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Goodwill (Details) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Goodwill [Line Items] |  |
Goodwill | $ 1,690.7 | $ 1,690.7
Gas Distribution Operations |  |
Goodwill [Line Items] |  |
Goodwill | 1,690.7 |
Electric Operations |  |
Goodwill [Line Items] |  |
Goodwill | 0.0 |
Corporate and Other |  |
Goodwill [Line Items] |  |
Goodwill | $ 0.0 |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975 + Details
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