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Share-based compensation plans - Narratives (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
share_based_plan
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
share_based_plan
Jun. 30, 2016
USD ($)
Share-based compensation plans        
Number of share-based compensation plans | share_based_plan 2   2  
Compensation cost $ 2,019 $ 1,507 $ 3,554 $ 3,149
Total unrecognized compensation cost related to unexercised stock options and unvested share awards $ 11,176   $ 11,176  
Total unrecognized compensation cost related to unexercised stock options and unvested share awards, recognition period (in months)     30 months  
Deferred shares        
Share-based compensation plans        
Maximum expected life of awards (in years)     4 years  
Restricted shares        
Share-based compensation plans        
Maximum expected life of awards (in years)     4 years  
Share options        
Share-based compensation plans        
Maximum expected life of awards (in years)     10 years