Employee Benefits - Unfunded Status (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ (15,300)	$ (14,800)	$ (16,500)
Non-current portion of unfunded status	(4,594)	(4,319)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(12,217)	(11,594)	(13,064)	$ (13,244)
Fair value of plan assets	10,493	10,033	11,060	10,892
Unfunded status	(1,724)	(1,561)
Current portion of unfunded status	0	0
Non-current portion of unfunded status	(1,724)	(1,561)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(3,037)	(2,977)	(3,375)	(3,413)
Fair value of plan assets	13	18	$ 23	$ 53
Unfunded status	(3,024)	(2,959)
Current portion of unfunded status	(224)	(252)
Non-current portion of unfunded status	$ (2,800)	$ (2,707)