AOCI (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Amounts reclassified from AOCI:
OCI	$ (61)	$ 52	$ (239)	$ 59
Unrecognized Net Pension and Postretirement Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(990)	(755)	(1,004)	(764)
OCI before reclassifications, net of tax	0	1	0	(1)
Amounts reclassified from AOCI:
OCI	13	12	27	21
AOCI, ending balance	(977)	(743)	(977)	(743)
Unrecognized Net Pension and Postretirement Costs \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	18	18	36	35
Tax effect	5	7	9	13
Amounts reclassified, net of tax	13	11	27	22
Unrealized Net Gains (Losses) on Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(14)	(94)	(92)	(92)
OCI before reclassifications, net of tax	23	(30)	93	(27)
Amounts reclassified from AOCI:
OCI	26	(34)	104	(36)
AOCI, ending balance	12	(128)	12	(128)
Unrealized Net Gains (Losses) on Cash Flow Hedges \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	3	(6)	14	(14)
Tax effect	0	(2)	3	(5)
Amounts reclassified, net of tax	3	(4)	11	(9)
Unrealized Net Gains (Losses) on AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(624)	(261)	(356)	(259)
OCI before reclassifications, net of tax	(100)	81	(382)	80
Amounts reclassified from AOCI:
OCI	(99)	74	(367)	72
AOCI, ending balance	(723)	(187)	(723)	(187)
Unrealized Net Gains (Losses) on AFS Securities \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	1	(12)	20	(13)
Tax effect	0	(5)	5	(5)
Amounts reclassified, net of tax	1	(7)	15	(8)
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(17)	(15)	(15)	(17)
OCI before reclassifications, net of tax	(2)	1	(4)	2
Amounts reclassified from AOCI:
OCI	(1)	0	(3)	2
AOCI, ending balance	(18)	(15)	(18)	(15)
Other, net \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	1	(1)	1	0
Tax effect	0	0	0	0
Amounts reclassified, net of tax	1	(1)	1	0
Total
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(1,645)	(1,125)	(1,467)	(1,132)
OCI before reclassifications, net of tax	(79)	53	(293)	54
Amounts reclassified from AOCI:
OCI	(61)	52	(239)	59
AOCI, ending balance	(1,706)	(1,073)	(1,706)	(1,073)
Total \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	23	(1)	71	8
Tax effect	5	0	17	3
Amounts reclassified, net of tax	$ 18	$ (1)	$ 54	$ 5

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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