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Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 26,001	$ 26,882	$ 34,569
Other comprehensive income (loss) before reclassification adjustments, pretax	(915)	(610)	(692)
Tax	284	161	(24)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(631)	(449)	(716)
Reclassification adjustments, pretax	253	(239)	424
Tax	(63)	40	(69)
Reclassification adjustments, net of taxes	190	(199)	355
Other comprehensive income (loss), net of taxes	(441)	(648)	(361)
Adoption of ASU 2018-02			(266)
Ending Balance	25,404	26,001	26,882
Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			48
Derivatives
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	31	166	(108)
Other comprehensive income (loss) before reclassification adjustments, pretax	(383)	86	228
Tax	84	(15)	(55)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(299)	71	173
Reclassification adjustments, pretax	2	(261)	157
Tax	0	55	(33)
Reclassification adjustments, net of taxes	2	(206)	124
Other comprehensive income (loss), net of taxes	(297)	(135)	297
Adoption of ASU 2018-02			(23)
Ending Balance	(266)	31	166
Derivatives | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0
Investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	18	(78)	(61)
Other comprehensive income (loss) before reclassification adjustments, pretax	3	140	(108)
Tax	0	0	1
Other comprehensive income (loss) before reclassification adjustments, net of taxes	3	140	(107)
Reclassification adjustments, pretax	(21)	(44)	97
Tax	0	0	0
Reclassification adjustments, net of taxes	(21)	(44)	97
Other comprehensive income (loss), net of taxes	(18)	96	(10)
Adoption of ASU 2018-02			1
Ending Balance	0	18	(78)
Investments | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(8)
Employee Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(4,261)	(3,556)	(2,787)
Other comprehensive income (loss) before reclassification adjustments, pretax	(599)	(948)	(728)
Tax	111	192	169
Other comprehensive income (loss) before reclassification adjustments, net of taxes	(488)	(756)	(559)
Reclassification adjustments, pretax	272	66	170
Tax	(63)	(15)	(36)
Reclassification adjustments, net of taxes	209	51	134
Other comprehensive income (loss), net of taxes	(279)	(705)	(425)
Adoption of ASU 2018-02			(344)
Ending Balance	(4,540)	(4,261)	(3,556)
Employee Benefit Plans | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0
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	5,400	5,100
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	(391)	(247)
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	(255)	(263)
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	(244)	(305)
Cumulative Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,981)	(2,077)	(1,954)
Other comprehensive income (loss) before reclassification adjustments, pretax	64	112	(84)
Tax	89	(16)	(139)
Other comprehensive income (loss) before reclassification adjustments, net of taxes	153	96	(223)
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	153	96	(223)
Adoption of ASU 2018-02			100
Ending Balance	(1,828)	(1,981)	(2,077)
Cumulative Translation Adjustment | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(6,193)	(5,545)	(4,910)
Ending Balance	$ (6,634)	$ (6,193)	(5,545)
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(274)
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (8)

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Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL116659650-108580

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