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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation expense $ 4,701 $ 3,810 $ 14,091 $ 10,374
Research and development        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation expense 2,268 2,093 7,329 5,972
General and administrative        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Total stock-based compensation expense $ 2,433 $ 1,717 $ 6,762 $ 4,402