Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 261.3	$ 277.6	$ 789.3	$ 824.1
Contracts with Customers – Non-utility	80.7	79.0	243.1	239.6
Other – Non-utility	6.0	5.9	17.9	17.7
Total Operating Revenue	348.0	362.5	1,050.3	1,081.4
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas – Utility	103.2	93.5	300.6	283.2
Transmission Services – Utility	17.9	18.9	53.1	53.1
Cost of Sales – Non-utility	38.7	36.0	108.6	105.9
Operating and Maintenance	80.3	81.1	253.6	251.4
Depreciation and Amortization	51.5	50.9	153.4	151.5
Taxes Other than Income Taxes	13.1	14.1	43.8	42.7
Total Operating Expenses	304.7	294.5	913.1	887.8
Operating Income	43.3	68.0	137.2	193.6
Other Income (Expense) [Abstract]
Interest Expense	(17.6)	(16.6)	(51.6)	(50.5)
Equity Earnings in ATC	4.0	5.9	13.0	17.3
Other	1.4	1.8	5.7	5.0
Total Other Expense	(12.2)	(8.9)	(32.9)	(28.2)
Income Before Income Taxes	31.1	59.1	104.3	165.4
Income Tax Expense (Benefit)	0.4	14.2	(8.7)	34.6
Net Income	$ 30.7	$ 44.9	$ 113.0	$ 130.8
Average Shares of Common Stock [Abstract]
Basic (Shares)	51.4	51.0	51.3	50.7
Diluted (Shares)	51.6	51.2	51.5	50.9
Basic Earnings Per Share of Common Stock	$ 0.59	$ 0.88	$ 2.20	$ 2.58
Diluted Earnings Per Share of Common Stock	0.59	0.88	2.19	2.57
Dividends Per Share of Common Stock	$ 0.56	$ 0.535	$ 1.68	$ 1.605