Debt (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Sep. 30, 2011 5.375% Senior Notes, due 2012 [Member]	Dec. 31, 2010 5.375% Senior Notes, due 2012 [Member]	Sep. 30, 2010 5.375% Senior Notes, due 2012 [Member]	Sep. 30, 2011 5.9% Senior Notes, due 2017 [Member]	Dec. 31, 2010 5.9% Senior Notes, due 2017 [Member]	Sep. 30, 2010 5.9% Senior Notes, due 2017 [Member]	Sep. 30, 2011 6.55% Senior Notes, due 2037 [Member]	Dec. 31, 2010 6.55% Senior Notes, due 2037 [Member]	Sep. 30, 2010 6.55% Senior Notes, due 2037 [Member]	Jul. 30, 2013 3 Year Facility [Member]
Short-term and long-term debt outstanding
Note payable	$ 100,000	$ 500,000	$ 100,000
Total debt	1,198,000,000	1,198,300,000	1,197,900,000	399,900,000	399,900,000	399,900,000	399,400,000	399,300,000	399,300,000	398,600,000	398,600,000	398,600,000
Less: short-term debt including current maturities	0	300,000	0
Long-term debt	1,198,000,000	1,198,000,000	1,197,900,000
Debt (Textuals) [Abstract]
Unamortized debt discount on senior notes				100,000			600,000			1,400,000
Line of Credit Facility [Line Items]
Size of total commercial paper program	$ 1,200,000,000												$ 1,200,000,000
Commitment fee	15.0 to 35.0 basis points, depending on credit rating and currently pay a commitment fee of 20.0 basis points
Commitment fee, minimum percentage to pay depending on credit rating	0.15%
Commitment fee, maximum percentage to pay depending on credit rating	0.35%
Commitment fee, current percentage	0.20%
Indebtedness to cash flow ratio	not greater than 4 to 1