Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 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
Disposal Group, Including Discontinued Operation, Revenue	$ 38.7	$ 123.8	$ 126.2	252.1
Disposal Group, Including Discontinued Operation, Restricted Cash	0.0		0.0		$ 1.5
Disposal Group, Including Discontinued Operation, Accounts, Notes and Loans Receivable, Net	22.1		22.1		37.9
Disposal Group, Including Discontinued Operation, Inventory	0.5		0.5		19.4
Disposal Group, Including Discontinued Operations, Taxes Applicable to Subsequent Years	0.9		0.9		7.4
Disposal Group, Including Discontinued Operation, Other Assets, Current	5.8		5.8		17.4
Disposal Group, Including Discontinued Operation, Property, Plant and Equipment	1.5		1.5		233.9
Disposal Group, Including Discontinued Operation, Intangible Assets	6.0		6.0		5.5
Disposal Group, Including Discontinued Operation, Other Assets, Noncurrent	0.0		0.0		0.6
Disposal Group, Including Discontinued Operation, Assets	36.8		36.8		323.6
Disposal Group, Including Discontinued Operation, Accounts Payable	9.6		9.6		25.1
Disposal Group, Including Discontinued Operation, Accrued Income Tax Payable	5.1		5.1		6.3
Disposal Group, Including Discontinued Operation, Other Liabilities, Current	19.2		19.2		30.0
Disposal Group, Including Discontinued Operations, Long-Term Debt	0.0		0.0		0.3
Disposal Group, Including Discontinued Operation, Deferred Tax Liabilities	(16.9)		(16.9)		(17.4)
Disposal Group, Including Discontinued Operation, Taxes Payable	0.6		0.6		7.4
Disposal Group, Including Discontinued Operation, Pension Plan Benefit Obligation	9.7		9.7		10.6
Asset Retirement Obligation, Held for Sale	116.8		116.8		116.6
Disposal Group, Including Discontinued Operation, Other Liabilities, Noncurrent	5.7		5.7		5.9
Disposal Group, Including Discontinued Operation, Liabilities	149.8		149.8		$ 184.8
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)
Debt Percentage of Rate Base				75.00%
Disposal Group, Including Discontinued Operation, Interest Expense		0.0		$ 0.2
Loss from discontinued operations before income taxes	10.0	17.8	31.7	(64.4)
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	0.0	0.0	1.9	0.0
Income tax benefit for discontinued operations	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)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	(0.8)	7.7	(0.4)	0.9
Cash flows from investing activities for discontinued operations	(2.0)	(5.3)	233.8	(18.0)
Subsidiaries [Member]
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
Debt Percentage of Rate Base				75.00%
Long-term Debt, Gross	596.0		596.0
Disposal Group, Including Discontinued Operation, Interest Expense		0.0		$ 0.2
Loss from discontinued operations before income taxes	0.0	11.2	0.0	(77.5)
Income tax benefit for discontinued operations	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)
Generation [Member] | Subsidiaries [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
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)
Loss from discontinued operations before income taxes		11.2		(77.5)
Income tax benefit for discontinued operations		6.4		(23.5)
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent		4.8		(54.0)
Cash Provided by (Used in) Operating Activities, Discontinued Operations		5.3		(6.5)
Cash flows from investing activities for discontinued operations		(2.9)		(10.6)
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