23,473
Employee-related expenses	19,638	15,838
Professional services	10,903	3,166
Other accrued expenses	237	124
Total accrued expenses	$ 51,994	$ 42,601

Commitments and Contingencies - Operating Leases - Additional Information (Detail) $ in Millions				1 Months Ended				3 Months Ended	12 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, 2018 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Rent expense \| $									$ 6.5	$ 3.7	$ 2.0
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease					58,442				58,442
Lease expire date					Aug. 31, 2024				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	Feb. 28, 2022	Aug. 31, 2024
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

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

Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - University of California License Agreements [Member] - USD ($)		1 Months Ended	12 Months Ended
	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	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			$ 200,000	$ 0	$ 0
Milestone payments			200,000	$ 0	$ 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] \| 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

Commitments and Contingencies - Washington University License Agreement - Additional Information (Detail) - Washington University License Agreement [Member] - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2013	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	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	$ 0
Milestone payments		0	$ 0	$ 0
Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		50,000
Milestone payments		$ 50,000
Maximum [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	700,000
Maximum [Member] \| Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 500,000

Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member] - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		$ 0	$ 0	$ 0
Milestone payments		0	$ 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		12 Months Ended
	Jun. 12, 2018	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 273,000	$ 90,000,000	$ 90,273,000
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		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

Preferred Stock - Additional Information (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (158,388)	$ (122,918)	$ (16,978)	$ (74,598)	$ (69,421)	$ (73,719)	$ (70,202)	$ (56,778)	$ (372,882)	$ (270,120)	$ (158,987)
Denominator:
Weighted average common stock outstanding - basic									46,121,194	38,113,678	33,492,795
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units									0	0	0
Weighted average common stock outstanding - diluted									46,121,194	38,113,678	33,492,795
Net loss per share—basic and diluted	$ (3.38)	$ (2.63)	$ (0.36)	$ (1.68)	$ (1.75)	$ (1.97)	$ (1.88)	$ (1.52)	$ (8.08)	$ (7.09)	$ (4.75)

Income Taxes - Summary of Significant Components of Company's Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating losses	$ 206,460	$ 131,151
Capitalized start-up costs	1,242	1,362
Tax credit carryforwards	63,653	54,523
Accrued expenses	4,498	3,057
Depreciation and amortization	1,001	685
Stock options	30,925	13,573
Others	568	1,090
Total net deferred tax asset before valuation allowance	308,347	205,441
Valuation allowance	$ (308,347)	$ (205,441)