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Other Comprehensive Income (Loss) - Changes in AOCI by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 34,569	$ 40,308	$ 44,767
Other comprehensive income (loss), net of taxes	(361)	316	(1,078)
Adoption of ASU 2018-02 (see Note 2)	(266)
Adoption of new accounting standards (see Note 2)	48
Ending Balance	26,882	34,569	40,308
Derivatives
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(108)	338	404
Other comprehensive income (loss) before reclassification adjustments, pretax	228	(561)	210
Tax	(55)	207	(72)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	173	(354)	138
Reclassification adjustments, pretax	157	(141)	(314)
Tax	(33)	49	110
Reclassification adjustments, net of taxes	124	(92)	(204)
Other comprehensive income (loss), net of taxes	297	(446)	(66)
Adoption of ASU 2018-02 (see Note 2)	(23)
Adoption of new accounting standards (see Note 2)				$ 0
Ending Balance	166	(108)	338
Investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(61)	(3)	41
Other comprehensive income (loss) before reclassification adjustments, pretax	(108)	212	(38)
Tax	1	(35)	16
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(107)	177	(22)
Reclassification adjustments, pretax	97	(291)	(31)
Tax	0	56	9
Reclassification adjustments, net of taxes	97	(235)	(22)
Other comprehensive income (loss), net of taxes	(10)	(58)	(44)
Adoption of ASU 2018-02 (see Note 2)	1
Adoption of new accounting standards (see Note 2)				(8)
Ending Balance	(78)	(61)	(3)
Employee Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,787)	(3,206)	(2,407)
Other comprehensive income (loss) before reclassification adjustments, pretax	(728)	438	(1,199)
Tax	169	(106)	363
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(559)	332	(836)
Reclassification adjustments, pretax	170	117	37
Tax	(36)	(30)	0
Reclassification adjustments, net of taxes	134	87	37
Other comprehensive income (loss), net of taxes	(425)	419	(799)
Adoption of ASU 2018-02 (see Note 2)	(344)
Adoption of new accounting standards (see Note 2)				0
Ending Balance	(3,556)	(2,787)	(3,206)
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	4,400	3,500
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	(170)	(16)
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	314	326
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	375	383
Cumulative Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,954)	(2,355)	(2,186)
Other comprehensive income (loss) before reclassification adjustments, pretax	(84)	235	(150)
Tax	(139)	166	(19)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(223)	401	(169)
Reclassification adjustments, pretax	0	0	0
Tax	0	0	0
Reclassification adjustments, net of taxes	0	0	0
Other comprehensive income (loss), net of taxes	(223)	401	(169)
Adoption of ASU 2018-02 (see Note 2)	100
Adoption of new accounting standards (see Note 2)				0
Ending Balance	(2,077)	(1,954)	(2,355)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(4,910)	(5,226)	(4,148)
Other comprehensive income (loss) before reclassification adjustments, pretax	(692)	324	(1,177)
Tax	(24)	232	288
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(716)	556	(889)
Reclassification adjustments, pretax	424	(315)	(308)
Tax	(69)	75	119
Reclassification adjustments, net of taxes	355	(240)	(189)
Other comprehensive income (loss), net of taxes	(361)	316	(1,078)
Adoption of new accounting standards (see Note 2)				$ (8)
Ending Balance	$ (5,545)	$ (4,910)	$ (5,226)

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