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Share-Based Compensation - Fair value of each stock option issued to employees - (Details) - Employee
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 110.80% 110.80%
Expected term (years) 6 years 3 months 21 days 6 years 10 days
Risk-free interest rate 1.06% 1.70%