Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 8 Months Ended 9 Months Ended
Feb. 11, 2015
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2015
Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [1] $ (16.7) $ (4.1)     $ (16.7)  
Predecessor net tax liabilities not assumed by Columbia OpCo [2] 1,222.2          
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]   (0.5)     (0.5)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1],[3]   0.7     1.3  
Other Comprehensive Income (Loss), Net of Tax 0.1 0.2 [1]   $ 0.7   $ 0.8 [1]
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest   0.2 [1]   0.0 (22.1) [1]  
Ending Balance [1]   (4.1)   (4.1) (4.1)  
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [1] (16.6) (4.0)     (16.6)  
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[2]         (10.2)  
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]   0.0     0.0  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1],[3]   0.4     1.0  
Other Comprehensive Income (Loss), Net of Tax [1]   0.4     1.0  
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest [1]   0.4     (21.8)  
Ending Balance [1]   (4.0)   (4.0) (4.0)  
Accumulated Defined Benefit Plans Adjustment [Member]            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [1] (0.1) (0.1)     (0.1)  
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[2]         (0.1)  
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]   (0.5)     (0.5)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1],[3]   0.3     0.3  
Other Comprehensive Income (Loss), Net of Tax [1]   (0.2)     (0.2)  
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest [1]   (0.2)     (0.3)  
Ending Balance [1]   $ (0.1)   (0.1) (0.1)  
Accumulated Other Comprehensive Loss            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Predecessor net tax liabilities not assumed by Columbia OpCo [2] (10.3)       (10.3) [1]  
Other Comprehensive Income (Loss), Net of Tax 0.1     0.1    
Allocation of Accumulated Other Comprehensive Loss To Noncontrolling Interest       $ 22.7    
Predecessor            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance $ (16.7)   $ (17.2) [1]   $ (16.7) (17.7) [1]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]     0.0     0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1]     0.3     0.8
Other Comprehensive Income (Loss), Net of Tax [1]     0.3     0.8
Ending Balance [1]     (16.9)     (16.9)
Predecessor | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [1]     (17.1)     (17.6)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]     0.0     0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1]     0.3     0.8
Other Comprehensive Income (Loss), Net of Tax [1]     0.3     0.8
Ending Balance [1]     (16.8)     (16.8)
Predecessor | Accumulated Defined Benefit Plans Adjustment [Member]            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [1]     (0.1)     (0.1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [1]     0.0     0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [1]     0.0     0.0
Other Comprehensive Income (Loss), Net of Tax [1]     0.0     0.0
Ending Balance [1]     $ (0.1)     $ (0.1)
[1] All amounts prior to the IPO are net of tax. Amounts in parentheses indicate debits.
[2] 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.
[3] Includes amounts allocated to noncontrolling interest.