Employee Benefits - Unfunded Status (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Non-current portion of unfunded status	$ (3,710)	$ (4,556)
Combined Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(9,678)	(12,202)	$ (12,217)	$ (11,594)
Fair value of plan assets (liabilities)	8,531	10,546	10,493	10,033
Unfunded status	(1,147)	(1,656)
Current portion of unfunded status	0	0
Non-current portion of unfunded status	(1,147)	(1,656)
Post-Retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(2,781)	(3,048)	(3,037)	$ (2,977)
Fair value of plan assets (liabilities)	5	5	$ 13
Unfunded status	(2,776)	(3,043)
Current portion of unfunded status	(212)	(228)
Non-current portion of unfunded status	$ (2,564)	$ (2,815)