Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Millions
 1 Months Ended 2 Months Ended 3 Months Ended
Feb. 11, 2015
Mar. 31, 2015
Mar. 31, 2016
Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning Balance $ (16.7) [1]   $ (4.0) [2] $ (16.7) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo 1,222.2      
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0.0 [2] 0.0 [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [3]     0.5 [2] 0.1 [1]
Other Comprehensive Income (Loss), Net of Tax 0.1 $ 0.1 0.5 [2] 0.1 [1]
Ending Balance   (4.2) [1] (3.9) [2] (4.2) [1]
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning Balance (16.6) [1]   (3.9) [2] (16.6) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[4]       (10.2)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0.0 [2] 0.0 [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [3]     0.5 [2] 0.1 [1]
Other Comprehensive Income (Loss), Net of Tax     0.5 [2] 0.1 [1]
Ending Balance   (4.1) [1] (3.8) [2] (4.1) [1]
Accumulated Defined Benefit Plans Adjustment        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning Balance (0.1) [1]   (0.1) [2] (0.1) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[4]       (0.1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax     0.0 [2] 0.0 [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [3]     0.0 [2] 0.0 [1]
Other Comprehensive Income (Loss), Net of Tax     0.0 [2] 0.0 [1]
Ending Balance   (0.1) [1] (0.1) [2] (0.1) [1]
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Noncontrolling Interest        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), Net of Tax     0.4 [2] (22.6) [1]
Accumulated Defined Benefit Plans Adjustment Attributable to Noncontrolling Interest        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), Net of Tax     0.0 [2] (0.1) [1]
AOCI Attributable to Noncontrolling Interest        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), Net of Tax     0.4 [2] (22.7) [1]
Accumulated Other Comprehensive Loss        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Predecessor net tax liabilities not assumed by Columbia OpCo (10.3)     $ (10.3) [1],[4]
Other Comprehensive Income (Loss), Net of Tax $ 0.1 $ 0.0 $ 0.1  
[1] All amounts prior to the IPO are net of tax. Amounts in parentheses indicate debits.
[2] Amounts in parentheses indicate debits.
[3] Includes amounts allocated to noncontrolling interest.
[4] 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.