AOCI (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amounts reclassified from AOCI:
OCI	$ (146)	$ 37	$ (385)	$ 96
Pension and OPEB Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(977)	(743)	(1,004)	(764)
OCI before reclassifications, net of tax	(27)	1	(27)	0
Amounts reclassified from AOCI:
OCI	(12)	8	15	29
AOCI, ending balance	(989)	(735)	(989)	(735)
Pension and OPEB Costs \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	19	11	55	46
Tax effect	4	4	13	17
Amounts reclassified, net of tax	15	7	42	29
Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	12	(128)	(92)	(92)
OCI before reclassifications, net of tax	20	1	113	(26)
Amounts reclassified from AOCI:
OCI	20	9	124	(27)
AOCI, ending balance	32	(119)	32	(119)
Cash Flow Hedges \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	0	13	14	(1)
Tax effect	0	5	3	0
Amounts reclassified, net of tax	0	8	11	(1)
AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(723)	(187)	(356)	(259)
OCI before reclassifications, net of tax	(162)	19	(544)	99
Amounts reclassified from AOCI:
OCI	(155)	18	(522)	90
AOCI, ending balance	(878)	(169)	(878)	(169)
AFS Securities \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	9	(2)	29	(15)
Tax effect	2	(1)	7	(6)
Amounts reclassified, net of tax	7	(1)	22	(9)
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(18)	(15)	(15)	(17)
OCI before reclassifications, net of tax	1	2	(3)	4
Amounts reclassified from AOCI:
OCI	1	2	(2)	4
AOCI, ending balance	(17)	(13)	(17)	(13)
Other, net \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	0	0	1	0
Tax effect	0	0	0	0
Amounts reclassified, net of tax	0	0	1	0
Total
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(1,706)	(1,073)	(1,467)	(1,132)
OCI before reclassifications, net of tax	(168)	23	(461)	77
Amounts reclassified from AOCI:
OCI	(146)	37	(385)	96
AOCI, ending balance	(1,852)	(1,036)	(1,852)	(1,036)
Total \| Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Before tax	28	22	99	30
Tax effect	6	8	23	11
Amounts reclassified, net of tax	$ 22	$ 14	$ 76	$ 19

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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