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Stock-Based Compensation - Summary of Option Activity for 2006 Plan and the 2016 Plan (Detail) - 2006 Plan and 2016 Plan - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Number of Shares    
Beginning balance (in shares) 4,369,042  
Granted (in shares) 502,500  
Forfeited (in shares) 0  
Exercised (in shares) (291,451)  
Ending balance (in shares) 4,580,091 4,369,042
Exercisable (in shares) 3,433,142  
Vested and expected to vest at end of period (in shares) 4,371,265  
Weighted Average Exercise Price at Grant Date    
Beginning balance (in dollars per share) $ 11.15  
Granted (in dollars per share) 19.95  
Forfeited (in dollars per share) 0  
Exercised (in dollars per share) 12.32  
Ending balance (in dollars per share) 12.04 $ 11.15
Exercisable (in dollars per share) 10.09  
Vested and expected to vest at end of period (in dollars per share) $ 11.68  
Weighted Average Remaining Term (Years)    
Weighted Average Remaining Term 5 years 7 months 9 days 5 years 3 months 10 days
Exercisable 4 years 6 months 18 days  
Vested and expected to vest at end of period 5 years 5 months 8 days  
Aggregate Intrinsic Value    
Beginning balance $ 65,438  
Exercised 960  
Ending balance 15,136 $ 65,438
Exercisable 14,626  
Vested and expected to vest at end of period $ 15,131