Property, Plant and Equipment (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($) generating_facility power_plant	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Property, Plant, and Equipment	$ 10.6	$ 0.1
Asset Retirement Obligations, Noncurrent	11.9	135.6	$ 15.1	$ 138.8
Asset Retirement Obligation, Accretion Expense	0.2
Asset Retirement Obligation, Reductions Due to Plant Sales	(3.4)
Asset Retirement Obligation, Held for Sale	(116.8)		(116.6)
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(1.1)	(16.2)
Subsidiaries [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Property, Plant, and Equipment	$ 10.6	0.1
Number Of Coal Fired Power Plants \| power_plant	1
Asset Retirement Obligation	$ 4.7	132.3	$ 8.0	$ 135.2
Asset Retirement Obligation, Revision of Estimate	$ 0.1
Number Of Generating Facilities \| generating_facility	1
Asset Retirement Obligation, Reductions Due to Plant Sales	$ (3.4)
Gain (Loss) on Sale of Assets and Asset Impairment Charges	$ (0.1)	$ (16.2)

Asset Retirement Obligation, Held for Sale

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- Definition

Asset Retirement Obligation, Reductions Due to Plant Sales

+ References

No definition available.

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- Definition

Number Of Coal Fired Power Plants

+ References

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Number Of Generating Facilities

+ References

No definition available.

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Amount of gain (loss) from the difference between the sale price or salvage price and the book value of an asset that was sold or retired, and gain (loss) from the write down of assets from their carrying value to fair value.

+ References

No definition available.

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- Definition

The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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