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Share-based compensation - Assumptions used in the Black Scholes option pricing model to determine the fair value of share options (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based compensation.      
Expected dividend yield 0.00%    
Stock option | Employees and directors      
Share-based compensation.      
Risk-free interest rate 1.30% 2.10% 2.70%
Expected volatility 74.80% 77.90% 78.60%
Expected term (in years) 5 years 11 months 23 days 5 years 10 months 10 days 6 years 26 days
Unrecognized compensation expense      
Total unrecognized compensation expense related to the unvested employee and director $ 13.7    
Unrecognized compensation cost expected to be recognized over a weighted average period 2 years 6 months