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Equity Incentive Plans - Valuation Assumptions for Stock Options (Details) - Stock options
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (years) 4 years 11 months 19 days 6 years 6 years
Expected volatility 77.00% 83.00% 81.00%
Risk-free interest rate 4.69% 1.07% 2.42%
Dividend yield 0.00% 0.00% 0.00%