Accumulated Other Comprehensive Income (Loss) (Components of Other Accumulated Comprehensive Income Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 7,943	$ 7,584
Other comprehensive (loss) gain before reclassifications	(180)	(492)
Amounts reclassified from accumulated other comprehensive income (loss)	125	84
Other comprehensive loss, net of tax	(55)	(408)	$ (590)
Ending balance	9,260	7,943	7,584
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(5,055)	(4,647)	(4,043)
Other comprehensive loss, net of tax	(55)	(408)	(590)
Ending balance	(5,110)	(5,055)	(4,647)
Pension and Post-Retirement Plans Losses
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(3,512)	(2,953)
Other comprehensive (loss) gain before reclassifications	(739)	(643)
Amounts reclassified from accumulated other comprehensive income (loss)	125	84
Other comprehensive loss, net of tax	(614)	(559)
Ending balance	(4,126)	(3,512)	(2,953)
Foreign Currency Translation Adjustments
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,543)	(1,694)
Other comprehensive (loss) gain before reclassifications	559	151
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Other comprehensive loss, net of tax	559	151
Ending balance	$ (984)	$ (1,543)	$ (1,694)