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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