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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Share-based compensation.
Risk-free interest rate	2.80%	1.00%	1.80%	0.50%
Expected volatility	82.90%	80.50%	81.90%	79.50%
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	$ 55.0		$ 55.0
Unrecognized compensation cost expected to be recognized over a weighted average period			3 years 2 months 12 days