Collaboration Agreements - Additional Information (Detail) - USD ($)			3 Months Ended				9 Months Ended			12 Months Ended
	Nov. 30, 2020	Jun. 30, 2018	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2018	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jun. 12, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue										$ 86,455,000	$ 7,686,000	$ 6,308,000
Collaboration receivable - related party			$ 83,009,000	$ 13,660,000						83,009,000	13,660,000
Shionogi Collaboration Agreement [Member] | Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue										200,000	0	0
Shionogi Collaboration Agreement [Member] | SAGE-217 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
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
Sale of stock, consideration received													$ 1,500,000,000
Public offering of common stock, net of offering costs, Shares													6,241,473
Purchase price													$ 650,000,000.0
Net reimbursement			8,000,000							(824,000)
Milestone achieved			75,000,000
Payments to related parties										0	0	0
Proceeds from related parties				$ 65,700,000	$ 80,300,000		$ 65,700,000	$ 80,300,000	$ 72,600,000
Biogen Collaboration Agreement [Member] | Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivable - related party			$ 83,000,000							83,000,000
Biogen Collaboration Agreement [Member] | License And Milestone Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue										$ 75,000,000	$ 0	$ 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] | First Commercial Sale of Zurzuvae for Treatment of MDD [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 150,000,000
Biogen Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member] | First Commercial Sale Of Zurzuvae For Treatment Of Women With Ppd [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	75,000,000
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
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
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
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
Biogen Stock Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment													875,000,000
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
Biogen stock purchase agreement, description										terminate on the earliest of (i) a specified regulatory milestone under the Biogen Collaboration Agreement, (ii) the date one year following the termination of the Biogen Collaboration Agreement and (iii) the seventh anniversary of the Effective Date.
Stock issued													$ 417,500,000