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Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015 | Jul. 01, 2015 | May 22, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Assets | $ 18,067.2 |  | $ 18,067.2 |  | $ 17,492.5 |  |
Transaction Costs Related to Separation |  | $ 5.5 |  | $ 54.4 |  |  |
Tax benefit ffrom favorable estimate-to-actual adjustments |  |  | 3.6 |  |  |  |
Columbia Pipeline Group Operations |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Assets | 0.0 |  | 0.0 |  |  | $ 9,200.0 |
Liabilities | 0.0 |  | 0.0 |  |  | 5,600.0 |
Equity |  |  |  |  |  | $ 3,600.0 |
Columbia Pipeline Group |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Related Party Transaction, Other Revenues from Transactions with Related Party | $ 31.9 | $ 31.6 | $ 107.5 | $ 105.4 |  |  |
Columbia Pipeline Group |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  | $ 2,750.0

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