Accumulated Other Comprehensive Loss - AOCI Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 10,374	$ 11,777
Other comprehensive income (loss) before reclassifications	(122)	437
Amounts reclassified from accumulated other comprehensive loss	411	622
Other comprehensive income	289	1,059	$ 655
Balance at End of Period	417	10,374	11,777
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(677)	(1,741)
Other comprehensive income (loss) before reclassifications	(121)	571
Amounts reclassified from accumulated other comprehensive loss	29	493
Other comprehensive income	(92)	1,064
Balance at End of Period	(769)	(677)	(1,741)
Defined Benefit Plans | Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(985)	(1,577)
Other comprehensive income (loss) before reclassifications	(110)	98
Amounts reclassified from accumulated other comprehensive loss	50	494
Other comprehensive income	(60)	592
Balance at End of Period	(1,045)	(985)	(1,577)
Defined Benefit Plans | Post-Retirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	308	(164)
Other comprehensive income (loss) before reclassifications	(11)	473
Amounts reclassified from accumulated other comprehensive loss	(21)	(1)
Other comprehensive income	(32)	472
Balance at End of Period	276	308	(164)
Foreign Currency Translation Adjustment and Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(422)	(400)
Other comprehensive income (loss) before reclassifications	(1)	(134)
Amounts reclassified from accumulated other comprehensive loss	382	112
Other comprehensive income	381	(22)
Balance at End of Period	(41)	(422)	(400)
Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	0	(17)
Other comprehensive income (loss) before reclassifications		0
Amounts reclassified from accumulated other comprehensive loss		17
Other comprehensive income		17
Balance at End of Period		0	(17)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(1,099)	(2,158)	(2,813)
Other comprehensive income	289	1,059	655
Balance at End of Period	$ (810)	$ (1,099)	$ (2,158)