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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based compensation.
Risk-free interest rate	0.30%	1.70%	1.40%	2.20%
Expected volatility	74.10%	77.00%	74.10%	78.30%
Expected term (in years)	6 years 1 month 6 days	6 years	6 years	5 years 10 months 24 days
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 14.7		$ 14.7
Unrecognized compensation cost expected to be recognized over a weighted average period			2 years 8 months 12 days