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Stockholders Equity and Stock Options - Assumptions used to Determine Fair Value of Stock Options Granted (Detail)
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Stockholders Equity Note [Abstract]    
Risk-free interest rate 0.88% 0.37%
Expected term (in years) 6 years 2 months 8 days 6 years 29 days
Expected volatility 91.00% 91.00%
Dividend yield 0.00% 0.00%