Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitments And Contingencies [Line Items]
Research and development expense		$ 368,815,000	$ 282,107,000	$ 210,277,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense		1,000,000.0
Milestone payments	$ 3,000,000.0
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] | Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense				0
Milestone payments		0		$ 0
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] | Brexanolone [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			800,000
Milestone payments			$ 800,000
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		2,300,000
Milestone payments		2,300,000
Intangible asset related to milestone		3,000,000.0
Milestone payments related to intangible assets		$ 3,000,000.0