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Stockholders' Equity - Stock-based Compensation Expenses (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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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

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