Generation Separation (Details) - Subsidiaries [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long Term Indebtedness, Less than or Equal to		$ 750.0
Loss from discontinued operations	$ 0.0	(88.7)
Discontinued Operation, Tax Effect of Discontinued Operation	0.0	(29.9)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	0.0	$ (58.8)
Debt Percentage of Rate Base		75.00%
Long-term Debt, Gross	$ 597.2
Transmission and Distribution [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt, Gross		$ 750.0
Generation [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Interest Expense		0.2
Cash Provided by (Used in) Operating Activities, Discontinued Operations		(11.8)
Cash Provided by (Used in) Investing Activities, Discontinued Operations		12.9
Cash Provided by (Used in) Financing Activities, Discontinued Operations		(1.1)
Disposal Group, Including Discontinued Operation, Revenue		121.0
Disposal Group, Including Discontinued Operation, Costs of Goods Sold		(69.8)
Disposal Group, Including Discontinued Operation, Operating and Other Expenses		(73.6)
Disposal Group, Including Discontinued Operation, Fixed-Asset Impairment		(66.3)
Loss from discontinued operations		(88.7)
Discontinued Operation, Tax Effect of Discontinued Operation		(29.9)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		$ (58.8)