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Stock Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 16,000,000      
Stock options vested during period less than 1 million in 2013 and 2012      
Compensation expense $ 1 $ 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 262,763 250,499 207,248  
Shares issued under employee purchase plan, Average price per share $ 40.70 $ 36.66 $ 36.07  
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 15,000,000      
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 $ 42.28 $ 39.65 $ 35.16  
Unrecognized compensation cost related to stock options expected to be recognized $ 4      
Weighted average period for recognizing unrecognized compensation cost 10 months      
Total intrinsic value of restricted stock units vested $ 17 $ 11 $ 12  
Dividend equivalents accrued on stock units 35,537      
Performance Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value of granted shares $ 45.97 $ 41.32 $ 38.94  
Total intrinsic value of performance units vested $ 17 $ 13 $ 6  
Unrecognized compensation cost related to stock options expected to be recognized $ 13      
Weighted average period for recognizing unrecognized compensation cost 1 year      
Dividend equivalents accrued on stock units 36,856      
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 3,500,000      
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%