Quarterly Financial Data (Unaudited) (Schedule Of Quarterly Financial Data) (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 2 Months Ended 3 Months Ended 11 Months Ended 12 Months Ended
Feb. 11, 2015
Mar. 31, 2015
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Data [Line Items]                            
Operating Revenues     $ 357.0 $ 320.0 $ 315.6 $ 339.2 $ 340.4 $ 317.6 $ 343.4 $ 345.5        
Operating Income     136.3 142.8 109.2 162.0 133.2 93.8 103.2 158.5   $ 550.3    
Net Income     146.6 144.6 107.8 $ 131.2 $ 64.4 $ 53.2 $ 59.0 $ 92.5     $ 269.1 $ 266.9
Income (Loss), Including Portion Attributable to Noncontrolling Interest $ 42.7   0.0 0.0 0.0           $ 487.5 530.2    
Net income attributable to noncontrolling interest in Columbia OpCo subsequent to IPO   $ 75.2 124.2 122.6 91.5             413.5    
Net income attributable to limited partners subsequent to the IPO   $ 13.3 $ 22.4 $ 22.0 $ 16.3           $ 74.0 $ 74.0    
Net Income (Loss), Per Outstanding Limited Partnership and General Partnership Unit, Basic and Diluted, Net of Tax                       $ 0.74    
Common Units                            
Quarterly Financial Data [Line Items]                            
Net income attributable to limited partners subsequent to the IPO                       $ 39.6    
Net Income (Loss), Per Outstanding Limited Partnership and General Partnership Unit, Basic and Diluted, Net of Tax     $ 0.22 $ 0.22 $ 0.17 $ 0.13           $ 0.74    
Subordinated Units                            
Quarterly Financial Data [Line Items]                            
Net income attributable to limited partners subsequent to the IPO                       $ 33.8    
Net Income (Loss), Per Outstanding Limited Partnership and General Partnership Unit, Basic and Diluted, Net of Tax     $ 0.22 $ 0.22 $ 0.16 $ 0.13           $ 0.72