Postemployment benefit plans (Details 10) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	$ 8,365	$ 9,357
U.S. Pension Benefits
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	3,891	4,833
Non-U.S. Pension Benefits
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	639	717
Pension Plan [Member]
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	4,530	5,550
Postretirement Health Coverage [Member]
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	3,335	3,376
Other Postretirement Benefits
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	109	106
Postemployment Retirement Benefits [Member]
Postemployment Benefit Plan [Line Items]
Liability for postemployment benefits	$ 391	$ 325