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Share-based Compensation Plans (Fair Value Assumptions) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Forfeiture rate 0.00%    
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 0.40% 0.29% 0.60%
Expected dividend yield 0.30% 0.30% 0.20%
Expected life 1 year 1 year 1 year
Volatility 25.00% 26.00% 23.00%
Forfeiture rate 0.00% 5.00% 5.00%
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 1.90% 1.60% 1.80%
Expected dividend yield 0.60% 0.50% 0.40%
Expected life 3 years 10 months 17 days 4 years 4 years
Volatility 29.00% 29.00% 26.00%
Forfeiture rate 0.00% 7.00% 7.00%