Derivative instruments (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets
Total derivative assets, fair value	$ 549	$ 349
Liabilities
Total derivative liabilities, fair value	74	103
Derivatives Designated as Hedging Instrument [Member]
Assets
Total derivative assets, fair value	549	349
Liabilities
Total derivative liabilities, fair value	74	103
Derivatives Not Designated as Hedging Instrument [Member]
Assets
Total derivative assets, fair value	0	0
Liabilities
Total derivative liabilities, fair value	0	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	195
Interest rate swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member] | Derivatives Designated as Hedging Instrument [Member]
Liabilities
Total derivative liabilities, fair value	0	0
Foreign currency contracts [Member] | Other current assets/Other noncurrent assets [Member] | Derivatives Designated as Hedging Instrument [Member]
Assets
Total derivative assets, fair value	172	154
Foreign currency contracts [Member] | Other current assets [Member] | Derivatives Not Designated as Hedging Instrument [Member]
Assets
Total derivative assets, fair value	0	0
Foreign currency contracts [Member] | Accrued liabilities/Other noncurrent liabilities [Member] | Derivatives Designated as Hedging Instrument [Member]
Liabilities
Total derivative liabilities, fair value	48	103
Foreign currency contracts [Member] | Accrued liabilities[ Member] | Derivatives Not Designated as Hedging Instrument [Member]
Liabilities
Total derivative liabilities, fair value	0	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	0
Cross currency swap contracts [Member] | Accrued liabilities/Other noncurrent liabilities[Member] | Derivatives Designated as Hedging Instrument [Member]
Liabilities
Total derivative liabilities, fair value	$ 26