Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 270.2	$ 281.6
Contracts with Customers – Non-utility	82.0	78.1
Other - Non-utility	6.0	5.9
Total Operating Revenue	358.2	365.6
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas – Utility	100.9	96.6
Transmission Services – Utility	18.4	16.6
Cost of Sales – Non-utility	32.9	31.5
Operating and Maintenance	86.5	84.4
Depreciation and Amortization	45.8	50.5
Taxes Other than Income Taxes	16.3	14.4
Total Operating Expenses	300.8	294.0
Operating Income	57.4	71.6
Other Income (Expense) [Abstract]
Interest Expense	(16.9)	(17.2)
Equity Earnings in ATC	4.7	6.1
Other	2.1	1.6
Total Other Expense	(10.1)	(9.5)
Income Before Income Taxes	47.3	62.1
Income Tax Expense (Benefit)	(3.7)	13.1
Net Income	$ 51.0	$ 49.0
Average Shares of Common Stock [Abstract]
Basic (Shares)	51.2	50.2
Diluted (Shares)	51.4	50.4
Basic Earnings Per Share of Common Stock	$ 1.00	$ 0.97
Diluted Earnings Per Share of Common Stock	0.99	0.97
Dividends Per Share of Common Stock	$ 0.56	$ 0.535