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, 2021	Mar. 31, 2020
Share-based compensation.
Risk-free interest rate	0.50%	1.60%
Expected volatility	79.50%	73.70%
Expected term (in years)	5 years 11 months 12 days	5 years 11 months 12 days
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 26.0
Unrecognized compensation cost expected to be recognized over a weighted average period	3 years 1 month 6 days