Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications out of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			$ (307)	$ (49)	$ (15)
Tax (Expense) Benefit			82	(19)	(4)
After-Tax Amount			(225)	(30)	(11)
Pension Lift Out Settlement Charge	$ (6)	$ (332)	(334)
Pension Settlement Charge, tax			94
Pension Lift Out Settlement Charge, net of tax	$ (4)	$ (239)	(240)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			5	(5)	(4)
Tax (Expense) Benefit			(2)	(2)	(1)
After-Tax Amount			3	(3)	(3)
Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			(346)	(1)	(20)
Tax (Expense) Benefit			98	0	(6)
After-Tax Amount			(248)	(1)	(14)
Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			34	(43)	9
Tax (Expense) Benefit			(14)	(17)	3
After-Tax Amount			20	(26)	6
Interest Expense | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			5	(5)	(4)
Tax (Expense) Benefit			(2)	(2)	(1)
After-Tax Amount			3	(3)	(3)
Amortization of Prior Service (Cost) Credit | Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			8	(21)	(21)
Tax (Expense) Benefit			(2)	(6)	(6)
After-Tax Amount			6	(15)	(15)
Amortization of Actuarial Loss | Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			(20)	22	41
Tax (Expense) Benefit			6	6	12
After-Tax Amount			(14)	16	29
Net Gains (Losses) on Trust Investments | Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount			34	(43)	9
Tax (Expense) Benefit			(14)	(17)	3
After-Tax Amount			$ 20	$ (26)	$ 6