Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Components of accumulated other comprehensive loss
Beginning balance	$ 1,534	$ 15,358
Changes in value (net of tax effect)		21
Changes in value, tax effect		3
Reclassifications into earnings (net of tax effect)	78	70
Reclassifications into earnings, tax effect	(23)	21
Ending balance	482	14,309
Accumulated Other Comprehensive Income
Components of accumulated other comprehensive loss
Beginning balance	(9,038)	(7,989)
Beginning balance, tax effect	1,764	1,549
Ending balance	(8,960)	(7,898)
Ending balance, tax effect	1,741	1,531
Pension and Other Benefit Liabilities
Components of accumulated other comprehensive loss
Beginning balance	(9,078)	(8,095)
Changes in value (net of tax effect)		0
Reclassifications into earnings (net of tax effect)	78	70
Ending balance	(9,000)	(8,025)
Deferred income tax expense in AOCI that will not be recognized until obligation is fully extinguished	750	750
Other
Components of accumulated other comprehensive loss
Beginning balance	40	106
Changes in value (net of tax effect)		21
Reclassifications into earnings (net of tax effect)	0	0
Ending balance	$ 40	$ 127