Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension and Other Postretirement Benefits
Pension and other postretirement benefit contributions
Contributions to pension and other postretirement benefit plans	$ 1,064	$ 324		$ 1,291	$ 522
Expected full year contributions to pension and other postretirement benefit plans during the year	1,362			1,362
UNITED STATES
Components of net periodic benefit cost:
Service cost	31	29		94	87
Interest cost	134	131		401	393
Expected return on plan assets	(202)	(184)		(607)	(551)
Amortization of prior service cost / (credit)	0	0		0	0
Curtailments and termination benefits			$ 9	0	9
Net periodic benefit cost (benefit)	(37)	(24)		$ (112)	$ (62)
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	22	23		$ 67	$ 70
Interest cost	25	23		74	73
Expected return on plan assets	(55)	(55)		(167)	(168)
Amortization of prior service cost / (credit)	0	0		0	(1)
Curtailments and termination benefits			$ 20	0	20
Net periodic benefit cost (benefit)	(8)	(9)		$ (26)	$ (6)
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%
Discretionary contribution
Pension and other postretirement benefit contributions
Contributions to pension and other postretirement benefit plans	1,000
Other Postretirement Benefits Plan
Components of net periodic benefit cost:
Service cost	22	19		$ 64	$ 58
Interest cost	31	33		93	98
Expected return on plan assets	(9)	(10)		(25)	(28)
Amortization of prior service cost / (credit)	(9)	(6)		(26)	(17)
Curtailments and termination benefits				0	0
Net periodic benefit cost (benefit)	$ 35	$ 36		$ 106	$ 111
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%