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Share-based Compensation - Valuation (Details) - Employee stock option
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock options:      
Expected volatility, minimum (as a percent) 51.00% 48.10% 48.00%
Expected volatility, maximum (as a percent) 58.00% 51.00% 49.20%
Risk-free interest rate, minimum (per annum) 0.66%    
Risk-free interest rate, maximum (per annum) 1.92%    
Minimum      
Stock options:      
Contractual term (in years) 10 years 9 years 11 months 1 day 10 years
Exercise multiple 2.2 2.2 2.2
Risk-free interest rate, minimum (per annum)   1.68% 2.70%
Maximum      
Stock options:      
Contractual term (in years) 10 years 10 years 10 years
Exercise multiple 2.8 2.8 2.8
Risk-free interest rate, maximum (per annum)   2.69% 2.90%