Postretirement benefits (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 31, 2016	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expenses recognized during period under the defined contributions plans			$ 171	$ 166	$ 150
Available for sale, Fair Value			78,470	78,939
Defined benefit plan, benefits paid inclusive of settlements			$ 209	200
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage			55.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage			65.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation			$ 4,000	4,300
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			22
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount		$ 159	143	157	159
Actuarial Loss (gain)			(20)	(2)
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation			23	21
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			1
Defined Benefit Plan, Plan Amendment [Abstract]
(Increase) decrease in other comprehensive income, curtailment			3	39	0
Amortization of Prior Service Cost (Credit)			85	89	15
Non-US [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount		962	1,008	1,172	962
Actuarial Loss (gain)			(71)	(4)
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation			115	152
Defined Benefit Plan, Plan Amendment [Abstract]
(Increase) decrease in other comprehensive income, curtailment			0	(6)	0
Amortization of Prior Service Cost (Credit)			0	0	1
UNITED STATES
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation		496
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount		2,765	2,784	3,109	2,765
Actuarial Loss (gain)			(214)	232
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation			3,066	3,223
Defined Benefit Plan, Plan Amendment [Abstract]
(Increase) decrease in other comprehensive income, curtailment			0	0	117
Effect of Plan Amendment on Accumulated Benefit Obligation		$ 113
Amortization of Prior Service Cost (Credit)			0	0	0
UNITED STATES | Postretirement Health Coverage [Member]
Defined Benefit Plan, Plan Amendment [Abstract]
(Increase) decrease in other comprehensive income, curtailment			3	39
Effect of Plan Amendment on Accumulated Benefit Obligation	$ 383
Amortization of Prior Service Cost (Credit)			80	$ 89	$ 15
Remaining Balance of Change in Benefit Obligation For Plan Amendment			$ 184
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member]
Defined Benefit Plan, Plan Amendment [Abstract]
Effect of Plan Amendment on Accumulated Benefit Obligation	$ 410