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Stockholders' Equity - Stock-based Compensation Expenses (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Stock-based compensation $ 745 $ 589 $ 1,463 $ 1,319
Research and Development Expense [Member]        
Stock-based compensation 251 84 424 177
General and Administrative Expense [Member]        
Stock-based compensation $ 494 $ 505 $ 1,039 $ 1,142