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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 19,330 $ 14,677 $ 9,089
Research and development      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense 10,135 8,249 5,031
General and administrative      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 9,195 $ 6,428 $ 4,058