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Share-based payments (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
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 $ 3.7  
Share-based incentives, shares issued 87  
Average grant price (in usd per share) $ 43.06  
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate fair values at grant date   $ 3.9
Share-based incentives, shares issued   90
Average grant price (in usd per share)   $ 43.33