Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and Other Postretirement Benefits
Pension and other postretirement benefit contributions
Contributions to pension and other postretirement benefit plans	$ 75	$ 92		$ 227	$ 198
Expected full year contributions to pension and other postretirement benefit plans during the year	365			365
UNITED STATES
Components of net periodic benefit cost:
Service cost	31	29		63	58
Interest cost	134	131		267	262
Expected return on plan assets	(203)	(183)		(405)	(367)
Amortization of prior service cost / (credit)	0	0		0	0
Curtailments and termination benefits			$ 9	0	9
Net periodic benefit cost (benefit)	(38)	(23)		$ (75)	$ (38)
Weighted-average assumptions used to determine net cost:
Discount rate used to measure service cost				3.70%	4.20%
Discount rate used to measure interest cost				3.20%	3.30%
Expected rate of return on plan assets (as a percent)				6.30%	6.70%
Rate of compensation increase (as a percent)				4.00%	4.00%
Foreign Plan
Components of net periodic benefit cost:
Service cost	23	24		$ 45	$ 47
Interest cost	24	25		49	50
Expected return on plan assets	(56)	(56)		(112)	(113)
Amortization of prior service cost / (credit)	0	(1)		0	(1)
Curtailments and termination benefits			$ 20	0	20
Net periodic benefit cost (benefit)	(9)	(8)		$ (18)	$ 3
Weighted-average assumptions used to determine net cost:
Discount rate used to measure service cost				2.30%	2.30%
Discount rate used to measure interest cost				2.20%	2.30%
Expected rate of return on plan assets (as a percent)				5.20%	5.90%
Rate of compensation increase (as a percent)				4.00%	4.00%
Other Postretirement Benefits Plan
Components of net periodic benefit cost:
Service cost	21	20		$ 42	$ 39
Interest cost	31	32		62	65
Expected return on plan assets	(8)	(9)		(16)	(18)
Amortization of prior service cost / (credit)	(8)	(6)		(17)	(11)
Curtailments and termination benefits				0	0
Net periodic benefit cost (benefit)	$ 36	$ 37		$ 71	$ 75
Weighted-average assumptions used to determine net cost:
Discount rate used to measure service cost				3.50%	3.90%
Discount rate used to measure interest cost				3.20%	3.30%
Expected rate of return on plan assets (as a percent)				7.50%	7.50%
Rate of compensation increase (as a percent)				4.60%	4.00%