Financing (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended
	Jan. 26, 2010	Jun. 30, 2011	Feb. 07, 2011
Financing
Shelf registration statement, amount			$ 400
Unsecured revolving credit facility	150
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement (as a percent)	62.50%
Minimum interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement	2
Number of trailing fiscal quarters used for calculating the interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement	4
Total indebtedness as a percent of total capitalization (as a percent)		52.00%
EBITDA coverage ratio over interest charges		5.2
Aggregate amount of default on other indebtedness subject to cross-default under the terms of the credit facility.	10
Amount of credit facility used to back up outstanding commercial paper borrowings		$ 18.5