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Stock-Based Awards - Summary of Stock-Based Compensation Expense Related to Stock Options, Restricted Common Stock and ESPP (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 [Line Items]        
Total stock-based compensation expense $ 604 $ 2,200 $ 4,555 $ 7,337
Research and Development [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense (359) 1,027 1,337 4,263
General and Administrative [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 963 $ 1,173 $ 3,218 $ 3,074