Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 11,162
Other comprehensive loss before reclassifications			(6)	$ (318)
Amounts reclassified from accumulated other comprehensive loss			109	96
Other comprehensive income (loss)	$ 135	$ 53	103	(222)
Balance at end of period	11,706	13,462	11,706	13,462
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			71	74
Other comprehensive income (loss)			71	74
Balance at end of period	(2,126)	(2,155)	(2,126)	(2,155)
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			7	6
Other comprehensive income (loss)			7	6
Balance at end of period	(265)	(178)	(265)	(178)
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			(6)	(230)
Amounts reclassified from accumulated other comprehensive loss			0	0
Other comprehensive income (loss)			(6)	(230)
Balance at end of period	(271)	(458)	(271)	(458)
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	(88)
Amounts reclassified from accumulated other comprehensive loss			31	16
Other comprehensive income (loss)			31	(72)
Balance at end of period	(48)	(111)	(48)	(111)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,845)	(2,955)	(2,813)	(2,680)
Other comprehensive income (loss)	135	53	103	(222)
Balance at end of period	$ (2,710)	$ (2,902)	$ (2,710)	$ (2,902)