Debt and Interest (Details) (USD $)									1 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Commercial paper	Aug. 31, 2013 Commercial paper	Dec. 31, 2012 Commercial paper	Sep. 30, 2013 Notes payable	Dec. 31, 2012 Notes payable	Sep. 30, 2013 Multi-year credit facility	Aug. 31, 2013 New 364 day credit facility
Components of the company's debt obligations, along with applicable interest rate
Short-term debt			$ 497,300,000		$ 593,700,000	$ 45,000,000	$ 44,500,000
Long-term debt, current maturities	170,900,000	167,600,000
Short-term debt including current maturities of long-term debt	713,200,000	805,800,000
Maximum borrowing capacity, commercial paper before expiration				2,000,000,000
Term of credit facility									364 days
Amount of commercial paper facility expired									500,000,000
Maximum borrowing capacity, commercial paper			$ 1,500,000,000					$ 1,500,000,000