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Share-based compensation - Share option valuation (Details) - Stock option - Employees and directors - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate 1.70% 3.00% 2.20% 2.50%
Expected volatility 77.00% 76.00% 78.30% 75.80%
Expected term (in years) 6 years 6 years 26 days 5 years 10 months 17 days 6 years 26 days
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized        
Total unrecognized compensation expense related to the unvested employee and director $ 11.4   $ 11.4  
Unrecognized compensation cost expected to be recognized over a weighted average period     2 years 9 months 18 days