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Retirement Benefits (Fair Value of Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 241 $ 257 $ 256
Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 20 33  
Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 144 154  
Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 1 1  
Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 77 70  
Level 1 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 164 187  
Level 1 [Member] | Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 20 33  
Level 1 [Member] | Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 144 154  
Level 1 [Member] | Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 1 1  
Level 1 [Member] | Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 2 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 77 70  
Level 2 [Member] | Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 2 [Member] | Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 2 [Member] | Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 2 [Member] | Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 77 70  
Level 3 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 3 [Member] | Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 3 [Member] | Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 3 [Member] | Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 3 [Member] | Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 0 $ 0