Debt and Interest (Details) (USD $)							1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Commercial paper	Dec. 31, 2011 Commercial paper	Sep. 30, 2012 Notes payable	Dec. 31, 2011 Notes payable	Apr. 30, 2012 364 day credit facility	Aug. 31, 2012 New 364 day credit facility
Components of the company's debt obligations, along with applicable interest rate
Short-term debt			$ 560,300,000	$ 916,100,000	$ 64,900,000	$ 100,300,000
Long-term debt, current maturities	5,800,000	6,600,000
Short-term debt including current maturities of long-term debt	631,000,000	1,023,000,000
Maximum borrowing capacity under the credit agreement before amendment							2,000,000,000
Maximum borrowing capacity under the credit agreement							$ 1,000,000,000	$ 500,000,000
Term of credit facility							364 days	364 days