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Stock-based Compensation - Stock Option Activity and Related Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Outstanding at December 31, 2015 (in shares) 5,196,253  
Granted (in shares) 1,186,944  
Canceled and forfeited (in shares) (910,243)  
Exercised (in shares) (430,602)  
Outstanding at December 31, 2016 (in shares) 5,042,352 5,196,253
Options vested and expected to vest at December 31, 2016 (in shares) 5,042,352  
Options exercisable at December 31, 2016 (in shares) 3,381,664  
Weighted- average exercise price per share    
Outstanding at December 31, 2015 (in dollars per share) $ 13.32  
Granted (in dollars per share) 8.69  
Canceled and forfeited (in dollars per share) 14.48  
Exercised (in dollars per share) 8.14  
Outstanding at December 31, 2016 (in dollars per share) 12.46 $ 13.32
Options vested and expected to vest at December 31, 2016 (in dollars per share) 12.46  
Options exercisable at December 31, 2016 (in dollars per share) $ 12.48  
Weighted- average remaining contractual term (in years)    
Outstanding 6 years 5 months 25 days 6 years 11 months 23 days
Options vested and expected to vest as December 31, 2016 6 years 5 months 25 days  
Options exercisable at December 31, 2016 5 years 5 months 27 days  
Aggregate intrinsic value (in thousands)    
Outstanding $ 18,255 $ 3,861
Options vested and expected to vest at December 31, 2016 18,255  
Options exercisable at December 31, 2016 $ 12,099