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Share-based payments (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 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     $ 4.8  
Share-based incentives, shares issued     110,600  
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       $ 5.1
Share-based incentives, shares issued       120,000
Average grant price (in usd per share)       $ 42.55