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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, 2019
Jun. 30, 2018
Jun. 30, 2019
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% 0.00% 0.00%  
2014 Plan          
Weighted-average assumptions used to estimate fair value of share options granted during year          
Authorized shares   8,601,471   8,601,471 3,000,000
2014 Plan | Share options          
Options          
Outstanding at beginning of year (in shares)     2,673,712    
Granted (in shares)     530,276    
Forfeited (in shares)     (41,602)    
Expired (in shares)     (543)    
Exercised (in shares)     (218,087)    
Outstanding at end of year (in shares) 2,943,756   2,943,756    
Fully vested and exercisable at end of year (in shares) 1,264,254   1,264,254    
Outstanding and expected to vest at end of year (in shares) 1,679,502   1,679,502    
Weighted average exercise price          
Outstanding at beginning of year (in dollars per share)     $ 15.09    
Granted (in dollars per share)     36.05    
Forfeited (in dollars per share)     11.88    
Expired (in dollars per share)     12.26    
Exercised (in dollars per share)     11.45    
Outstanding at end of year (in dollars per share) $ 19.16   19.16    
Fully vested and exercisable at end of year (in dollars per share) $ 12.37   12.37    
Outstanding and expected to vest at end of year (in dollars per share)     $ 24.27    
Total weighted average grant date fair value of options issued during the period (in $ millions)     $ 11.2    
Proceeds from option sales     $ 2.5    
Weighted-average assumptions used to estimate fair value of share options granted during year          
Expected volatility (as a percent) 75.00% 75.00% 75.00% 75.00%  
Expected terms (in years) 10 years 10 years 10 years 10 years  
Risk free interest rate, minimum (as a percent) 2.17% 2.99% 2.17% 2.77%  
Risk free interest rate, maximum (as a percent) 2.63% 3.07% 2.87% 3.07%  
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