Short-Term Debt (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Abstract]
Commercial Paper	$ 141.60	$ 257.40
Short-term Debt, Weighted Average Interest Rate	0.30%	0.40%
Bank Loans	0	0
Line of Credit Facility, Current Borrowing Capacity	250.00
Line of Credit Facility, Expiration Date	May 31, 2013
Line of Credit Facility, Maximum Borrowing Capacity	400.00
Line of Credit Facility, Amount Outstanding	$ 0
Line of Credit Facility, Covenant Terms	The syndicated credit agreement requires that we maintain credit ratings with Standard & Poor’s (S&P) and Moody’s Investors Service, Inc. (Moody’s) and notify the lenders of any change in our senior unsecured debt ratings by such rating agencies. A change in our debt ratings is not an event of default, nor is the maintenance of a specific minimum level of debt rating a condition of drawing upon the credit facility. However, interest rates on any loans outstanding under the credit facility are tied to debt ratings, which would increase or decrease the cost of any loans under the credit facility when ratings are changed. There were no changes in our credit ratings during 2011. The syndicated credit agreement also requires us to maintain a consolidated indebtedness to total capitalization ratio of 70 percent or less. Failure to comply with this covenant would entitle the lenders to terminate their lending commitments and accelerate the maturity of all amounts outstanding. We were in compliance with this covenant at December 31, 2011 and 2010.