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, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based compensation.
Expected dividend yield	0.00%
Stock option | Employees and directors
Share-based compensation.
Risk-free interest rate	4.00%	2.20%	0.60%
Expected volatility	82.90%	82.50%	79.80%
Expected term (in years)	6 years 1 month 6 days	6 years	6 years
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 54.0
Unrecognized compensation cost expected to be recognized over a weighted average period	2 years 7 months 6 days