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Stock-Based Compensation - Schedule of Employee Stock Option Activity (Detail) - Employee Stock Options [Member] - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Number of shares, Outstanding Beginning Balance 1,456,956  
Number of shares, Options granted 525,571  
Number of shares, Outstanding Ending Balance 1,982,527  
Number of shares, Options vested and exercisable Ending Balance 320,365  
Weighted average exercise price, Outstanding Beginning Balance $ 8.38  
Exercise price 10.95 $ 6.52
Weighted average exercise price, Outstanding Ending Balance 9.06  
Weighted average exercise price, Options vested and exercisable Ending Balance $ 6.55  
Weighted average remaining contractual life (in years), Outstanding 9 years 2 months 12 days  
Weighted average remaining contractual life (in years), Options vested and exercisable 8 years 10 months 24 days  
Aggregate intrinsic value, Outstanding Ending Balance $ 4,947,269  
Aggregate intrinsic value, Options vested and exercisable Ending Balance $ 1,363,844