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Share-based compensation - Option activity and weighted-average assumptions (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Weighted average exercise price        
Total weighted average grant date fair value of options issued during the period (in $ millions)     $ 14.2  
Proceeds from option sales during the period (in $ millions)     $ 3.4  
2014 Plan | Share options        
Options        
Outstanding at beginning of year (in shares)     2,683,104  
Granted (in shares)     470,327  
Forfeited (in shares)     (69,758)  
Exercised (in shares)     (203,940)  
Outstanding at end of year (in shares) 2,879,733   2,879,733  
Thereof, fully vested and exercisable at end of period (in shares) 1,530,724   1,530,724  
Thereof, outstanding and expected to vest at end of period (in shares) 1,349,009   1,349,009  
Weighted average exercise price        
Outstanding at beginning of year (in dollars per share)     $ 21.29  
Granted (in dollars per share)     53.26  
Forfeited (in dollars per share)     36.13  
Exercised (in dollars per share)     16.68  
Outstanding at end of year (in dollars per share) $ 26.47   26.47  
Thereof, fully vested and exercisable at end of year (in dollars per share) $ 16.06   16.06  
Outstanding and expected to vest at end of year (in dollars per share)     $ 38.30  
Weighted-average assumptions used to estimate fair value of share options granted during year        
Expected volatility (as a percent) 70.00% 75.00% 70.00% 75.00%
Expected terms (in years) 10 years 10 years 10 years 10 years
Risk free interest rate, minimum (as a percent) 0.76% 2.17% 0.76% 2.17%
Risk free interest rate, maximum (as a percent) 0.83% 2.63% 1.44% 2.87%
Expected dividend (as a percent) 0.00% 0.00% 0.00% 0.00%
2014 Plan | Non-executive directors        
Weighted average exercise price        
Vesting percentage per year     25.00%  
Vesting period (in years)     4 years  
2014 Plan | One year from grant date        
Weighted average exercise price        
Vesting period (in years)     1 year