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Share-based compensation - RSU activity (Details) - 2014 Plan - $ / shares
1 Months Ended 6 Months Ended
Mar. 31, 2017
Jun. 30, 2019
Non-executive directors    
Other disclosure    
Vesting period (in years)   4 years
Restricted share units ("RSUs")    
Number of shares    
Non-vested at beginning of year (in shares)   412,321
Restricted stock granted (in shares)   145,004
Vested (in shares)   (186,417)
Forfeited (in shares)   (14,516)
Non-vested at end of year (in shares)   356,392
Weighted average grant-date fair value    
Non-vested at beginning of year (in dollars per share)   $ 16.49
Granted (in dollars per share)   33.62
Vested (in dollars per share)   12.78
Forfeited (in dollars per share)   12.90
Non-vested at end of year (in dollars per share)   25.55
Restricted share units ("RSUs") | Non-executive directors    
Other disclosure    
Vesting period (in years) 1 year  
Restricted share units ("RSUs") | Directors and Officers    
Weighted average grant-date fair value    
Granted (in dollars per share)   $ 4,900,000
Minimum | Restricted share units ("RSUs")    
Other disclosure    
Vesting period (in years)   1 year
Maximum | Restricted share units ("RSUs")    
Other disclosure    
Vesting period (in years)   3 years