Leases, Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Commitments And Contingencies [Line Items]
Research and development expense	$ 83,497,000	$ 74,078,000		$ 207,723,000	$ 211,008,000
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			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			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			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			8,500,000
CyDex License Agreement [Member] | Clinical Development [Member] | Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense					0
Milestone payments					0
Milestone payments related to intangible assets					0
CyDex License Agreement [Member] | Clinical Development [Member] | SAGE-689 [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense	100,000			100,000
Milestone payments	100,000			100,000
CyDex License Agreement [Member] | Clinical Development [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000			800,000
CyDex License Agreement [Member] | Regulatory Milestones [Member] | Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense					0
Milestone payments					0
Intangible asset related to milestone		$ 0			0
Milestone payments related to intangible assets					$ 0
CyDex License Agreement [Member] | Regulatory Milestones [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 1,800,000			$ 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