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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 [Line Items]        
Stock based compensation expense $ 185 $ 660 $ 1,735 $ 2,358
Research and Development [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock based compensation expense 14 244 81 760
General and Administrative [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock based compensation expense $ 171 $ 416 $ 1,654 $ 1,598