Employee Benefits - Unfunded Status (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Non-current portion of unfunded status	$ (2,490)	$ (2,391)
Combined Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(5,212)	(5,295)	$ (9,678)	$ (12,202)
Fair value of plan assets (liabilities)	4,476	4,715	8,531	10,546
Unfunded status	(736)	(580)
Current portion of unfunded status	0	0
Non-current portion of unfunded status	(736)	(580)
Post-Retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(1,919)	(1,995)	(2,781)	$ (3,048)
Fair value of plan assets (liabilities)	1	5	$ 5
Unfunded status	(1,918)	(1,990)
Current portion of unfunded status	(193)	(210)
Non-current portion of unfunded status	$ (1,725)	$ (1,780)