Additional milestone payments based on achievement of certain milestones.

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- Definition

Average percentage on tiered royalties.

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- Definition

Cash consideration received on sale of stock and upfront payment.

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- Definition

Collaboration agreement efective date.

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- Definition

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

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- Definition

The maximum amount of milestones that could receive in the future.

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Collaborative agreement transaction price, total.

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- Definition

Payments to related parties.

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- Definition

Premium amount on equity investment.

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Premium on per share price, percentage.

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- Definition

Proceeds from related parties.

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- Definition

Purchase price for new share issued.

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- Definition

Amount of reimbursement from collaboration agreement.

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- Definition

Up front payment.

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No definition available.

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- Definition

Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

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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.

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No definition available.

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- Definition

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

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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).

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Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

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No definition available.

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The fair value of stock issued in noncash financing activities.

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Number of new stock issued during the period.

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Collaboration Agreements - Additional Information (Detail) - USD ($)			3 Months Ended			12 Months Ended
	Nov. 30, 2020	Jun. 30, 2018	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2018	Dec. 31, 2020	Jun. 12, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares issued						6,241,473
Collaboration receivable - related party			$ 24,766,000
Shionogi Collaboration Agreement [Member] \| Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue			0	$ 0
Shionogi Collaboration Agreement [Member] \| SAGE-217 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement effective date		Jun. 30, 2018
Upfront payment		$ 90,000,000.0
Average percentage on tiered royalties		20.00%
Revenue					$ 90,000,000.0
Standalone selling price of license performance obligation		$ 90,000,000.0			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			$ 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
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement effective date	Dec. 28, 2020
Upfront payment						$ 875,000,000.0
Revenue						1,100,000,000
Standalone selling price of license performance obligation						1,100,000,000
Sale of stock, and upfront payment consideration received						$ 1,500,000,000
Number of shares issued						6,241,473
Purchase price						$ 650,000,000.0
Reimbursement income			24,800,000
Collaboration receivable - related party			24,800,000
Proceeds from related parties			0
Payments to related parties			0
Biogen Collaboration Agreement [Member] \| Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue			$ 0
Biogen Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payment receivable							$ 1,600,000,000
Biogen Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Sales Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 300,000,000.0
Biogen Collaboration Agreement [Member] \| SAGE-217 [Member] \| Maximum [Member] \| Regulatory and Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	475,000,000.0
Biogen Collaboration Agreement [Member] \| SAGE-324 [Member] \| Maximum [Member] \| Sales Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	300,000,000.0
Biogen Collaboration Agreement [Member] \| SAGE-324 [Member] \| Maximum [Member] \| Regulatory and Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 520,000,000.0
Biogen Stock Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment						875,000,000.0
Purchase price						650,000,000.0
Transaction price, total						1,100,000,000
Premium amount on equity investment						$ 232,500,000
Price per share						$ 104.14
Premium on per share price						40.00%
Stock issued						$ 417,500,000