CONDENSED CONSOLIDATED STATEMENTS OF CAPITALIZATION (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
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	756.3	555.4
Retained earnings	487.9	470.5
Total common stock equity	1,339.8	1,121.5
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	511,882
Total preferred stock	51.2	51.2
Total long-term debt to parent	7.0	7.2
Total First Mortgage Bonds and Senior Notes	850.1	872.1
Unamortized discount on long-term debt	(0.7)	(0.7)
Total	849.4	871.4
Current portion of long-term debt	(125.0)	(147.0)
Total long-term debt	724.4	724.4
Total capitalization	2,122.4	1,904.3
Long Term debt to parent, 8.76% Series, Year Due, 2015
Schedule of Capitalization
Interest rate, (as a percent)	8.76%	8.76%
Total long-term debt to parent	2.7	2.8
Long Term debt to parent, 7.35% Series, Year Due, 2016
Schedule of Capitalization
Interest rate, (as a percent)	7.35%	7.35%
Total long-term debt to parent	4.3	4.4
Long Term debt, 7.125% Series, Year Due, 2023
Schedule of Capitalization
Interest rate, (as a percent)	7.125%	7.125%
First Mortgage Bonds	0.1	0.1
Long Term debt, 3.95% Series, Year Due, 2013
Schedule of Capitalization
Interest rate, (as a percent)	3.95%	3.95%
Senior Notes		22.0
Long Term debt, 4.80% Series, Year Due, 2013
Schedule of Capitalization
Interest rate, (as a percent)	4.80%	4.80%
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%	6.375%
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%	5.65%
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%	6.08%
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%	5.55%
Senior Notes	125.0	125.0
Long Term debt, 3.671% Series, Year Due, 2042
Schedule of Capitalization
Interest rate, (as a percent)	3.671%	3.671%
Senior Notes	300.0	300.0
Preferred stock, 5.00% Series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.00%	5.00%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	131,916	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%	5.04%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	29,983	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%	5.08%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	49,983	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%	6.76%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	150,000	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%	6.88%
Cumulative; $100 par value; 1,000,000 shares authorized with no mandatory redemption - Shares Outstanding	150,000	150,000
Total preferred stock	$ 15.0	$ 15.0