Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property in Service by Segment
Property In Service	$ 15,509.0us-gaap_PropertyPlantAndEquipmentOther	$ 14,966.3us-gaap_PropertyPlantAndEquipmentOther
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	191.8us-gaap_ConstructionInProgressGross	149.6us-gaap_ConstructionInProgressGross
Goodwill related to acquisition of Wisconsin Gas	441.9us-gaap_Goodwill	441.9us-gaap_Goodwill
Utility Energy [Member]
Property in Service by Segment
Property In Service	12,290.7	11,779.8
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	170.1	132.7
Non-Utility Energy [Member]
Property in Service by Segment
Property In Service	3,127.8	3,091.3
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	21.1	16.5
Corporate and Other [Member]
Property in Service by Segment
Property In Service	90.5	95.2
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	$ 0.6	$ 0.4

Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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