Postretirement Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 1
Interest cost	659	705
Expected return on plan assets	(829)	(861)
Amortization of prior service credits	(20)	(20)
Recognized net actuarial loss/(gain)	67	42
Net periodic benefit income	(121)	(133)
Postretirement
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	12
Interest cost	31	37
Expected return on plan assets	(2)	(2)
Amortization of prior service credits	(3)	(6)
Recognized net actuarial loss/(gain)	(44)	(44)
Net periodic benefit income	(6)	(3)
Operating Income (Loss) | Pension
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	2	1
Operating Income (Loss) | Postretirement
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	11	15
Other Income | Pension
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	(123)	(134)
Other Income | Postretirement
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	(18)	(15)
Operating Income (Loss) Before Taxes | Pension
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	(121)	(133)
Operating Income (Loss) Before Taxes | Postretirement
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (loss)/earnings from operations, other income, and loss before income taxes	$ (7)	$ 0