Collaboration Agreements - Additional Information (Detail) - USD ($)				3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 30, 2020	Feb. 27, 2019	Jun. 30, 2018	Jun. 30, 2018	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jun. 12, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue						$ 6,308,000	$ 1,114,200,000	$ 6,868,000
Public offering of common stock, net of offering costs, Shares		3,833,334					6,241,473
Collaboration receivable - related party						18,506,000
Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue								$ 2,911,000
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						0	1,100,000,000
Standalone selling price of license performance obligation							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
Operating expenses						91,130,000
Collaboration receivable - related party						18,500,000
Proceeds from related parties					$ 72,600,000
Payments To Related Parties						$ 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%
Biogen stock purchase agreement, description						The standstill restrictions 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. BIMA has also agreed not to, and to cause its affiliates not to, sell or transfer any of the Biogen Shares for a period of eighteen months from the closing of the sale of the Biogen Shares, which expires June 30, 2022, and to limit sales and transfers of the Biogen Shares for an additional eighteen-month period, in each case subject to specified conditions and exceptions.
Stock issued							$ 417,500,000