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

Additional milestone payments based on achievement of certain milestones.

No definition available.

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- Definition

Average percentage on tiered royalties.

+ References

No definition available.

+ Details

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- Definition

Collaboration agreement efective date.

+ References

No definition available.

+ Details

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- Definition

Collaboration agreement regulatory and commercial event based additional milestone payment receivable.

+ References

No definition available.

+ Details

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- Definition

Up front payment.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

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