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Share-based compensation - Option activity and weighted-average assumptions (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Weighted average exercise price    
Total weighted average grant date fair value of options issued during the period (in $ millions) $ 11.5  
Proceeds from option sales during the period (in $ millions) $ 0.9  
Weighted-average assumptions used to estimate fair value of share options granted during year    
Risk free interest rate (as a percent) 1.44%  
Risk free interest rate, maximum (as a percent)   2.87%
2014 Plan | Share options    
Options    
Outstanding at beginning of year (in shares) 2,683,104  
Granted (in shares) 393,302  
Forfeited (in shares) (8,214)  
Exercised (in shares) (64,762)  
Outstanding at end of year (in shares) 3,003,430  
Thereof, fully vested and exercisable at end of period (in shares) 1,528,351  
Thereof, outstanding and expected to vest at end of period (in shares) 1,475,079  
Weighted average exercise price    
Outstanding at beginning of year (in dollars per share) $ 21.29  
Granted (in dollars per share) 51.81  
Forfeited (in dollars per share) 40.82  
Exercised (in dollars per share) 14.38  
Outstanding at end of year (in dollars per share) 25.38  
Thereof, fully vested and exercisable at end of year (in dollars per share) 15.75  
Outstanding and expected to vest at end of year (in dollars per share) $ 35.36  
Weighted-average assumptions used to estimate fair value of share options granted during year    
Expected volatility (as a percent) 70.00% 75.00%
Expected terms (in years) 10 years 10 years
Risk free interest rate, minimum (as a percent) 1.44% 2.80%
Expected dividend (as a percent) 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