Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Tax Contingency [Line Items]        
Increase in valuation allowance $ 4,800 $ 6,500    
Unrecognized tax benefits 10,116 $ 4,061 $ 332 $ 287
Employee Stock Option [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards 2,100      
Federal [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards 68,400      
Federal [Member] | Research and Development Tax Credits [Member]        
Income Tax Contingency [Line Items]        
Tax credits $ 955      
Tax credit, expiration Begin to expire in 2029      
Federal [Member] | Orphan Drug Credit [Member]        
Income Tax Contingency [Line Items]        
Tax credits $ 20,900      
Tax credit, expiration Begins to expire in 2033      
State [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards $ 55,100      
Operating loss carryforwards, expiration Begin to expire in 2029      
State [Member] | Research and Development Tax Credits [Member]        
Income Tax Contingency [Line Items]        
Tax credits $ 1,616      
Foreign [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards $ 17,400      
Operating loss carryforwards, expiration Begin to expire in 2023