Derivative instruments (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets
Total derivative assets, fair value	$ 99	$ 549
Liabilities
Total derivative liabilities, fair value	82	74
Derivatives designated as hedging instrument [Member]
Assets
Total derivative assets, fair value	94	549
Liabilities
Total derivative liabilities, fair value	81	74
Derivatives not designated as hedging instrument [Member]
Assets
Total derivative assets, fair value	5	0
Liabilities
Total derivative liabilities, fair value	1	0
Interest rate swap contracts [Member] | Other current assets/Other noncurrent assets [Member] | Derivatives designated as hedging instrument [Member]
Assets
Total derivative assets, fair value		377
Interest rate swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member] | Derivatives designated as hedging instrument [Member]
Liabilities
Total derivative liabilities, fair value		0
Cross currency swap contracts [Member] | Other current assets/Other noncurrent assets [Member] | Derivatives designated as hedging instrument [Member]
Assets
Total derivative assets, fair value	12	0
Cross currency swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member] | Derivatives designated as hedging instrument [Member]
Liabilities
Total derivative liabilities, fair value	27	26
Foreign currency contracts [Member] | Other current assets/Other noncurrent assets [Member] | Derivatives designated as hedging instrument [Member]
Assets
Total derivative assets, fair value	82	172
Foreign currency contracts [Member] | Other current assets [Member] | Derivatives not designated as hedging instrument [Member]
Assets
Total derivative assets, fair value	5	0
Foreign currency contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member] | Derivatives designated as hedging instrument [Member]
Liabilities
Total derivative liabilities, fair value	54	48
Foreign currency contracts [Member] | Accrued liabilities [Member] | Derivatives not designated as hedging instrument [Member]
Liabilities
Total derivative liabilities, fair value	$ 1	$ 0