AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Amounts reclassified from AOCI:
OCI	$ 1,742	$ 294
Pension and OPEB Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(1,122)	(1,164)
OCI before reclassifications, net of tax	0	0
Amounts reclassified from AOCI:
OCI	15	17
AOCI, ending balance	(1,107)	(1,147)
Pension and OPEB Costs | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	20	23
Tax effect	5	6
Amounts reclassified, net of tax	15	17
Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(101)	(31)
OCI before reclassifications, net of tax	0	(30)
Amounts reclassified from AOCI:
OCI	11	(34)
AOCI, ending balance	(90)	(65)
Cash Flow Hedges | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	15	(5)
Tax effect	4	(1)
Amounts reclassified, net of tax	11	(4)
AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	380	(500)
OCI before reclassifications, net of tax	1,690	314
Amounts reclassified from AOCI:
OCI	1,721	309
AOCI, ending balance	2,101	(191)
AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	41	(6)
Tax effect	10	(1)
Amounts reclassified, net of tax	31	(5)
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(1)	(20)
OCI before reclassifications, net of tax	(5)	2
Amounts reclassified from AOCI:
OCI	(5)	2
AOCI, ending balance	(6)	(18)
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	(844)	(1,715)
OCI before reclassifications, net of tax	1,685	286
Amounts reclassified from AOCI:
OCI	1,742	294
AOCI, ending balance	898	(1,421)
Total | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	76	12
Tax effect	19	4
Amounts reclassified, net of tax	$ 57	$ 8