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Retirement Benefits (Fair Value of Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Benefit Obligation $ 208 $ 208 $ 198
Defined Benefit Plan, Plan Assets, Amount 291 270 241
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   270  
Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 45 43  
Cash and Cash Equivalents [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 18 14  
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 202 177  
Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 20 19  
Fixed Income Securities [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 71 79  
Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 24 23  
Cash and Cash Equivalents [Domain] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount   1  
Level 1 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 220 191  
Level 1 [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 21 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 18 14  
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 202 177  
Level 1 [Member] | Mutual Fund [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 20 19  
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 0  
Level 1 [Member] | Cash and Cash Equivalents [Domain] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount   1  
Level 2 [Member] | Other Postretirement Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 71 79  
Level 2 [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 24 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 71 79  
Level 2 [Member] | Fixed Income Securities [Member] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 24 23  
Level 2 [Member] | Cash and Cash Equivalents [Domain] | Pension Benefits      
Fair Value of Plan Assets [Line Items]      
Defined Benefit Plan, Plan Assets, Amount   0  
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 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 0  
Level 3 [Member] | Cash and Cash Equivalents [Domain] | 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 55 51  
Defined Benefit Plan, Benefit Obligation 55 52 1
Defined Benefit Plan, Plan Assets, Amount 45 43 1
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets   43  
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 31 34  
Defined Benefit Plan, Benefit Obligation 31 34 37
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