Consolidated Statement of Income (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating Revenue	$ 251.0	$ 248.8	$ 750.4	$ 705.2
Operating Expenses [Abstract]
Fuel and Purchased Power	80.5	79.5	245.7	228.7
Operating and Maintenance	102.7	98.7	311.2	294.8
Depreciation	29.4	25.0	86.3	74.4
Total Operating Expenses	212.6	203.2	643.2	597.9
Operating Income	38.4	45.6	107.2	107.3
Other Income (Expense) [Abstract]
Interest Expense	(12.7)	(12.3)	(37.8)	(33.4)
Equity Earnings in ATC	4.9	4.9	15.1	14.3
Other	3.3	1.5	7.5	3.4
Total Other Expense	(4.5)	(5.9)	(15.2)	(15.7)
Income Before Income Taxes	33.9	39.7	92.0	91.6
Income Tax Expense	8.7	10.3	20.3	23.4
Net Income	$ 25.2	$ 29.4	$ 71.7	$ 68.2
Average Shares of Common Stock [Abstract]
Basic	39.8	37.7	39.4	37.3
Diluted	39.9	37.8	39.5	37.3
Basic Earnings Per Share of Common Stock	$ 0.63	$ 0.78	$ 1.82	$ 1.83
Diluted Earnings Per Share of Common Stock	$ 0.63	$ 0.78	$ 1.81	$ 1.83
Dividends Per Share of Common Stock	$ 0.475	$ 0.46	$ 1.425	$ 1.38