Additional milestone payments based on achievement of certain milestones.

No definition available.

Name:	sage_AdditionalMilestonePaymentsBasedOnAchievementOfCertainMilestones
Namespace Prefix:	sage_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Average percentage on tiered royalties.

+ References

No definition available.

+ Details

Name:	sage_AveragePercentageOnTieredRoyalties
Namespace Prefix:	sage_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Collaboration agreement efective date.

+ References

No definition available.

+ Details

Name:	sage_CollaborationAgreementEffectiveDate
Namespace Prefix:	sage_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	sage_CollaborationAgreementRegulatoryAndCommercialEventBasedAdditionalMilestonePaymentReceivable
Namespace Prefix:	sage_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Up front payment.

+ References

No definition available.

+ Details

Name:	sage_UpFrontPayment
Namespace Prefix:	sage_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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=118956577&loc=SL49130556-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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://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 3: 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

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-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=116634182&loc=SL114868664-224227

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=sage_ShionogiCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=sage_LicenseThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=sage_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=sage_CommercialMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=sage_SalesMilestonesMember
Namespace Prefix:
Data Type:	na
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Collaboration Agreement - Additional Information (Detail) - USD ($)		3 Months Ended	9 Months Ended
	Jun. 12, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 3,570,000	$ 4,908,000	$ 90,000,000
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement effective date	Jun. 12, 2018
Upfront payment	$ 90,000,000.0
Average percentage on tiered royalties	20.00%
Standalone selling price of license performance obligation		$ 90,000,000.0	90,000,000.0
Revenue			$ 90,000,000.0
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payment receivable	$ 485,000,000.0
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Regulatory Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	70,000,000.0
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	30,000,000.0
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Sales Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 385,000,000.0