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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, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Outstanding at December 31, 2015 (in shares) 4,721,941  
Granted (in shares) 1,180,086  
Canceled and forfeited (in shares) (478,078)  
Exercised (in shares) (227,696)  
Outstanding at December 31, 2016 (in shares) 5,196,253 4,721,941
Options vested and expected to vest at December 31, 2016 (in shares) 4,792,694  
Options exercisable at December 31, 2016 (in shares) 3,437,421  
Weighted- average exercise price per share    
Outstanding at December 31, 2015 (in dollars per share) $ 12.00  
Granted (in dollars per share) 17.90  
Canceled and forfeited (in dollars per share) 15.90  
Exercised (in dollars per share) 4.78  
Outstanding at December 31, 2016 (in dollars per share) 13.32 $ 12.00
Options vested and expected to vest at December 31, 2016 (in dollars per share) 13.07  
Options exercisable at December 31, 2016 (in dollars per share) $ 11.90  
Weighted- average remaining contractual term (in years)    
Outstanding 6 years 11 months 23 days 7 years 4 months 28 days
Options vested and expected to vest as December 31, 2016 6 years 10 months 2 days  
Options exercisable at December 31, 2016 6 years 2 months 9 days  
Aggregate intrinsic value (in thousands)    
Outstanding $ 3,861 $ 48,670
Options vested and expected to vest at December 31, 2016 3,861  
Options exercisable at December 31, 2016 $ 3,861