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Stock-Based Compensation Plans - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Stock-Based Compensation Plans      
Shares authorized for future grant 2,251,507    
Additional share-based compensation disclosures      
Proceed from option exercises $ 15.1 $ 28.8 $ 70.1
Stock options      
Number of shares      
Outstanding at the beginning of the period (in shares) 1,327,412 1,937,241 3,405,500
Exercised (in shares) (390,606) (593,204) (1,437,053)
Expired or forfeited (in shares) (2,481) (16,625) (31,206)
Outstanding at the end of the period (in shares) 934,325 1,327,412 1,937,241
Exercisable at the end of the period (in shares) 934,325    
Weighted Average Exercise Price      
Outstanding at the beginning of the period, Weighted Average Exercise Price (in dollars per share) $ 49.66 $ 49.35 $ 49.12
Exercised, Weighted Average Exercise Price (in dollars per share) 48.19 48.58 48.78
Expired or forfeited, Weighted Average Exercise Price (in dollars per share) 51.96 52.13 51.37
Outstanding at the end of the period, Weighted Average Exercise Price (in dollars per share) 50.26 $ 49.66 $ 49.35
Exercisable at the end of the period, Weighted Average Exercise Price (in dollars per share) $ 50.26    
Weighted Average Remaining Contractual Term      
Weighted Average Remaining Contractual Term, stock options outstanding (in years) 1 year 9 months 18 days    
Weighted Average Remaining Contractual Term, stock options exercisable (in years) 1 year 9 months 18 days    
Aggregate Intrinsic Value (In millions)      
Aggregate Intrinsic Value, options outstanding $ 7.4    
Aggregate Intrinsic Value, options exercisable $ 7.4    
Vesting period (in years) 4 years    
Award expiration term (in years) 7 years    
Changes in non-vested stock options      
Non-vested at the beginning of the period (in shares) 222,413    
Forfeited (in shares) (625)    
Vested (in shares) (221,788)    
Non-vested at the end of the period (in shares)   222,413  
Weighted Average Grant Date Fair Value      
Non-vested options at the beginning of the period (in dollars per share) $ 26.98    
Forfeited (in dollars per shares) 26.98    
Vested (in dollars per shares) $ 26.98    
Non-vested options at the end of the period (in dollars per shares)   $ 26.98  
Additional share-based compensation disclosures      
Proceed from option exercises $ 15.1 $ 28.8 $ 70.1
Total intrinsic value of all options exercised 4.8 13.5 29.0
Tax benefit realized from option exercises $ 7.6 $ 10.7 $ 11.5
Stock options | Non-employee director      
Number of shares      
Outstanding at the end of the period (in shares) 102,000    
Aggregate Intrinsic Value (In millions)      
Vesting period (in years) 1 year    
Options with ten-year term | Non-employee director      
Aggregate Intrinsic Value (In millions)      
Award expiration term (in years) 10 years