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Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 and 2017 Plan (Detail) - 2015 and 2017 Plan - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding Beginning balance, Number of options 1,267,221  
Granted, Number of options 919,496  
Exercised, Number of options (19,830)  
Forfeited, Number of options (394,136)  
Expired, Number of options (32,867)  
Outstanding Ending balance, Number of options 1,739,884 1,267,221
Number of options, Vested or expected to vest 1,739,884  
Number of options, Exercisable 489,236  
Beginning balance, Weighted-average price $ 13.62  
Granted, Weighted-average price 9.71  
Exercised, Weighted-average price 12.28  
Forfeited, Weighted-average price 12.09  
Expired, Weighted-average price 13.53  
Ending balance, Weighted-average price 11.92 $ 13.62
Weighted-average price Vested or expected to vest 11.92  
Weighted-average price, Exercisable $ 13.20  
Outstanding, weighted average remaining contractual term (Year) 9 years 9 years 7 months 6 days
Weighted average remaining contractual term, Vested or expected to vest 9 years  
Weighted average remaining contractual term, Exercisable 8 years 4 months 24 days