AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Amounts reclassified from AOCI:
OCI	$ (178)	$ 7
Unrecognized Net Pension and Postretirement Costs
AOCI, Net of Tax [Roll Forward]
Beginning balance	(1,004)	(764)
OCI before reclassifications, net of tax	0	(2)
Amounts reclassified from AOCI:
OCI	14	9
Ending balance	(990)	(755)
Unrecognized Net Pension and Postretirement Costs | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	18	17
Tax effect	4	6
Amounts reclassified, net of tax	14	11
Unrealized Net Gains (Losses) on Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
Beginning balance	(92)	(92)
OCI before reclassifications, net of tax	70	3
Amounts reclassified from AOCI:
OCI	78	(2)
Ending balance	(14)	(94)
Unrealized Net Gains (Losses) on Cash Flow Hedges | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	11	(8)
Tax effect	3	(3)
Amounts reclassified, net of tax	8	(5)
Unrealized Net Gains (Losses) on AFS Securities
AOCI, Net of Tax [Roll Forward]
Beginning balance	(356)	(259)
OCI before reclassifications, net of tax	(282)	(1)
Amounts reclassified from AOCI:
OCI	(268)	(2)
Ending balance	(624)	(261)
Unrealized Net Gains (Losses) on AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	19	(1)
Tax effect	5	0
Amounts reclassified, net of tax	14	(1)
Other, net
AOCI, Net of Tax [Roll Forward]
Beginning balance	(15)	(17)
OCI before reclassifications, net of tax	(2)	1
Amounts reclassified from AOCI:
OCI	(2)	2
Ending balance	(17)	(15)
Other, net | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	0	1
Tax effect	0	0
Amounts reclassified, net of tax	0	1
Total
AOCI, Net of Tax [Roll Forward]
Beginning balance	(1,467)	(1,132)
OCI before reclassifications, net of tax	(214)	1
Amounts reclassified from AOCI:
OCI	(178)	7
Ending balance	(1,645)	(1,125)
Total | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	48	9
Tax effect	12	3
Amounts reclassified, net of tax	$ 36	$ 6