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Retirement Benefits (Fair Value of Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Benefit Obligation $ 208 $ 198 $ 156
Defined Benefit Plan, Plan Assets, Amount 270 241 257
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   241  
Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 43    
Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 14 20  
Cash and Cash Equivalents [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 1    
Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 177 144  
Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 19 1  
Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 79 77  
Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 23    
Level 1 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 191 164  
Level 1 [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 20    
Level 1 [Member] | Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 14 20  
Level 1 [Member] | Cash and Cash Equivalents [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 1    
Level 1 [Member] | Mutual Fund [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 177 144  
Level 1 [Member] | Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 19 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 1 [Member] | Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0    
Level 2 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 79 77  
Level 2 [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 23    
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] | Cash and Cash Equivalents [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 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 79 77  
Level 2 [Member] | Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 23    
Level 3 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Level 3 [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 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] | Cash and Cash Equivalents [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 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  
Level 3 [Member] | Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0    
Funded Plans [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 51 0  
Defined Benefit Plan, Benefit Obligation 52 1 1
Defined Benefit Plan, Plan Assets, Amount 43 1 1
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   1  
Unfunded Plans [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 34 37  
Defined Benefit Plan, Benefit Obligation 34 37 47
Defined Benefit Plan, Plan Assets, Amount $ 0 0 $ 0
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   $ 0