Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended				9 Months Ended			12 Months Ended
	Nov. 30, 2020	Jun. 30, 2018	Jan. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2018	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2020	Jun. 12, 2018
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue											$ 41,243,000	$ 86,455,000	$ 7,686,000
Collaboration receivable - related party				$ 83,009,000							9,134,000	83,009,000
Other Collaboration Revenue [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue											1,582,000	177,000
Shionogi Collaboration Agreement [Member] | Collaboration Revenue [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue											600,000	200,000	0
Shionogi Collaboration Agreement [Member] | SAGE-217 [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [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] | Regulatory Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments												55,000,000.0
Shionogi Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [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 Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments		70,000,000.0
Shionogi Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member] | Commercial Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments		30,000,000.0
Shionogi Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member] | Sales Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments		$ 385,000,000.0
Biogen Collaboration Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement effective date	Dec. 28, 2020
Upfront payment														$ 875,000,000
Revenue														1,100,000,000
Sale of stock, consideration received														$ 1,500,000,000
Issuance of common stock upon public offering, net of issuance costs, Shares														6,241,473
Purchase price														$ 650,000,000
Net reimbursement				8,000,000							(36,087,000)	(824,000)
Milestone achieved											75,000,000
Payments to related parties											0	0	0
Proceeds from related parties				118,100,000	$ 65,700,000	$ 80,300,000		$ 118,100,000	$ 65,700,000	$ 80,300,000
Biogen Collaboration Agreement [Member] | Related Party [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration receivable - related party				83,000,000							9,100,000	83,000,000
Biogen Collaboration Agreement [Member] | Other Collaboration Revenue [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue											900,000	$ 0	0
Biogen Collaboration Agreement [Member] | License And Milestone Revenue - Related Party [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue				75,000,000							$ 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 Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments				150,000,000
Milestone achieved				$ 75,000,000
Biogen Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [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 Arrangement and Arrangement Other than Collaborative [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 Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments	75,000,000
Biogen Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member] | Sales Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments	300,000,000
Biogen Collaboration Agreement [Member] | SAGE-217 [Member] | Maximum [Member] | Regulatory and Commercial Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments	475,000,000
Biogen Collaboration Agreement [Member] | SAGE-324 [Member] | Maximum [Member] | Sales Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments	300,000,000
Biogen Collaboration Agreement [Member] | SAGE-324 [Member] | Maximum [Member] | Regulatory and Commercial Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone payments	$ 520,000,000
Biogen Stock Purchase Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [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