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Share-Based Payments (Summary of Stock Option and Performance Share Unit Pricing Assumptions) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 2.00% 1.20% 1.80%
Expected dividend yield 2.15% 2.66% 2.18%
Expected lives 5 years 5 years 5 years
Expected volatility 27.80% 26.50% 26.40%
Sign-on Grant Performance Share Unit Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.60%    
Expected dividend yield 2.54%    
Expected lives 4 years    
Expected volatility 28.60%    
Stock Options Assumed [Member] | Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 0.80%    
Expected dividend yield 2.19%    
Expected lives 4 months 24 days    
Expected volatility 32.10%    
Performance Share Units, Liability Classified Awards [Member] | Senior Executive and Employee Performance Share Unit Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.50% 1.20% 1.30%
Expected dividend yield 2.01% 2.40% 2.17%
Expected lives 3 years 3 years 3 years
Expected volatility 28.90% 29.70% 24.40%
Average expected volatility of comparable companies 33.40% 37.00% 32.80%
Performance Share Units, Equity Classified Awards [Member] | Senior Executive and Employee Performance Share Unit Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.40% 1.20%  
Expected dividend yield 1.92% 2.50%  
Expected lives 3 years 3 years  
Expected volatility 28.20% 29.90%  
Average expected volatility of comparable companies 37.00% 37.00%