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Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 26,882	$ 34,569	$ 40,308
Other comprehensive income (loss) before reclassification adjustments, pretax	(610)	(692)	324
Tax	161	(24)	232
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(449)	(716)	556
Reclassification adjustments, pretax	(239)	424	(315)
Tax	40	(69)	75
Reclassification adjustments, net of taxes	(199)	355	(240)
Other comprehensive income (loss), net of taxes	(648)	(361)	316
Adoption of ASU 2018-02		(266)
Adoption of new accounting standards			48
Ending Balance	26,001	26,882	34,569
Derivatives
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	166	(108)	338
Other comprehensive income (loss) before reclassification adjustments, pretax	86	228	(561)
Tax	(15)	(55)	207
Other comprehensive income (loss) before reclassification adjustments, net of taxes	71	173	(354)
Reclassification adjustments, pretax	(261)	157	(141)
Tax	55	(33)	49
Reclassification adjustments, net of taxes	(206)	124	(92)
Other comprehensive income (loss), net of taxes	(135)	297	(446)
Adoption of ASU 2018-02		(23)
Adoption of new accounting standards				$ 0
Ending Balance	31	166	(108)
Investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(78)	(61)	(3)
Other comprehensive income (loss) before reclassification adjustments, pretax	140	(108)	212
Tax	0	1	(35)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	140	(107)	177
Reclassification adjustments, pretax	(44)	97	(291)
Tax	0	0	56
Reclassification adjustments, net of taxes	(44)	97	(235)
Other comprehensive income (loss), net of taxes	96	(10)	(58)
Adoption of ASU 2018-02		1
Adoption of new accounting standards				(8)
Ending Balance	18	(78)	(61)
Employee Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,556)	(2,787)	(3,206)
Other comprehensive income (loss) before reclassification adjustments, pretax	(948)	(728)	438
Tax	192	169	(106)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(756)	(559)	332
Reclassification adjustments, pretax	66	170	117
Tax	(15)	(36)	(30)
Reclassification adjustments, net of taxes	51	134	87
Other comprehensive income (loss), net of taxes	(705)	(425)	419
Adoption of ASU 2018-02		(344)
Adoption of new accounting standards				0
Ending Balance	(4,261)	(3,556)	(2,787)
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	5,100	4,400
Other Postretirement Benefit Plan Net Gain
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Pension and other postretirement benefit plans, net loss and prior service (credit) included in AOCI	(247)	(170)
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	(263)	(314)
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	(305)	(375)
Cumulative Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,077)	(1,954)	(2,355)
Other comprehensive income (loss) before reclassification adjustments, pretax	112	(84)	235
Tax	(16)	(139)	166
Other comprehensive income (loss) before reclassification adjustments, net of taxes	96	(223)	401
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	96	(223)	401
Adoption of ASU 2018-02		100
Adoption of new accounting standards				0
Ending Balance	(1,981)	(2,077)	(1,954)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(5,545)	(4,910)	(5,226)
Adoption of new accounting standards			(274)	$ (8)
Ending Balance	$ (6,193)	$ (5,545)	$ (4,910)

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