Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Pension Lift Out Settlement Charge	$ (332)		$ (332)
Pension Settlement Charge, tax	93		93
Pension Lift Out Settlement Charge, net of tax	(239)		(239)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(338)	$ (15)	(365)	$ (36)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax	95	5	105	13
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(243)	(10)	(260)	(23)
Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Cash Flow Hedge, Pre-tax	3	(1)	3	(3)
Cash Flow Hedge, Tax	1	0	1	(1)
Cash Flow Hedge, After Tax	2	(1)	2	(2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			3		$ (5)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	(1)	2	(2)	$ (3)
Pension and OPEB Plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax	(333)	0	(343)	(1)
Reclassification Adjustment from AOCI, Pension and OPEB, Tax	93	0	96	0
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax	(240)	0	(247)	(1)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(240)	0	(247)	(1)
Available-for-Sale Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Reclassification for Available for Sale Securities, Pre-Tax	(8)	(14)	(25)	(32)
Reclassification for Available for Sale Securities, Tax	3	5	10	12
Reclassification for Available for Sale Securities, After-Tax	(5)	(9)	(15)	(20)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(5)	(9)	(15)	(20)
Interest Expense [Member] | Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Cash Flow Hedge, Pre-tax	3	(1)	3	(3)
Cash Flow Hedge, Tax	(1)	0	(1)	1
Cash Flow Hedge, After Tax	2	1	2	(2)
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	5	6	15
Amortization of Prior Service (Cost) Credit, Tax	(1)	(1)	(2)	(4)
Amortization of Prior Service (Cost) Credit, After-Tax	1	4	4	11
Amortization of Actuarial Loss, Pre-Tax	(3)	(5)	(17)	(16)
Amortization of Actuarial Loss, Tax	1	1	5	4
Amortization of Actuarial Loss, After-Tax	(2)	(4)	(12)	(12)
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	(8)	(14)	(25)	(32)
Reclassification for Available for Sale Securities, Tax	3	5	10	12
Reclassification for Available for Sale Securities, After-Tax	$ (5)	$ (9)	$ (15)	$ (20)