Fair value measurements (Details Textual)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2011 Cross currency swap contracts [Member] USD ($)	Dec. 31, 2011 Cross currency swap contracts [Member] GBP (£)	Dec. 31, 2011 Foreign currency forward contracts [Member] USD ($)	Dec. 31, 2010 Foreign currency forward contracts [Member] USD ($)	Dec. 31, 2011 Foreign currency option contracts [Member] USD ($)	Dec. 31, 2010 Foreign currency option contracts [Member] USD ($)
Fair Value Measurement (Textual) [Abstract]
Foreign currency open contracts - cash flow hedges - notional amounts				$ 748,000,000	£ 475,000,000	$ 3,500,000,000	$ 3,200,000,000	$ 292,000,000	$ 398,000,000
Foreign currency open contracts - not designated as hedges - notional amounts						389,000,000	670,000,000
Interest rate swap contracts - fair value hedge - notional amounts	3,600,000,000	3,600,000,000	1,500,000,000
Aggregate fair value of long-term debt, including current portion	23,000,000,000	14,500,000,000
Carrying value of long-term debt, including current portion	$ 21,428,000,000	$ 13,362,000,000