Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 13,470	$ 19,828
Other comprehensive (loss) income before reclassifications	(341)	(396)
Amounts reclassified from accumulated other comprehensive loss	208	177
Other comprehensive loss	(133)	(219)	$ (59)
Balance at end of period	11,162	13,470	19,828
Defined Benefit Plans | Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,229)	(2,173)
Other comprehensive (loss) income before reclassifications	(115)	(219)
Amounts reclassified from accumulated other comprehensive loss	147	163
Other comprehensive loss	32	(56)
Balance at end of period	(2,197)	(2,229)	(2,173)
Defined Benefit Plans | Post-Retirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(184)	(58)
Other comprehensive (loss) income before reclassifications	(103)	(138)
Amounts reclassified from accumulated other comprehensive loss	15	12
Other comprehensive loss	(88)	(126)
Balance at end of period	(272)	(184)	(58)
Foreign Currency Translation Adjustment and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(228)	(230)
Other comprehensive (loss) income before reclassifications	(37)	2
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive loss	(37)	2
Balance at end of period	(265)	(228)	(230)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(39)	0
Other comprehensive (loss) income before reclassifications	(86)	(41)
Amounts reclassified from accumulated other comprehensive loss	46	2
Other comprehensive loss	(40)	(39)
Balance at end of period	(79)	(39)	0
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,680)	(2,461)	(1,995)
Other comprehensive loss	(133)	(219)	(59)
Balance at end of period	$ (2,813)	$ (2,680)	$ (2,461)