Leases, Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Commitments And Contingencies [Line Items]
Research and development expense	$ 283,166,000	$ 292,714,000	$ 368,815,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense	1,000,000.0
CyDex License Agreement [Member] | First and Second Clinical Development Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] | First and Second Regulatory Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	3,800,000
CyDex License Agreement [Member] | Third and Fourth Clinical Development Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	1,300,000
CyDex License Agreement [Member] | Third and Fourth Regulatory Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	8,500,000
CyDex License Agreement [Member] | Clinical Development [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] | Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Construction Expense Related To Milestone Payment	1,300,000
CyDex License Agreement [Member] | Regulatory Milestones [Member] | Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense	100,000
Milestone payments	100,000
Intangible asset related to milestone			$ 3,000,000.0
CyDex License Agreement [Member] | Regulatory Milestones [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	1,800,000
CyDex License Agreement [Member] | Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense	3,700,000
Milestone payments	3,700,000
Intangible asset related to milestone	$ 3,000,000.0