Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Property in Service by Segment
Property In Service	$ 14,966.3	$ 14,238.8
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	149.6	315.9
Goodwill related to acquisition of Wisconsin Gas	441.9	441.9
Utility Energy [Member]
Property in Service by Segment
Property In Service	11,779.8	11,080.9
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	132.7	298.2
Non-Utility Energy [Member]
Property in Service by Segment
Property In Service	3,091.3	3,068.5
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	16.5	13.3
Corporate and Other [Member]
Property in Service by Segment
Property In Service	95.2	89.4
Construction Work in Progress (CWIP) by segment
Construction Work in Progress (CWIP)	$ 0.4	$ 4.4

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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