CONDENSED CONSOLIDATED STATEMENTS OF CAPITALIZATION (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Schedule of Capitalization
Common stock - $4 par value; 32,000,000 shares authorized; 23,896,962 shares outstanding	$ 95.6	$ 95.6
Additional paid-in capital	553.9	626.7
Retained earnings	434.5	424.9
Total common stock equity	1,084.0	1,147.2
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	511,882
Total preferred stock	51.2	51.2
Total long-term debt to parent	8.3	8.6
Long-term debt
Total First Mortgage Bonds and Senior Notes	872.1	872.1
Unamortized discount on long-term debt	(0.9)	(1.0)
Total	871.2	871.1
Current portion	(150.0)	(150.0)
Total long-term debt	721.2	721.1
Total capitalization	1,864.7	1,928.1
Preferred stock, 5.00% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.00%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	131,916
Total preferred stock	13.2	13.2
Preferred stock, 5.04% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.04%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	29,983
Total preferred stock	3.0	3.0
Preferred stock, 5.08% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.08%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	49,983
Total preferred stock	5.0	5.0
Preferred stock, 6.76% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	6.76%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	150,000
Total preferred stock	15.0	15.0
Preferred stock, 6.88% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	6.88%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption -	150,000
Total preferred stock	15.0	15.0
Long-term debt to parent, 8.76% Series, Year Due, 2015
Schedule of Capitalization
Interest rate, (as a percent)	8.76%
Total long-term debt to parent	3.3	3.4
Long-term debt to parent, 7.35% Series, Year Due, 2016
Schedule of Capitalization
Interest rate, (as a percent)	7.35%
Total long-term debt to parent	5.0	5.2
Long-term debt, 7.125% Series, Year Due, 2023
Schedule of Capitalization
Interest rate, (as a percent)	7.125%
Long-term debt
First Mortgage Bonds	0.1	0.1
Long-term debt, 6.125% Series, Year Due, 2011
Schedule of Capitalization
Interest rate, (as a percent)	6.125%
Long-term debt
Senior Notes	150.0	150.0
Long-term debt, 4.875% Series, Year Due, 2012
Schedule of Capitalization
Interest rate, (as a percent)	4.875%
Long-term debt
Senior Notes	150.0	150.0
Long-term debt, 3.95% Series, Year Due, 2013
Schedule of Capitalization
Interest rate, (as a percent)	3.95%
Long-term debt
Senior Notes	22.0	22.0
Long-term debt, 4.80% Series, Year Due, 2013
Schedule of Capitalization
Interest rate, (as a percent)	4.80%
Long-term debt
Senior Notes	125.0	125.0
Long-term debt, 6.375% Series, Year Due, 2015
Schedule of Capitalization
Interest rate, (as a percent)	6.375%
Long-term debt
Senior Notes	125.0	125.0
Long-term debt, 5.65% Series, Year Due, 2017
Schedule of Capitalization
Interest rate, (as a percent)	5.65%
Long-term debt
Senior Notes	125.0	125.0
Long-term debt, 6.08% Series, Year Due, 2028
Schedule of Capitalization
Interest rate, (as a percent)	6.08%
Long-term debt
Senior Notes	50.0	50.0
Long-term debt, 5.55% Series, Year Due, 2036
Schedule of Capitalization
Interest rate, (as a percent)	5.55%
Long-term debt
Senior Notes	$ 125.0	$ 125.0