Other Commitments And Contingencies (Narrative) (Details)
$ in Millions
 12 Months Ended
Oct. 01, 2015
Aug. 18, 2015
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 05, 2014
USD ($)
Other Commitments And Contingencies [Line Items]            
Guarantees     $ 2,768.1      
Line of Credit Facility, Maximum Borrowing Capacity     2,600.0      
Ozone Level Maximum 70 ppb          
Percentage Reduction in Methane Emissions   95.00%        
Hours of Operation   26,000        
Operating lease payments     18.5 $ 14.9 $ 13.4  
Replacement Years   3 years        
Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Commercial Paper     0.0      
Line of Credit Facility Used For Credit Support for Subsidiaries     750.0      
Line of Credit Facility Used For Corporate Purposes     750.0      
Guarantee of Indebtedness of Others            
Other Commitments And Contingencies [Line Items]            
Guarantees     2,750.0      
Letters Of Credit            
Other Commitments And Contingencies [Line Items]            
Guarantees     18.1      
Revolving Credit Facility            
Other Commitments And Contingencies [Line Items]            
Credit facilities borrowings     15.0 $ 0.0    
Revolving Credit Facility | Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Credit facilities borrowings     0.0      
Letters Of Credit            
Other Commitments And Contingencies [Line Items]            
Letters of Credit Outstanding, Amount     0.0      
Letters Of Credit | Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Letters of Credit Outstanding, Amount     18.1      
Commercial Paper | Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity     1,000.0      
Revolving Credit Facility            
Other Commitments And Contingencies [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity     500.0     $ 1,500.0
Revolving Credit Facility | Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity     1,500.0      
Letters Of Credit            
Other Commitments And Contingencies [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity     50.0      
Letters Of Credit | Columbia Pipeline Group            
Other Commitments And Contingencies [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity     $ 250.0