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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 - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding Beginning balance, Number of options 1,739,884  
Granted, Number of options 1,171,030  
Forfeited, Number of options (508,915)  
Outstanding Ending balance, Number of options 2,401,999 1,739,884
Number of options, Vested or expected to vest 2,401,999  
Number of options, Exercisable 805,772  
Beginning balance, Weighted-average price $ 11.92  
Granted, Weighted-average price 8.08  
Forfeited, Weighted-average price 11.76  
Ending balance, Weighted-average price 10.08 $ 11.92
Weighted-average price Vested or expected to vest 10.08  
Weighted-average price, Exercisable $ 11.86  
Outstanding, weighted average remaining contractual term (Year) 8 years 9 months 18 days 9 years
Weighted average remaining contractual term, Vested or expected to vest 8 years 9 months 18 days  
Weighted average remaining contractual term, Exercisable 8 years 3 months 18 days  
Forfeited, Aggregate Intrinsic value $ 25