Leases, Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Commitments And Contingencies [Line Items]
Research and development expense	$ 66,170,000	$ 73,320,000	$ 124,226,000	$ 136,930,000
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] | 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	$ 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			3,600,000
Milestone payments			3,600,000
Intangible asset related to milestone	$ 3,000,000.0		$ 3,000,000.0