Pension and Other Postretirement Benefits - Change in Plan Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension
Change in benefit obligation:
Benefit obligation at January 1	$ 2,661	$ 2,562
Service cost	60	63	$ 62
Interest cost	72	72	92
Plan participants’ contributions	2	2
Amendments	9	0
Actuarial (gain) loss	(162)	26
Benefits paid	(165)	(152)
Medicare subsidy received	0	0
Liabilities from other immaterial plans	5	0
Foreign currency translation	(53)	88
Benefit obligation at December 31	2,429	2,661	2,562
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at January 1	546	551
Service cost	8	9	9
Interest cost	18	19	24
Plan participants’ contributions	12	12
Amendments	0	0
Actuarial (gain) loss	(35)	(5)
Benefits paid	(40)	(41)
Medicare subsidy received	2	1
Liabilities from other immaterial plans	0	0
Foreign currency translation	0	0
Benefit obligation at December 31	$ 511	$ 546	$ 551