Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes to asset retirement obligations
Balance as of January 1	$ 557.7	$ 571.2	$ 43.6
Integrys subsidiaries	0.0	0.0	491.0
Accretion	27.5	28.3	14.5
Additions and revisions to estimated cash flows	26.5	0.0	35.5
Liabilities settled	(38.0)	(41.8)	(13.4)
Balance as of December 31	573.7	$ 557.7	571.2
ARO increase due to revisions made to estimated cash flows	20.5		10.4
Rothschild Biomass Plant
Changes to asset retirement obligations
ARO additions	$ 5.5
Fly-ash landfills
Changes to asset retirement obligations
ARO additions			16.1
Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities rule
Changes to asset retirement obligations
ARO additions			$ 9.0

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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- 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 asset retirement obligations incurred during the period.

+ References

+ Details

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- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

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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
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

The asset retirement obligations assumed as a result of a business combination.

+ References

No definition available.

+ Details

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