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Fair Values of Stock Options Estimated Using Black-Scholes Valuation Model (Detail) - Employee Stock Option - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Black Scholes Method Used [Line Items]        
Risk-free rate 0.91% 0.39% 0.76% 0.96%
Expected term (in years) 6 years 6 years 1 month 6 days 6 years 5 years 9 months 18 days
Dividend yield 0.00% 0.00% 0.00% 0.00%
Volatility 90.95% 115.62% 93.34% 112.87%
Weighted-average fair value of stock options granted $ 3.23 $ 4.61 $ 4.31 $ 5.17