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Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Equity-based compensation expense $ 954 $ 919
Research and Development Expense    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Equity-based compensation expense 284 377
General and Administrative Expense    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Equity-based compensation expense $ 670 $ 542