Postemployment benefit plans (Details 2) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation:
Actuarial losses (gains)	$ 495	$ 301	$ 495	$ 301	$ 985
U.S. pensions
Change in benefit obligation:
Benefit obligation, beginning of year			17,326	16,218
Service cost			125	115	119
Interest cost			534	525	517
Plan amendments			0	0
Actuarial losses (gains)			(1,058)	1,439
Foreign currency exchange rates			0	0
Participant contributions			0	0
Benefits paid - gross			(971)	(977)
Less: federal subsidy on benefits paid			0	0
Curtailments, settlements and termination benefits			(3)	6
Benefit obligation, end of year	15,953	17,326	15,953	17,326	16,218
Accumulated benefit obligation, end of year	$ 15,877	$ 17,175	$ 15,877	$ 17,175
Weighted-average assumptions used to determine benefit obligation:
Discount rate (as a percent)	4.20%	3.50%	4.20%	3.50%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%	4.00%
Non-U.S. pensions
Change in benefit obligation:
Benefit obligation, beginning of year			$ 4,606	$ 4,472
Service cost			89	95	92
Interest cost			96	101	117
Plan amendments			26	(1)
Actuarial losses (gains)			(88)	(75)
Foreign currency exchange rates			(205)	312
Participant contributions			6	6
Benefits paid - gross			(277)	(203)
Less: federal subsidy on benefits paid			0	0
Curtailments, settlements and termination benefits			(38)	(101)
Benefit obligation, end of year	$ 4,215	$ 4,606	4,215	4,606	4,472
Accumulated benefit obligation, end of year	$ 4,038	$ 4,335	$ 4,038	$ 4,335
Weighted-average assumptions used to determine benefit obligation:
Discount rate (as a percent)	2.50%	2.40%	2.50%	2.40%
Rate of compensation increase (as a percent)	3.00%	4.00%	3.00%	4.00%
Other postretirement benefits
Change in benefit obligation:
Benefit obligation, beginning of year			$ 4,002	$ 4,088
Service cost			83	78	82
Interest cost			125	130	131
Plan amendments			(25)	(79)
Actuarial losses (gains)			(195)	71
Foreign currency exchange rates			(28)	4
Participant contributions			51	59
Benefits paid - gross			(369)	(361)
Less: federal subsidy on benefits paid			7	10
Curtailments, settlements and termination benefits			(2)	2
Benefit obligation, end of year	$ 3,649	$ 4,002	$ 3,649	$ 4,002	$ 4,088
Weighted-average assumptions used to determine benefit obligation:
Discount rate (as a percent)	4.20%	3.60%	4.20%	3.60%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%	4.00%