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Share-based compensation plans - Stock activity - DSUs (Details)
1 Months Ended 6 Months Ended 12 Months Ended
Jan. 31, 2016
director
$ / shares
shares
Jun. 30, 2016
$ / shares
shares
Dec. 31, 2015
$ / shares
shares
Deferred Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years)     1 year
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, beginning balance, shares | shares 21,400 21,400  
Granted, shares | shares   26,000  
Forfeited, shares | shares   0  
Vested, shares | shares   (21,400)  
Nonvested, ending balance, shares | shares   26,000 21,400
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Nonvested, beginning balance (dollars per share) | $ / shares $ 32.79 $ 32.79  
Granted (dollars per share) | $ / shares   29.35  
Forfeited (dollars per share) | $ / shares   0.00  
Vested (dollars per share) | $ / shares   31.72  
Nonvested, ending balance (dollars per share) | $ / shares   $ 29.35 $ 32.79
Director | Severance Costs | Executive Leadership and Board of Directors Restructuring      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of directors retired | director 2