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Share-based compensation - 2014 Plan Share Options (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Jun. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Options          
Granted (in shares) 10,000        
Weighted average exercise price          
Vesting period 3 years        
Weighted-average assumptions used to estimate fair value of share options granted during year          
Expected volatility (as a percent)   80.00% 80.00% 80.00%  
Expected terms (in years)   10 years 10 years 10 years 10 years
Risk free interest rate, minimum (as a percent)   3.10%   2.67% 2.40%
Risk free interest rate, maximum (as a percent)       3.10% 2.81%
Expected dividend (as a percent)   0.00% 0.00% 0.00% 0.00%
2014 Plan          
Options          
Outstanding at beginning of year (in shares)       72,818  
Outstanding at end of year (in shares)   32,567   32,567  
Weighted-average assumptions used to estimate fair value of share options granted during year          
Increase in authorized shares 30,000,000        
Authorized shares   8,601,471   8,601,471  
2014 Plan | Share options          
Options          
Outstanding at beginning of year (in shares)       2,456,433  
Granted (in shares)       906,582  
Forfeited (in shares)       (266,222)  
Expired (in shares)       (9,915)  
Exercised (in shares)       (379,340)  
Outstanding at end of year (in shares)   2,707,538   2,707,538  
Fully vested and exercisable at end of year (in shares)   977,409   977,409  
Outstanding and expected to vest at end of year (in shares)   1,730,129   1,730,129  
Weighted average exercise price          
Outstanding at beginning of year (in dollars per share)       $ 10.06  
Granted (in dollars per share)       26.05  
Forfeited (in dollars per share)       13.18  
Expired (in dollars per share)       13.49  
Exercised (in dollars per share)       11.42  
Outstanding at end of year (in dollars per share)   $ 14.91   14.91  
Fully vested and exercisable at end of year (in dollars per share)   $ 11.03   11.03  
Outstanding and expected to vest at end of year (in dollars per share)       $ 17.10  
Total weighted average grant date fair value of options issued during the period (in $ millions)       $ 15.8  
Proceeds from option sales       $ 4.6  
2014 Plan | Non-executive directors          
Weighted average exercise price          
Vesting percentage per year       25.00%  
Vesting period       4 years  
2014 Plan | Directors and Officers | Share options          
Options          
Granted (in shares)       315,156  
Weighted average exercise price          
Total weighted average grant date fair value of options issued during the period (in $ millions)       $ 4.7  
2014 Plan | One year from grant date          
Weighted average exercise price          
Vesting period       1 year