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Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax $ (4) $ (16)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax 2 6  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (2) (10)  
Cash Flow Hedges [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Cash Flow Hedge, Pre-tax 3 (1)  
Cash Flow Hedge, Tax 1 0  
Cash Flow Hedge, After Tax 2 (1)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax 3   $ 5
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 2 (1) $ 3
Pension and OPEB Plans [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (5) (5)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 2 2  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (3) (3)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax (3)    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (2) (3)  
Reclassification from AOCI, Current Period, Tax 1    
Available-for-Sale Securities [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (4) (10)  
Reclassification for Available for Sale Securities, Tax 2 4  
Reclassification for Available for Sale Securities, After-Tax (2) (6)  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (2) (6)  
Interest Expense [Member] | Cash Flow Hedges [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Cash Flow Hedge, Pre-tax 3 (1)  
Cash Flow Hedge, Tax (1) 0  
Cash Flow Hedge, After Tax 2 (1)  
Non-Operating Pension and OPEB Credits (Costs) [Member] | Pension and OPEB Plans [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of Prior Service (Cost) Credit, Pre-Tax   2  
Amortization of Prior Service (Cost) Credit, Tax   0  
Amortization of Prior Service (Cost) Credit, After-Tax   2  
Amortization of Actuarial Loss, Pre-Tax (3) (7)  
Amortization of Actuarial Loss, Tax 1 2  
Amortization of Actuarial Loss, After-Tax (2) (5)  
Net Gains (Losses) on Trust Investments [Member] | Available-for-Sale Securities [Member]      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (4) (10)  
Reclassification for Available for Sale Securities, Tax 2 4  
Reclassification for Available for Sale Securities, After-Tax $ (2) $ (6)