Generation Separation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Interest Expense		$ 0.0		$ 0.2
Long Term Indebtedness, Less than or Equal to				750.0
Cash Provided by (Used in) Operating Activities, Discontinued Operations	$ (0.8)	7.7	$ (0.4)	0.9
Cash Provided by (Used in) Investing Activities, Discontinued Operations	(2.0)	(5.3)	233.8	(18.0)
Disposal Group, Including Discontinued Operation, Revenue	38.7	123.8	126.2	252.1
Disposal Group, Including Discontinued Operation, Costs of Goods Sold	(19.5)	(62.5)	(60.6)	(135.2)
Disposal Group, Including Discontinued Operation, Operating and Other Expenses	(9.2)	(43.5)	(33.9)	(181.3)
Loss from discontinued operations	10.0	17.8	31.7	(64.4)
Discontinued Operation, Tax Effect of Discontinued Operation	1.1	10.9	4.9	(20.0)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	8.9	6.9	24.9	$ (44.4)
Debt Percentage of Rate Base				75.00%
Subsidiaries [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Interest Expense		0.0		$ 0.2
Long Term Indebtedness, Less than or Equal to				750.0
Loss from discontinued operations	0.0	11.2	0.0	(77.5)
Discontinued Operation, Tax Effect of Discontinued Operation	0.0	6.4	0.0	(23.5)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	0.0	4.8	0.0	$ (54.0)
Debt Percentage of Rate Base				75.00%
Long-term Debt, Gross	$ 596.0		$ 596.0
Generation [Member] | Subsidiaries [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Provided by (Used in) Operating Activities, Discontinued Operations		5.3		$ (6.5)
Cash Provided by (Used in) Investing Activities, Discontinued Operations		(2.9)		(10.6)
Disposal Group, Including Discontinued Operation, Revenue		115.9		236.9
Disposal Group, Including Discontinued Operation, Costs of Goods Sold		(59.8)		(129.6)
Disposal Group, Including Discontinued Operation, Operating and Other Expenses		(44.9)		(118.5)
Disposal Group, Including Discontinued Operation, Fixed-Asset Impairment		0.0		(66.3)
Loss from discontinued operations		11.2		(77.5)
Discontinued Operation, Tax Effect of Discontinued Operation		6.4		(23.5)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		4.8		(54.0)
Utility [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt, Gross		750.0		750.0
Utility [Member] | Subsidiaries [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt, Gross		$ 750.0		$ 750.0