Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax
Beginning balance		$ 15,358	$ 13,687	$ 12,530
Changes in value (net of tax effect)		(1,252)	(415)	(287)
Changes in value, tax effect		(384)	(133)	(88)
Reclassifications into retained earnings (net of tax effect)				(106)
Reclassifications into retained earnings, tax effect				(61)
Reclassification into earnings (net of tax effect)		203	251	189
Reclassifications into earnings, tax effect		169	76	57
Ending balance		1,534	15,358	13,687
Deferred income tax expense in AOCI that will not be recognized until obligation is fully extinguished		750
Tax benefit		3,202	(1,431)	(1,216)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax
Beginning balance		(7,989)	(7,825)	(7,621)
Beginning balance, tax effect		(1,549)	(1,492)	(1,400)
Ending balance		(9,038)	(7,989)	(7,825)
Ending balance, tax effect		(1,764)	(1,549)	(1,492)
Pension and Other Benefits Liabilities
AOCI Attributable to Parent, Net of Tax
Beginning balance		(8,095)	(7,925)	(7,812)
Changes in value (net of tax effect)		(1,269)	(422)	(294)
Reclassifications into retained earnings (net of tax effect)				0
Reclassification into earnings (net of tax effect)		286	252	181
Ending balance		(9,078)	(8,095)	(7,925)
Other
AOCI Attributable to Parent, Net of Tax
Beginning balance		106	100	85
Changes in value (net of tax effect)		17	7	7
Reclassifications into retained earnings (net of tax effect)				0
Reclassification into earnings (net of tax effect)		(83)	(1)	8
Ending balance		40	106	100
Other | Reclassification out of Accumulated Other Comprehensive Income [Member]
AOCI Attributable to Parent, Net of Tax
Tax benefit	$ 83
Available-for-Sale Investments
AOCI Attributable to Parent, Net of Tax
Beginning balance		0	0	106
Changes in value (net of tax effect)		0	0	0
Reclassifications into retained earnings (net of tax effect)				(106)
Reclassification into earnings (net of tax effect)		0	0	0
Ending balance		$ 0	$ 0	$ 0