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		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Share-based compensation.
Risk-free interest rate	3.00%	0.90%	2.00%	0.60%
Expected volatility	84.10%	80.20%	82.20%	79.60%
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	$ 52.7		$ 52.7
Unrecognized compensation cost expected to be recognized over a weighted average period			3 years