Employee Benefits - Unfunded Status (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Non-current portion of unfunded status	$ (4,556)	$ (4,594)
Combined Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(12,202)	(12,217)	$ (11,594)	$ (13,064)
Fair value of plan assets (liabilities)	10,546	10,493	10,033	11,060
Unfunded status	(1,656)	(1,724)
Current portion of unfunded status	0	0
Non-current portion of unfunded status	(1,656)	(1,724)
Post-Retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(3,048)	(3,037)	(2,977)	$ (3,375)
Fair value of plan assets (liabilities)	5	13	$ 18
Unfunded status	(3,043)	(3,024)
Current portion of unfunded status	(228)	(224)
Non-current portion of unfunded status	$ (2,815)	$ (2,800)