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
	Mar. 31, 2022	Mar. 31, 2021
Share-based compensation.
Risk-free interest rate	1.50%	0.50%
Expected volatility	81.50%	79.50%
Expected term (in years)	6 years	6 years
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 54.5
Unrecognized compensation cost expected to be recognized over a weighted average period	3 years 3 months 18 days