Derivative instruments (Fair Value of Derivatives) (Details) - Derivatives designated as hedging instruments [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Total derivative assets, fair value	$ 286	$ 244
Liabilities
Total derivative liabilities, fair value	485	534
Foreign currency contracts [Member] | Other current assets/Other noncurrent assets [Member]
Assets
Total derivative assets, fair value	6	203
Foreign currency contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member]
Liabilities
Total derivative liabilities, fair value	204	4
Cross-currency swap contracts [Member] | Other current assets/ Other noncurrent assets [Member]
Assets
Total derivative assets, fair value	270	0
Cross-currency swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member]
Liabilities
Total derivative liabilities, fair value	220	523
Interest rate swap contracts [Member] | Other current assets/ Other noncurrent assets [Member]
Assets
Total derivative assets, fair value	10	41
Interest rate swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member]
Liabilities
Total derivative liabilities, fair value	$ 61	$ 7