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Stock-based Compensation - Expense Related to Stock Options (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense related to stock options $ 398 $ 410
Research and development, net    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense related to stock options 126 136
Selling, general and administrative    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense related to stock options $ 272 $ 274