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, 2016
Sep. 30, 2015
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [2] $ (16.7) [1] $ (3.8) $ (4.1)   $ (4.0) $ (16.7) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo [3] 1,222.2          
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [2]   0.2 (0.5)   0.2 (0.5) [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [2],[4]   0.5 0.7   1.5 1.3 [1]
Other Comprehensive Income (Loss) 0.1 0.7 [2] 0.2 [2] $ 0.7 1.7 [2] 0.8 [1],[2]
Ending Balance [2]   (3.7) (4.1) [1] (4.1) [1] (3.7) (4.1) [1]
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [2] (16.6) [1] (3.7) (4.0)   (3.9) (16.6) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[2],[5]           (10.2)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [2]   0.0 0.0   0.0 0.0 [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [2],[4]   0.6 0.4   1.6 1.0 [1]
Other Comprehensive Income (Loss) [2]   0.6 0.4   1.6 1.0 [1]
Ending Balance [2]   (3.6) (4.0) [1] (4.0) [1] (3.6) (4.0) [1]
Accumulated Defined Benefit Plans Adjustment            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Beginning Balance [2] (0.1) [1] (0.1) (0.1)   (0.1) (0.1) [1]
Predecessor net tax liabilities not assumed by Columbia OpCo [1],[2],[5]           (0.1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax [2]   0.2 (0.5)   0.2 (0.5) [1]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [2],[4]   (0.1) 0.3   (0.1) 0.3 [1]
Other Comprehensive Income (Loss) [2]   0.1 (0.2)   0.1 (0.2) [1]
Ending Balance [2]   (0.1) (0.1) [1] (0.1) [1] (0.1) (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) [2]   0.5 0.4   1.3 (21.8) [1]
Accumulated Defined Benefit Plans Adjustment Attributable to Noncontrolling Interest            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Other Comprehensive Income (Loss) [2]   0.1 (0.2)   0.1 (0.3) [1]
AOCI Attributable to Noncontrolling Interest            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Other Comprehensive Income (Loss) [2]   $ 0.6 $ 0.2   1.4 (22.1) [1]
Accumulated Other Comprehensive Loss            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Predecessor net tax liabilities not assumed by Columbia OpCo (10.3) [3]         $ (10.3) [1],[2],[5]
Other Comprehensive Income (Loss) $ 0.1     $ 0.1 $ 0.3  
[1] All amounts prior to the IPO are net of tax.
[2] Amounts in parentheses indicate debits.
[3] 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.
[4] Includes amounts allocated to noncontrolling interest.
[5] 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.