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Stock Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Excess Tax Benefits $ 4 $ 4 $ 3
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 16,000,000    
Compensation expense   $ 1 $ 1
Percentage Of Fair Market Value Being Expected Purchase Price Of Employee Stock Purchase Plan 95.00%    
Minimum Holding Period for Stock Purchased through Employee Stock Purchase Plan 3 months    
Percentage Of Fair Market Value Being Expected Purchase Price Of Employee Stock Purchase Plan Non Represented 90.00%    
Maximum Percentage Limit Of Base Pay For Employees For Purchasing Shares 10.00%    
Shares issued under employee stock purchase plan 288,527 262,763 250,499
Shares issued under employee purchase plan, Average price per share $ 42.07 $ 40.70 $ 36.66
Various [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 14,000,000    
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value of granted shares $ 44.33 $ 42.28 $ 39.65
Unrecognized compensation cost related to stock options expected to be recognized $ 3    
Weighted average period for recognizing unrecognized compensation cost 10 months    
Total intrinsic value of restricted stock units vested $ 13 $ 17 $ 11
Dividend equivalents accrued on stock units 30,066    
Performance Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value of granted shares $ 45.02 $ 45.97 $ 41.32
Total intrinsic value of performance units vested $ 18 $ 17 $ 13
Unrecognized compensation cost related to stock options expected to be recognized $ 16    
Weighted average period for recognizing unrecognized compensation cost 1 year    
Dividend equivalents accrued on stock units 38,425    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 100.00%    
Employee Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 3,200,000    
Minimum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period 1 year    
Minimum [Member] | Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options vesting period 3 years    
Minimum [Member] | Performance Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 0.00%    
Maximum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options vesting period 4 years    
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period 10 years    
Maximum [Member] | Performance Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options vesting period 3 years    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 200.00%