Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable To Parent, Tax [Roll Forward]
Net Change in AOCL	$ 21	$ (297)	$ (222)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	10,374	11,777
Recognition of Net Periodic Benefits Expense	411	622
Deferrals	(122)	437
Net Change in AOCL	289	1,059	655
Balance at End of Period	417	10,374	11,777
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(677)	(1,741)
Recognition of Net Periodic Benefits Expense	29	493
Deferrals	(121)	571
Net Change in AOCL	(92)	1,064
Balance at End of Period	(769)	(677)	(1,741)
Combined Pension Plan | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable To Parent, Tax [Roll Forward]
Balance at Beginning of Period	367	559
Recognition of Net Periodic Benefits Expense	(23)	(166)
Deferrals	37	(26)
Net Change in AOCL	14	(192)
Balance at End of Period	381	367	559
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	(985)	(1,577)
Recognition of Net Periodic Benefits Expense	50	494
Deferrals	(110)	98
Net Change in AOCL	(60)	592
Balance at End of Period	(1,045)	(985)	(1,577)
Combined Pension Plan | Net actuarial (loss) gain
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	(1,752)	(2,564)
Recognition of Net Periodic Benefits Expense	80	688
Deferrals	(147)	124
Net Change in AOCL	(67)	812
Balance at End of Period	(1,819)	(1,752)	(2,564)
Combined Pension Plan | Settlement charge
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	383	383
Recognition of Net Periodic Benefits Expense	0	0
Deferrals	0	0
Net Change in AOCL	0	0
Balance at End of Period	383	383	383
Combined Pension Plan | Prior service benefit (cost)
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	17	45
Recognition of Net Periodic Benefits Expense	(7)	(28)
Deferrals	0	0
Net Change in AOCL	(7)	(28)
Balance at End of Period	10	17	45
Post-Retirement Benefit Plans | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable To Parent, Tax [Roll Forward]
Balance at Beginning of Period	(104)	54
Recognition of Net Periodic Benefits Expense	7	1
Deferrals	3	(159)
Net Change in AOCL	10	(158)
Balance at End of Period	(94)	(104)	54
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at Beginning of Period	308	(164)
Recognition of Net Periodic Benefits Expense	(21)	(1)
Deferrals	(11)	473
Net Change in AOCL	(32)	472
Balance at End of Period	276	308	(164)
Post-Retirement Benefit Plans | Net actuarial (loss) gain
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	371	(217)
Recognition of Net Periodic Benefits Expense	(20)	(3)
Deferrals	(14)	591
Net Change in AOCL	(34)	588
Balance at End of Period	337	371	(217)
Post-Retirement Benefit Plans | Prior service benefit (cost)
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	37	(5)
Recognition of Net Periodic Benefits Expense	(8)	1
Deferrals	0	41
Net Change in AOCL	(8)	42
Balance at End of Period	29	37	(5)
Post-Retirement Benefit Plans | Curtailment loss
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at Beginning of Period	4	4
Recognition of Net Periodic Benefits Expense	0	0
Deferrals	0	0
Net Change in AOCL	0	0
Balance at End of Period	$ 4	$ 4	$ 4