Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments And Contingencies [Line Items]
Research and development expense	$ 68,980,000	$ 55,900,000	$ 118,250,000	$ 101,100,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense			1,000,000
Research and development expense related to milestone expense	800,000	0	800,000	0
Milestone payments	800,000	$ 0	800,000	$ 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] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000		800,000
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			2,300,000
Milestone payments			$ 2,300,000