Consolidated Statement of Income (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating Revenue	$ 263.8	$ 240.0
Operating Expenses [Abstract]
Fuel and Purchased Power	86.5	77.1
Operating and Maintenance	104.7	99.9
Depreciation	28.2	24.6
Total Operating Expenses	219.4	201.6
Operating Income	44.4	38.4
Other Income (Expense) [Abstract]
Interest Expense	(12.3)	(11.0)
Equity Earnings in ATC	5.2	4.6
Other	2.7	0.7
Total Other Expense	(4.4)	(5.7)
Income Before Income Taxes	40.0	32.7
Income Tax Expense	7.5	8.3
Net Income	$ 32.5	$ 24.4
Average Shares of Common Stock [Abstract]
Basic	38.9	36.8
Diluted	39.0	36.9
Basic Earnings Per Share of Common Stock	$ 0.83	$ 0.66
Diluted Earnings Per Share of Common Stock	$ 0.83	$ 0.66
Dividends Per Share of Common Stock	$ 0.475	$ 0.46