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BIAL Agreement - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2017
License Agreement [Line Items]    
Milestone payables   $ 132
B I A L    
License Agreement [Line Items]    
License agreement date   Feb. 09, 2017
Up-front license fees $ (30)  
B I A L | Regulatory Approval and Net Sales Milestones    
License Agreement [Line Items]    
Milestone payables 115  
B I A L | Regulatory and Clinical Results and FDA Acceptance Milestones    
License Agreement [Line Items]    
Milestone payables $ 20