AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
AOCI, Net of Tax [Roll Forward]
OCI	$ (716)	$ 1,020
Pension and OPEB Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(1,079)	(1,535)
OCI before reclassifications, net of tax	1	(26)
OCI	1	(14)
AOCI, ending balance	(1,078)	(1,549)
Pension and OPEB Costs | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	0	16
Tax effect	0	4
Amounts reclassified, net of tax	0	12
Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(300)	(78)
OCI before reclassifications, net of tax	(232)	125
OCI	(190)	125
AOCI, ending balance	(490)	47
Cash Flow Hedges | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	55	0
Tax effect	13	0
Amounts reclassified, net of tax	42	0
AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(8,778)	(9,395)
OCI before reclassifications, net of tax	(455)	903
OCI	(576)	853
AOCI, ending balance	(9,354)	(8,542)
AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	(158)	(65)
Tax effect	(37)	(15)
Amounts reclassified, net of tax	(121)	(50)
HTM Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(2,347)	(2,588)
OCI before reclassifications, net of tax	0	0
OCI	51	55
AOCI, ending balance	(2,296)	(2,533)
HTM Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	66	70
Tax effect	15	15
Amounts reclassified, net of tax	51	55
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(2)	(5)
OCI before reclassifications, net of tax	(2)	1
OCI	(2)	1
AOCI, ending balance	(4)	(4)
Other, net | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	0	0
Tax effect	0	0
Amounts reclassified, net of tax	0	0
Total
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(12,506)	(13,601)
OCI before reclassifications, net of tax	(688)	1,003
OCI	(716)	1,020
AOCI, ending balance	(13,222)	(12,581)
Total | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	(37)	21
Tax effect	(9)	4
Amounts reclassified, net of tax	$ (28)	$ 17