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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based compensation.
Risk-free interest rate	0.40%	2.20%	1.50%	2.20%
Expected volatility	76.80%	78.00%	74.10%	78.10%
Expected term (in years)	6 years 1 month 6 days	5 years 9 months 18 days	6 years	5 years 10 months 24 days
Unrecognized compensation expense
Total unrecognized compensation expense related to the unvested employee and director	$ 14.5		$ 14.5
Unrecognized compensation cost expected to be recognized over a weighted average period			2 years 9 months 18 days