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Stock-Based Awards - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 2,671 $ 2,102 $ 5,137 $ 4,103
Research and Development [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 1,664 877 3,236 1,733
General and Administrative [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 1,007 $ 1,225 $ 1,901 $ 2,370