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Share-based Compensation - Valuation assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation.      
Expected volatility     57.30%
Weighted average volatility     57.30%
Expected dividends   0.00% 0.00%
Risk-free rate     3.10%
Contractual term (in years) 0 years 0 years 10 years
Enterprise value     $ 0.65