Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 11,840
Other comprehensive income before reclassifications	67	$ (86)
Amounts reclassified from accumulated other comprehensive loss	44	54
Net current-period other comprehensive income (loss)	111	(32)
Balance at end of period	12,282	11,303
Defined Benefit Plan | Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,577)	(2,197)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	26	35
Net current-period other comprehensive income (loss)	26	35
Balance at end of period	(1,551)	(2,162)
Defined Benefit Plan | Post-Retirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(164)	(272)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	1	4
Net current-period other comprehensive income (loss)	1	4
Balance at end of period	(163)	(268)
Foreign Currency Translation Adjustment and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(400)	(265)
Other comprehensive income before reclassifications	67	(86)
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	67	(86)
Balance at end of period	(333)	(351)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(17)	(79)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	17	15
Net current-period other comprehensive income (loss)	17	15
Balance at end of period	0	(64)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,158)	(2,813)
Net current-period other comprehensive income (loss)	111	(32)
Balance at end of period	$ (2,047)	$ (2,845)