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Share-based payments (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Board of Directors          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares granted 14,300 13,000      
Aggregate fair value of awards at grant dates $ 0.6 $ 0.6      
Time-Based Restricted Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting period         3 years
Performance-Based Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting period         3 years
Restricted Stock Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate fair values at grant date     $ 4.7    
Share-based incentives, shares issued     109,000    
Average grant price (in usd per share)     $ 43.39    
Restricted Stock Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate fair values at grant date       $ 4.7  
Share-based incentives, shares issued       107,000  
Average grant price (in usd per share)       $ 44.21