Fair value measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended												12 Months Ended											12 Months Ended											12 Months Ended
	Dec. 31, 2011 Total Losses	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1)	Sep. 30, 2012 Fair value measurements using quoted prices in active markets for identical assets (Level 1) Recurring	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1) Recurring	Sep. 30, 2012 Fair value measurements using quoted prices in active markets for identical assets (Level 1) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1) U.S. treasuries with original maturities of three months or less Recurring	Sep. 30, 2012 Fair value measurements using quoted prices in active markets for identical assets (Level 1) U.S. treasuries and government agency securities Recurring	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1) U.S. treasuries and government agency securities Recurring	Sep. 30, 2012 Fair value measurements using quoted prices in active markets for identical assets (Level 1) ARS Recurring	Dec. 31, 2011 Fair value measurements using quoted prices in active markets for identical assets (Level 1) ARS Recurring	Sep. 30, 2012 Fair value measurements using quoted prices in active markets for identical assets (Level 1) Foreign exchange contract derivatives Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2)	Sep. 30, 2012 Fair value measurements using significant other observable inputs (Level 2) Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2) Recurring	Sep. 30, 2012 Fair value measurements using significant other observable inputs (Level 2) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2) U.S. treasuries with original maturities of three months or less Recurring	Sep. 30, 2012 Fair value measurements using significant other observable inputs (Level 2) U.S. treasuries and government agency securities Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2) U.S. treasuries and government agency securities Recurring	Sep. 30, 2012 Fair value measurements using significant other observable inputs (Level 2) ARS Recurring	Dec. 31, 2011 Fair value measurements using significant other observable inputs (Level 2) ARS Recurring	Sep. 30, 2012 Fair value measurements using significant other observable inputs (Level 2) Foreign exchange contract derivatives Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3)	Sep. 30, 2012 Fair value measurements using significant unobservable inputs (Level 3) Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3) Recurring	Sep. 30, 2012 Fair value measurements using significant unobservable inputs (Level 3) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3) Money market funds Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3) U.S. treasuries with original maturities of three months or less Recurring	Sep. 30, 2012 Fair value measurements using significant unobservable inputs (Level 3) U.S. treasuries and government agency securities Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3) U.S. treasuries and government agency securities Recurring	Sep. 30, 2012 Fair value measurements using significant unobservable inputs (Level 3) ARS Recurring	Dec. 31, 2011 Fair value measurements using significant unobservable inputs (Level 3) ARS Recurring	Sep. 30, 2012 Fair value measurements using significant unobservable inputs (Level 3) Foreign exchange contract derivatives Recurring	Dec. 31, 2011 Fair value	Sep. 30, 2012 Fair value Recurring	Dec. 31, 2011 Fair value Recurring	Sep. 30, 2012 Fair value Money market funds Recurring	Dec. 31, 2011 Fair value Money market funds Recurring	Dec. 31, 2011 Fair value U.S. treasuries with original maturities of three months or less Recurring	Sep. 30, 2012 Fair value U.S. treasuries and government agency securities Recurring	Dec. 31, 2011 Fair value U.S. treasuries and government agency securities Recurring	Sep. 30, 2012 Fair value ARS Recurring	Dec. 31, 2011 Fair value ARS Recurring	Sep. 30, 2012 Fair value Foreign exchange contract derivatives Recurring
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total recurring fair value measurements			$ 3,191	$ 3,215	$ 2,774	$ 2,869	$ 2	$ 417	$ 344	$ 0	$ 0	$ 0		$ 3	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 3		$ 19	$ 16	$ 0	$ 0	$ 0	$ 0	$ 0	$ 19	$ 16	$ 0		$ 3,213	$ 3,231	$ 2,774	$ 2,869	$ 2	$ 417	$ 344	$ 19	$ 16	$ 3
Goodwill	(12)	0											0											7,111											7,111
Total non-recurring fair value measurements	$ (12)	$ 0											$ 0											$ 7,111											$ 7,111