Other Comprehensive Income (Loss) - Changes in AOCI by Component (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 40,308	$ 44,767	$ 48,791
Accumulated Other Comprehensive Income (Loss), ending balance	(4,910)	(5,226)
Ending Balance	34,569	40,308	44,767
Derivatives
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	338	404	530
Other comprehensive income (loss) before reclassification adjustments, pretax	(561)	210	526
Tax	207	(72)	(177)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(354)	138	349
Reclassification adjustments, pretax	(141)	(314)	(731)
Tax	49	110	256
Reclassification adjustments, net of taxes	(92)	(204)	(475)
Other comprehensive income (loss), net of taxes	(446)	(66)	(126)
Ending Balance	(108)	338	404
Investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3)	41	111
Other comprehensive income (loss) before reclassification adjustments, pretax	212	(38)	(9)
Tax	(35)	16	(13)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	177	(22)	(22)
Reclassification adjustments, pretax	(291)	(31)	(73)
Tax	56	9	25
Reclassification adjustments, net of taxes	(235)	(22)	(48)
Other comprehensive income (loss), net of taxes	(58)	(44)	(70)
Ending Balance	(61)	(3)	41
Employee Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,206)	(2,407)	(2,986)
Other comprehensive income (loss) before reclassification adjustments, pretax	438	(1,199)	710
Tax	(106)	363	(272)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	332	(836)	438
Reclassification adjustments, pretax	117	37	203
Tax	(30)	0	(62)
Reclassification adjustments, net of taxes	87	37	141
Other comprehensive income (loss), net of taxes	419	(799)	579
Ending Balance	(2,787)	(3,206)	(2,407)
Pension Plan Net Loss
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI	3,500	3,900
Other Postretirement Benefit Plan Net Loss
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI	(16)	115
Pension Plan Prior Service Credit
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI	326	361
Other Postretirement Benefit Plan Prior Service Credit
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Pension and other postretirement benefit plans, net loss and prior service credit included in AOCI	383	466
Cumulative Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,355)	(2,186)	(1,978)
Other comprehensive income (loss) before reclassification adjustments, pretax	235	(150)	(158)
Tax	166	(19)	(28)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	401	(169)	(186)
Reclassification adjustments, pretax	0	0	(22)
Tax	0	0	0
Reclassification adjustments, net of taxes	0	0	(22)
Other comprehensive income (loss), net of taxes	401	(169)	(208)
Ending Balance	(1,954)	(2,355)	(2,186)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(5,226)	(4,148)	(4,323)
Other comprehensive income (loss) before reclassification adjustments, pretax	324	(1,177)	1,069
Tax	232	288	(490)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	556	(889)	579
Reclassification adjustments, pretax	(315)	(308)	(623)
Tax	75	119	219
Reclassification adjustments, net of taxes	(240)	(189)	(404)
Other comprehensive income (loss), net of taxes	316	(1,078)	175
Ending Balance	$ (4,910)	$ (5,226)	$ (4,148)

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, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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

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

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