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Share-based compensation - Option activity and weighted-average assumptions (Details) - 2014 Plan - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Jun. 30, 2018
May 31, 2018
Weighted-average assumptions used to estimate fair value of share options granted during year        
Expected dividend (as a percent) 0.00% 0.00%    
Authorized shares     8,601,471 3,000,000
Share options        
Options        
Outstanding at beginning of year (in shares) 2,673,712      
Granted (in shares) 502,526      
Forfeited (in shares) (41,479)      
Expired (in shares) (75)      
Exercised (in shares) (165,240)      
Outstanding at end of year (in shares) 2,969,444      
Fully vested and exercisable at end of year (in shares) 1,176,022      
Outstanding and expected to vest at end of year (in shares) 1,793,422      
Weighted average exercise price        
Outstanding at beginning of year (in dollars per share) $ 15.09      
Granted (in dollars per share) 34.35      
Forfeited (in dollars per share) 11.77      
Expired (in dollars per share) 7.49      
Exercised (in dollars per share) 12.35      
Outstanding at end of year (in dollars per share) 18.54      
Fully vested and exercisable at end of year (in dollars per share) 11.73      
Outstanding and expected to vest at end of year (in dollars per share) $ 23.01      
Total weighted average grant date fair value of options issued during the period (in $ millions) $ 10.1      
Proceeds from option sales $ 2.0      
Weighted-average assumptions used to estimate fair value of share options granted during year        
Expected volatility (as a percent) 75.00% 75.00%    
Expected terms (in years) 10 years 10 years    
Risk free interest rate, minimum (as a percent) 2.80% 2.77%    
Risk free interest rate, maximum (as a percent) 2.87% 2.99%    
Non-executive directors        
Weighted average exercise price        
Vesting percentage per year 25.00%      
Vesting period (in years) 4 years      
One year from grant date        
Weighted average exercise price        
Vesting period (in years) 1 year