Debt (Details 3) (USD $) In Millions, unless otherwise specified	1 Months Ended			1 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2012	Sep. 30, 2012	Dec. 31, 2011	Feb. 29, 2012 1.50% Series B Convertible Senior Notes due December 2037	Sep. 30, 2011 1.50% Series B Convertible Senior Notes due December 2037	Sep. 30, 2011 1.50% Series B Convertible Senior Notes due December 2037	Dec. 31, 2011 1.50% Series B Convertible Senior Notes due December 2037	Sep. 30, 2012 1.50% Series C Convertible Senior Notes due December 2037	Sep. 30, 2011 1.50% Series C Convertible Senior Notes due December 2037	Sep. 30, 2012 1.50% Series C Convertible Senior Notes due December 2037	Sep. 30, 2011 1.50% Series C Convertible Senior Notes due December 2037	Dec. 31, 2011 1.50% Series C Convertible Senior Notes due December 2037
Carrying amounts of Convertible Senior Notes
Convertible debt instrument effective interest rate (as a percent)												5.28%
Remaining period over which discount will be amortized										1 year
Interest expense					$ 21	$ 61		$ 22	$ 21	$ 65	$ 62
Principal amount		14,104					30	1,722		1,722		1,722
Unamortized discount								(13)		(13)		(59)
Total debt		14,117	13,536				30	1,709		1,709		1,663
Carrying amount of equity component							4	276		276		276
Aggregate principal amount of debt repurchased	148			30
Aggregate cash payment made for debt repurchased				$ 30