Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Noncurrent deferred tax assets:        
Capitalized research and development expenses $ 59,489 $ 46,568    
Net operating loss carryforwards 14,991 11,186    
Tax credit carryforwards 14,297 11,570    
Capitalized legal expenses 2,119 1,773    
Stock-based compensation 6,027 2,780    
Accrued expenses 783 648    
Other temporary differences 14 16    
Total noncurrent deferred tax assets 97,720 74,541    
Total gross deferred tax assets 97,720 74,541    
Valuation allowance (97,720) (74,541) $ (42,398) $ (27,831)
Net deferred tax assets $ 0 $ 0