Other Commitments And Contingencies (Narrative) (Details)
$ in Millions
 9 Months Ended
Sep. 30, 2016
USD ($)
Jun. 24, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 05, 2014
USD ($)
Other Commitments And Contingencies [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity $ 2,600.0      
Short-term borrowings-affiliated $ 1,121.5   $ 42.1  
Ozone Level Maximum 70 ppb      
Dwellings within the potential impact area 5      
Gathering line nominal design inches 8      
Feet under the surface water 150      
Columbia Pipeline Group        
Other Commitments And Contingencies [Line Items]        
Commercial Paper $ 0.0      
Revolving Credit Facility        
Other Commitments And Contingencies [Line Items]        
Short-term borrowings-affiliated 5.0      
Credit facilities borrowings     $ 15.0  
Revolving Credit Facility | Columbia Energy Group        
Other Commitments And Contingencies [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity   $ 50.0    
Letters of Credit        
Other Commitments And Contingencies [Line Items]        
Letters of Credit Outstanding, Amount 0.0      
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      
Guarantee of Indebtedness of Others        
Other Commitments And Contingencies [Line Items]        
Guarantees $ 2,750.0