CONSOLIDATED STATEMENTS OF CAPITALIZATION (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
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	782.0	723.5
Retained earnings	521.8	496.3
Total common stock equity	1,399.4	1,315.4
Preferred stock, shares outstanding	511,882	511,882
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	51.2	51.2
Total long-term debt to parent (including current portion)	5.4	6.3
Current portion of long-term debt to parent	(2.5)	0
Total long-term debt to parent	2.9	6.3
Total First Mortgage Bonds and Senior Notes	1,175.1	1,175.1
Unamortized discount on long-term debt	(0.6)	(0.6)
Total	1,174.5	1,174.5
Current portion of long-term debt	(125.0)	0
Total long-term debt	1,049.5	1,174.5
Total capitalization	2,503.0	2,547.4
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 (including current portion)	2.0	2.4
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 (including current portion)	3.4	3.9
Long Term debt, 7.125% Series, Year Due, 2023
Schedule of Capitalization
Interest rate, (as a percent)	7.125%
First Mortgage Bonds	0.1	0.1
Long Term debt, 6.375% Series, Year Due, 2015
Schedule of Capitalization
Interest rate, (as a percent)	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%
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%
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%
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%
Senior Notes	300.0	300.0
Long Term debt 4.752% Series, Year Due 2044
Schedule of Capitalization
Interest rate, (as a percent)	4.752%
Senior Notes	450.0	450.0
5.00% preferred stock series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.00%
Preferred stock, shares outstanding	131,916	131,916
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	13.2	13.2
5.04% preferred stock series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.04%
Preferred stock, shares outstanding	29,983	29,983
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	3.0	3.0
5.08% preferred stock series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	5.08%
Preferred stock, shares outstanding	49,983	49,983
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	5.0	5.0
6.76% preferred stock series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	6.76%
Preferred stock, shares outstanding	150,000	150,000
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	15.0	15.0
6.88% preferred stock series
Schedule of Capitalization
Preferred stock, Dividend Rate, (as a percent)	6.88%
Preferred stock, shares outstanding	150,000	150,000
Preferred stock – $100 par value; 1,000,000 shares authorized; 511,882 shares issued and outstanding	$ 15.0	$ 15.0