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Share-Based Compensation Plans - Terms and Assumptions (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Market Share Units MSU      
Weighted-average assumptions used to estimate fair value:      
Expected dividend yield     1.50%
Expected stock price volatility     30.00%
Risk-free interest rate     1.00%
Contractual term in years     2 years 9 months 18 days
Weighted-average fair value estimate at grant date     $ 3.3
Weighted-average fair value estimate per share (in dollars per share)     $ 30.37
Performance Shares 2015      
Weighted-average assumptions used to estimate fair value:      
Expected dividend yield     1.50%
Expected stock price volatility     30.00%
Risk-free interest rate     1.00%
Contractual term in years     2 years 9 months 18 days
Weighted-average fair value estimate at grant date     $ 5.8
Weighted-average fair value estimate per share (in dollars per share)     $ 29.10
TSR PSU      
Weighted-average assumptions used to estimate fair value:      
Expected dividend yield 0.80% 1.40%  
Expected stock price volatility 30.60% 29.10%  
Risk-free interest rate 1.40% 0.80%  
Contractual term in years 2 years 10 months 24 days 2 years 8 months 12 days  
Weighted-average fair value estimate at grant date $ 2.0 $ 2.3  
Weighted-average fair value estimate per share (in dollars per share) $ 67.81 $ 31.64