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Share-based compensation plans - Stock activity - RSUs, PSUs, MSUs(Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, beginning balance, shares 296,500    
Granted, shares 91,700    
Forfeited, shares (10,800)    
Vested, shares (88,600)    
Nonvested, ending balance, shares 288,800   296,500
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) $ 27.84    
Granted (dollars per share) 51.96    
Forfeited (dollars per share) 27.31    
Vested (dollars per share) 27.47    
Nonvested, ending balance (dollars per share) $ 35.63   $ 27.84
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, beginning balance, shares 603,200    
Granted, shares 214,100    
Forfeited, shares (12,100)    
Vested, shares (134,300)    
Nonvested, ending balance, shares 670,900   603,200
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) $ 28.02    
Granted (dollars per share) 53.66    
Forfeited (dollars per share) 27.53    
Vested (dollars per share) 24.39    
Nonvested, ending balance (dollars per share) $ 36.94   $ 28.02
Number of shares earned and distributed   252,000,000  
Market Share Units MSU      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Nonvested, beginning balance, shares 141,700    
Granted, shares 0    
Forfeited, shares 0    
Vested, shares (67,500)    
Nonvested, ending balance, shares 74,200   141,700
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) $ 27.02    
Granted (dollars per share) 0.00    
Forfeited (dollars per share) 0.00    
Vested (dollars per share) 23.34    
Nonvested, ending balance (dollars per share) $ 30.37   $ 27.02
Number of shares earned and distributed     81,800,000