Employee Benefits - Unfunded Status (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ (14,800)	$ (16,500)	$ (16,700)
Non-current portion of unfunded status	(4,319)	(5,178)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(11,594)	(13,064)	(13,244)	$ (13,287)
Fair value of plan assets	10,033	11,060	10,892	11,072
Unfunded status	(1,561)	(2,004)
Current portion of unfunded status	0	0
Non-current portion of unfunded status	(1,561)	(2,004)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	(2,977)	(3,375)	(3,413)	(3,567)
Fair value of plan assets	18	23	$ 53	$ 193
Unfunded status	(2,959)	(3,352)
Current portion of unfunded status	(252)	(262)
Non-current portion of unfunded status	$ (2,707)	$ (3,090)