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Stock-Based Compensation - Schedule of Estimated Grant-date Fair Values of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 5 years 3 months 18 days 5 years 3 months 18 days 5 years 3 months 18 days
Expected volatility, Minimum 87.00% 89.00% 88.00%
Expected volatility, Maximum 90.00% 94.00% 91.00%
Risk-free interest rate, Minimum 0.30% 1.60% 2.60%
Risk-free interest rate, Maximum 1.20% 2.60% 3.10%
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 7 years 6 years 10 months 24 days 7 years