Postemployment benefit plans (Details 8) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net periodic benefit cost:
Defined benefit plan curtailment and settlement losses	$ (92)	$ (11)
U.S. Pension Benefits
Components of net periodic benefit cost:
Service cost	181	157	$ 196
Interest cost	608	648	581
Expected return on plan assets	(879)	(885)	(832)
Other adjustments	0	0	31
Defined benefit plan curtailment and settlement losses	52	0	0
Amortization of:
Transition obligation / (asset)	0	0	0
Prior service cost / (credit)	1	17	18
Net actuarial loss / (gain)	490	392	546
Total cost included in operating profit	453	329	540
Other changes in plan assets and benefit obligations recognized in other comprehensive income (pre-tax):
Current year actuarial loss (gain)	701	2,030	(2,344)
Amortization of actuarial (loss) gain	(490)	(392)	(546)
Current year prior service cost (credit)	0	0	0
Amortization of prior service (cost) credit	(1)	(17)	(18)
Amortization of transition (obligation) asset	0	0	0
Total recognized in other comprehensive income	210	1,621	(2,908)
Total recognized in net periodic cost and other comprehensive income	$ 663	$ 1,950	$ (2,368)
Weighted-average assumptions used to determine net cost:
Discount rate (as a percent)	3.80%	4.60%	3.70%
Expected return on plan assets (as a percent)	7.40%	7.80%	7.80%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.50%
Expected return on plan assets, next fiscal year (as a percent)	6.90%
Percentage of the highest and lowest yielding bonds that were excluded from the discount rate calculation (as a percent)	10.00%
Additional percentage amount added to long-term passive rate of returns to arrive at the long-term expected rate of return (as a percent)	0.95%	1.00%	1.00%
Non-U.S. Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 110	$ 109	$ 120
Interest cost	146	185	166
Expected return on plan assets	(260)	(258)	(225)
Other adjustments	0	0	0
Defined benefit plan curtailment and settlement losses	14	14	2
Amortization of:
Transition obligation / (asset)	0	0	0
Prior service cost / (credit)	0	0	1
Net actuarial loss / (gain)	99	86	128
Total cost included in operating profit	109	136	192
Other changes in plan assets and benefit obligations recognized in other comprehensive income (pre-tax):
Current year actuarial loss (gain)	(95)	207	(406)
Amortization of actuarial (loss) gain	(99)	(86)	(128)
Current year prior service cost (credit)	(8)	(4)	(7)
Amortization of prior service (cost) credit	0	0	(1)
Amortization of transition (obligation) asset	0	0	0
Total recognized in other comprehensive income	(202)	117	(542)
Total recognized in net periodic cost and other comprehensive income	$ (93)	$ 253	$ (350)
Weighted-average assumptions used to determine net cost:
Discount rate (as a percent)	3.30%	4.10%	3.70%
Expected return on plan assets (as a percent)	6.80%	6.90%	6.80%
Rate of compensation increase (as a percent)	4.00%	4.20%	3.90%
Expected return on plan assets, next fiscal year (as a percent)	6.10%
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	$ 101	$ 82	$ 108
Interest cost	181	213	195
Expected return on plan assets	(53)	(52)	(56)
Other adjustments	0	0	(22)
Defined benefit plan curtailment and settlement losses	32	(2)	0
Amortization of:
Transition obligation / (asset)	0	0	2
Prior service cost / (credit)	(54)	(55)	(73)
Net actuarial loss / (gain)	52	41	107
Total cost included in operating profit	259	227	261
Other changes in plan assets and benefit obligations recognized in other comprehensive income (pre-tax):
Current year actuarial loss (gain)	(577)	179	(759)
Amortization of actuarial (loss) gain	(52)	(41)	(107)
Current year prior service cost (credit)	16	(2)	2
Amortization of prior service (cost) credit	54	55	73
Amortization of transition (obligation) asset	0	0	(2)
Total recognized in other comprehensive income	(559)	191	(793)
Total recognized in net periodic cost and other comprehensive income	$ (300)	$ 418	$ (532)
Weighted-average assumptions used to determine net cost:
Discount rate (as a percent)	3.90%	4.60%	3.70%
Expected return on plan assets (as a percent)	7.80%	7.80%	7.80%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.40%
Assumed increase in health care trend rate
Assumed increase in health care trend rate over the current period to calculate benefit expenses (as a percent)	6.60%
Assumed increase in health care trend rate for the next year to calculate benefit expenses (as a percent)	6.50%
Year that heath care trend rate is assumed to reach ultimate trend rate (year)	2021
Ultimate health care cost trend rate (as a percent)	5.00%
General inflation rate that forms a part of ultimate health care trend rate (as a percent)	3.00%
Additional healthcare inflation rate that forms a part of ultimate health care trend rate (as a percent)	2.00%