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Stock-Based Compensation - Weighted Average Fair Value Assumptions (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected stock option life     5 years 1 month 24 days
Expected volatility     48.82%
Risk-free interest rate     2.35%
Expected dividend yield 0.00% 0.00% 0.00%
2018 Equity Incentive Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected stock option life   8 years 11 months 19 days 5 years 9 months 14 days
Expected volatility   61.33% 58.54%
Risk-free interest rate   2.10% 2.92%
Expected dividend yield 0.00% 0.00% 0.00%