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Share-based compensation - Share option valuation (Details) - Stock option - Employees and directors - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate 2.20% 1.30% 2.20% 1.30%
Expected volatility 78.00% 75.10% 78.10% 75.10%
Expected term (in years) 5 years 9 months 29 days 6 years 26 days 5 years 10 months 6 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 $ 15.9   $ 15.9  
Unrecognized compensation cost expected to be recognized over a weighted average period     3 years