Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 11,162
Other comprehensive loss before reclassifications	(86)	$ (319)
Amounts reclassified from accumulated other comprehensive loss	54	44
Other comprehensive loss	(32)	(275)
Balance at end of period	11,303	13,291
Defined Benefit Plan | Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,197)	(2,229)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	35	36
Other comprehensive loss	35	36
Balance at end of period	(2,162)	(2,193)
Defined Benefit Plan | Post-Retirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(272)	(184)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	4	3
Other comprehensive loss	4	3
Balance at end of period	(268)	(181)
Foreign Currency Translation Adjustment and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(265)	(228)
Other comprehensive loss before reclassifications	(86)	(239)
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive loss	(86)	(239)
Balance at end of period	(351)	(467)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(79)	(39)
Other comprehensive loss before reclassifications	0	(80)
Amounts reclassified from accumulated other comprehensive loss	15	5
Other comprehensive loss	15	(75)
Balance at end of period	(64)	(114)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,813)	(2,680)
Other comprehensive loss	(32)	(275)
Balance at end of period	$ (2,845)	$ (2,955)