Share-based compensation - Assumptions used in the Black Scholes option pricing model to determine the fair value of share options (Details) - Stock option - Employees and directors - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based compensation.
Risk-free interest rate	1.00%	0.40%	0.50%	1.50%
Expected volatility	80.50%	76.80%	79.50%	74.10%
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years	6 years
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 25.7		$ 25.7