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Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Commitments And Contingencies [Line Items]    
Research and development expense $ 49,270,000 $ 45,200,000
CyDex License Agreement [Member]    
Commitments And Contingencies [Line Items]    
Research and development expense 1,000,000  
Research and development expense related to milestone expense 0 0
Milestone payments 0 $ 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] | 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 1,500,000  
Milestone payments $ 1,500,000