AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Amounts reclassified from AOCI:
OCI	$ (2,232)	$ 1,742
Pension and OPEB Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(875)	(1,122)
OCI before reclassifications, net of tax	28	0
Amounts reclassified from AOCI:
OCI	35	15
AOCI, ending balance	(840)	(1,107)
Pension and OPEB Costs | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	9	20
Tax effect	2	5
Amounts reclassified, net of tax	7	15
Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(64)	(101)
OCI before reclassifications, net of tax	0	0
Amounts reclassified from AOCI:
OCI	36	11
AOCI, ending balance	(28)	(90)
Cash Flow Hedges | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	47	15
Tax effect	11	4
Amounts reclassified, net of tax	36	11
AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	1,654	380
OCI before reclassifications, net of tax	(2,408)	1,690
Amounts reclassified from AOCI:
OCI	(2,304)	1,721
AOCI, ending balance	(650)	2,101
AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	136	41
Tax effect	32	10
Amounts reclassified, net of tax	104	31
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	1	(1)
OCI before reclassifications, net of tax	1	(5)
Amounts reclassified from AOCI:
OCI	1	(5)
AOCI, ending balance	2	(6)
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	716	(844)
OCI before reclassifications, net of tax	(2,379)	1,685
Amounts reclassified from AOCI:
OCI	(2,232)	1,742
AOCI, ending balance	(1,516)	898
Total | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	192	76
Tax effect	45	19
Amounts reclassified, net of tax	$ 147	$ 57