Pension and Other Postretirement Benefits - Change in Plan Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Change in benefit obligation:
Benefit obligation at January 1	$ 2,562	$ 2,462
Service cost	63	62	$ 70
Interest cost	72	92	92
Plan participants’ contributions	2	2
Actuarial (gain) loss	26	216
Acquisitions and divestitures	0	7
Benefits paid	(152)	(150)
Medicare subsidy received	0	0
Foreign currency translation	88	(129)
Benefit obligation at December 31	2,661	2,562	2,462
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at January 1	551	552
Service cost	9	9	11
Interest cost	19	24	24
Plan participants’ contributions	12	12
Actuarial (gain) loss	(5)	(5)
Acquisitions and divestitures	0	0
Benefits paid	(41)	(43)
Medicare subsidy received	1	2
Foreign currency translation	0	0
Benefit obligation at December 31	$ 546	$ 551	$ 552