Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended							6 Months Ended		12 Months Ended
	Nov. 30, 2020	Jun. 30, 2018	Jan. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2018	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2020	Jun. 12, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				$ 8,654,000			$ 2,474,000				$ 16,556,000	$ 5,768,000
Collaboration receivable - related party				9,943,000		$ 83,009,000					9,943,000		$ 83,009,000
Shionogi Collaboration Agreement [Member] | Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				600,000			14,000				600,000	14,000
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
Issuance of common stock, net of issuance costs ,Shares														6,241,473
Purchase price														$ 650,000,000.0
Net reimbursement				(7,420,000)							(13,633,000)		8,000,000
Milestone achieved													75,000,000
Payments to related parties											0	0
Proceeds from related parties					$ 93,100,000	93,100,000		$ 27,800,000	$ 27,800,000
Biogen Collaboration Agreement [Member] | Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivable - related party				9,900,000		83,000,000					9,900,000		$ 83,000,000
Biogen Collaboration Agreement [Member] | License And Milestone Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				$ 0		75,000,000	$ 0				$ 0	$ 0
Milestone payments			$ 75,000,000
Biogen Collaboration Agreement [Member] | SAGE-217 [Member] | First Commercial Sale Of Zurzuvae For Treatment Of Women With Ppd [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone achieved						$ 75,000,000
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] | Treatment of Essential Tremor [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	150,000,000
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
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