Concentration of Credit Risk (Schedule of Concentration Of Credit Risk) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Concentration Risk [Line Items]        
Revenues $ 326.5 $ 320.0 $ 1,003.2 $ 974.8
Credit Concentration Risk        
Concentration Risk [Line Items]        
Revenues [1] $ 35.6 $ 35.3 $ 121.1 $ 120.4
Concentration Risk, Percentage [1] 10.90% 11.00% 12.10% 12.40%
[1] Represents the gross amount of revenue contracted for with Columbia Gas of Ohio and, therefore, subject to risk at the loss of this customer. Columbia Gas of Ohio has entered into certain capacity release arrangements with third parties which ultimately can decrease the net revenue amount the Partnership receives from Columbia Gas of Ohio in any given period.