Impairment, Disposition and Assets and Liabilities Held for Sale (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income	$ 0.0	$ (2.5)	$ 0.0	$ (2.5)
Total Assets Classified as Held for Sale on the Balance Sheets					$ 1,951.2
Total Liabilities Classified as Held for Sale on the Balance Sheets					235.9
Discontinued Operations and Disposal Groups (Textuals)
Asset Impairments and Other Related Charges	1.9	0.0	13.1	0.0
Generation And Marketing [Member]
Total Income on Discontinued Operations as Presented on the Condensed Consolidated Statements of Income	$ 0.0	0.0	0.0	0.0
Fuel					145.5
Materials and Supplies					49.4
Property, Plant and Equipment - Net					1,756.2
Other Class of Assets That Are Not Major					0.1
Total Assets Classified as Held for Sale on the Balance Sheets					1,951.2
Long-term Debt					134.8
Waterford Plant Upgrade Liability					52.2
Asset Retirement Obligations					36.7
Other Classes of Liabilities That Are Not Major					12.2
Total Liabilities Classified as Held for Sale on the Balance Sheets					$ 235.9
Discontinued Operations and Disposal Groups (Textuals)
Cash Proceeds from Sale of Plants Gross			2,200.0
Cash Proceeds from Sale of Disposition Plants, Net			1,200.0
Pre-tax Gain on Sale of Plants			226.0
Income from Continuing Operations before Income Tax Expense and Equity Earnings of the Plants Sold		$ 81.0	42.0	$ 196.0
Generation And Marketing [Member] | Merchant Coal-Fired Generation Assets [Member]
Discontinued Operations and Disposal Groups (Textuals)
Asset Impairments and Other Related Charges			4.0
Generation And Marketing [Member] | Merchant Coal-Fired Generation Assets [Member] | Wm. H. Zimmer Generating Station [Member]
Discontinued Operations and Disposal Groups (Textuals)
Asset Impairments and Other Related Charges			$ 9.0