Financing (Details 2) (Unsecured revolving credit facility, USD $)	3 Months Ended
	Mar. 31, 2013 item
Unsecured revolving credit facility
SHORT-TERM BORROWINGS
Unsecured revolving credit facility	$ 150,000,000
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement	0.625
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	49.70%
EBITDA coverage ratio over interest charges	5.0
Aggregate amount of default on other indebtedness subject to cross-default under the terms of the credit facility.	10,000,000
Outstanding borrowings	0
Amount of credit facility used to back up outstanding commercial paper borrowings	$ 23,000,000