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Share-based compensation - 2014 Plan RSUs (Details) - 2014 Plan - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Mar. 31, 2017
Mar. 31, 2018
Non-executive directors    
Other disclosure    
Vesting period   4 years
Restricted share units ("RSUs")    
Number of shares    
Non-vested at beginning of year (in shares)   683,663
Granted (in shares)   207,599
Vested (in shares)   (218,500)
Forfeited (in shares)   (10,950)
Non-vested at end of year (in shares)   661,812
Weighted average grant-date fair value    
Non-vested at beginning of year (in dollars per share)   $ 6.38
Granted (in dollars per share)   19.39
Vested (in dollars per share)   6.11
Forfeited (in dollars per share)   9.26
Non-vested at end of year (in dollars per share)   $ 10.51
Other disclosure    
Total fair value of units awarded during the period (in million)   $ 4.0
Restricted share units ("RSUs") | Chief Executive Officer    
Other disclosure    
Vesting period 2 years  
Restricted share units ("RSUs") | Non-executive directors    
Other disclosure    
Vesting period 1 year  
Restricted share units ("RSUs") | Directors and Officers    
Number of shares    
Granted (in shares)   98,808
Other disclosure    
Total fair value of units awarded during the period (in million)   $ 1.9
Minimum | Restricted share units ("RSUs")    
Other disclosure    
Vesting period   1 year
Maximum | Restricted share units ("RSUs")    
Other disclosure    
Vesting period   3 years