Accumulated Other Comprehensive Income (Loss), Net of Tax - Schedule of Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (173)	$ (515)	$ (179)	$ (550)	$ (550)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	32	59	35	77
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1)	243	2	260
Other Comprehensive Income (Loss), net of tax	31	302	37	337
Accumulated Other Comprehensive Income (Loss), Ending Balance	(142)	(213)	(142)	(213)	(179)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	20	7	3	(3)	(3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(5)	13	17	23	9
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(3)	(2)	(8)	(2)	(3)
Other Comprehensive Income (Loss), net of tax	(8)	11	9	21
Accumulated Other Comprehensive Income (Loss), Ending Balance	12	18	12	18	3
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(98)	(419)	(102)	(426)	(426)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	82	0	82
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	240	6	247
Other Comprehensive Income (Loss), net of tax	2	322	6	329
Accumulated Other Comprehensive Income (Loss), Ending Balance	(96)	(97)	(96)	(97)	(102)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(95)	(103)	(80)	(121)	(121)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	37	(36)	18	(28)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	5	4	15
Other Comprehensive Income (Loss), net of tax	37	(31)	22	(13)
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ (58)	$ (134)	$ (58)	$ (134)	$ (80)