Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes in AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (550)	$ (350)	$ (504)
Other Comprehensive Income (Loss) before Reclassifications	146	(230)	143
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	225	30	11
Net Current Period Other Comprehensive Income (Loss)	371	(200)	154
Ending Balance	(179)	(550)	(350)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(3)	(6)	(9)
Other Comprehensive Income (Loss) before Reclassifications	9	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(3)	3	3
Net Current Period Other Comprehensive Income (Loss)	6	3	3
Ending Balance	3	(3)	(6)
Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(426)	(355)	(545)
Other Comprehensive Income (Loss) before Reclassifications	76	(72)	176
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	248	1	14
Net Current Period Other Comprehensive Income (Loss)	324	(71)	190
Ending Balance	(102)	(426)	(355)
Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(121)	11	50
Other Comprehensive Income (Loss) before Reclassifications	61	(158)	(33)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(20)	26	(6)
Net Current Period Other Comprehensive Income (Loss)	41	(132)	(39)
Ending Balance	$ (80)	$ (121)	$ 11