Debt (Details 2) (USD $)	Sep. 30, 2012 Fixed rate senior notes	Dec. 31, 2011 Fixed rate senior notes	Dec. 31, 2011 4.25% senior notes due September 2012	Sep. 30, 2012 3.65% senior notes due June 2022	Mar. 31, 2012 3.65% senior notes due June 2022	Sep. 30, 2012 5.15% senior notes due June 2042	Mar. 31, 2012 5.15% senior notes due June 2042
Debt
Senior notes				$ 750,000,000	$ 750,000,000	$ 500,000,000	$ 500,000,000
Senior Notes, Current			500,000,000
Debt instrument, interest rate (as a percent)			4.25%	3.65%	3.65%	5.15%	5.15%
Percentage of face value at which senior notes are sold					99.823%		99.755%
Debt instrument face value	5,500,000,000	4,800,000,000
Debt instrument fair value	$ 6,500,000,000	$ 5,400,000,000