Financing (Details 2) (Unsecured revolving credit facility, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 quarter
Unsecured revolving credit facility
SHORT-TERM BORROWINGS
Unsecured revolving credit facility	$ 150
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement	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	50.40%
EBITDA coverage ratio over interest charges	4.88
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	$ 17.9