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Stock-Based Compensation Plans - Performance Share Awards and Restricted Stock Units (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Performance Share Awards      
Stock-Based Compensation Plans - Disclosures      
Performance period for goals 3 years    
Change in nonvested units outstanding      
Awards or units outstanding at beginning of period (in shares) 0.9    
Awards or units granted (in shares) 0.2 0.3 0.3
Awards or units vested (in shares) 0.3    
Awards or units outstanding at end of period (in shares) 0.8 0.9  
Awards or units outstanding at beginning of period, Weighted-average grant-date fair value (in dollars per share) $ 44.23    
Awards or units granted, Weighted-average grant-date fair value (in dollars per share) 62.98 $ 37.38 $ 51.33
Awards or units vested, Weighted-average grant-date fair value (in dollars per share) 44.88    
Awards or units outstanding at end of period, Weighted-average grant-date fair value (in dollars per share) $ 48.82 $ 44.23  
Other award and unit disclosures      
Intrinsic value from awards or units vested $ 15.9 $ 18.1 $ 20.1
Lower limit multiple of initial target awards (as a percent) 0.00%    
Upper limit multiple of initial target awards (as a percent) 150.00%    
Unrecognized compensation costs $ 4.2    
Weighted-average service period over which unrecognized compensation costs will be recognized 1 year 2 months 12 days    
Tax benefits realized for the tax deductions from awards or units paid out $ 5.4 $ 4.8 $ 7.4
Restricted Stock Units      
Change in nonvested units outstanding      
Awards or units outstanding at beginning of period (in shares) 3.0    
Awards or units granted (in shares) 0.8 1.3 0.9
Awards or units vested (in shares) 0.9    
Awards or units canceled (in shares) 0.1    
Awards or units outstanding at end of period (in shares) 2.8 3.0  
Awards or units outstanding at beginning of period, Weighted-average grant-date fair value (in dollars per share) $ 43.77    
Awards or units granted, Weighted-average grant-date fair value (in dollars per share) 62.85 $ 37.59 $ 51.35
Awards or units vested, Weighted-average grant-date fair value (in dollars per share) 45.46    
Awards or units canceled, Weighted-average grant-date fair value (in dollars per share) 46.81    
Awards or units outstanding at end of period, Weighted-average grant-date fair value (in dollars per share) $ 48.70 $ 43.77  
Other award and unit disclosures      
Intrinsic value from awards or units vested $ 59.9 $ 46.2 $ 53.6
Unrecognized compensation costs $ 43.9    
Weighted-average service period over which unrecognized compensation costs will be recognized 1 year 8 months 12 days    
Tax benefits realized for the tax deductions from awards or units paid out $ 20.8 $ 16.1 $ 18.8
2014 Stock Incentive Plan, Amended and Restated 2010 Stock Incentive Plan and Stock Incentive Plan | Restricted Stock Units      
Other award and unit disclosures      
Vesting period 3 years    
2014 Directors Stock Plan and 2005 Directors Stock Plan | Restricted Stock Units      
Other award and unit disclosures      
Vesting period 1 year