Accumulated Other Comprehensive Income (Loss), Net of Tax (Changes of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (179)	$ (550)	$ (550)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	8	18
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	10
Other Comprehensive Income (Loss), net of tax	10	28
Accumulated Other Comprehensive Income (Loss), Ending Balance	(169)	(522)	(179)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	3	(3)	(3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	20	(2)	9
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)	1	(3)
Other Comprehensive Income (Loss), net of tax	18	(1)
Accumulated Other Comprehensive Income (Loss), Ending Balance	21	(4)	3
Pension and OPEB Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(102)	(426)	(426)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	3
Other Comprehensive Income (Loss), net of tax	2	3
Accumulated Other Comprehensive Income (Loss), Ending Balance	(100)	(423)	(102)
Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	(80)	(121)	(121)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(12)	20
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	2	6
Other Comprehensive Income (Loss), net of tax	(10)	26
Accumulated Other Comprehensive Income (Loss), Ending Balance	$ (90)	$ (95)	$ (80)