Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) (USD $)
In Millions, unless otherwise specified
 2 Months Ended 1 Months Ended 3 Months Ended
Mar. 31, 2015
Feb. 11, 2015
Mar. 31, 2015
Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Predecessor net tax liabilities not assumed by Columbia OpCo(1)   $ 1,222.2 [1]    
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest 0      
Beginning Balance   (16.7) (16.7) (17.7)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax     0.1 [2] 0.3
Other Comprehensive Income (Loss), Net of Tax 0.1 0.1 0.1 0.3
Ending Balance (4.2)   (4.2) (17.4)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Predecessor net tax liabilities not assumed by Columbia OpCo(1)     (10.2) [3]  
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest     22.6  
Beginning Balance   (16.6) (16.6) (17.6)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0 0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax     0.1 [2] 0.3
Other Comprehensive Income (Loss), Net of Tax     0.1 0.3
Ending Balance (4.1)   (4.1) (17.3)
Accumulated Defined Benefit Plans Adjustment [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Predecessor net tax liabilities not assumed by Columbia OpCo(1)     (0.1) [3]  
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest     0.1  
Beginning Balance   (0.1) (0.1) (0.1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0 0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax     0 [2] 0
Other Comprehensive Income (Loss), Net of Tax     0 0
Ending Balance (0.1)   (0.1) (0.1)
Accumulated Other Comprehensive Income (Loss)        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Predecessor net tax liabilities not assumed by Columbia OpCo(1)   (10.3) [1] (10.3) [3]  
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest 22.7   22.7  
Other Comprehensive Income (Loss), Net of Tax $ 0 $ 0.1    
[1] Reflects the non-cash elimination of all historical current and deferred income taxes other than Tennessee state income taxes that continue to be borne by the Partnership post-IPO, as well as associated regulatory assets and liabilities.
[2] Reflects amounts allocated to noncontrolling interest.
[3] Reflects the non-cash elimination of all historical current and deferred income taxes other than Tennessee state income taxes that will continue to be borne by the Partnership post-IPO.