Initial Public Offering (Schedule of Change From Net Income Attributable to the Partnership and Transfers to Noncontrolling Interest) (Details) - USD ($)
$ in Millions
 3 Months Ended 8 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2015
Sep. 30, 2015
 [1]
Initial Public Offering [Line Items]      
Net Income (Loss) Allocated to Limited Partners $ 22.0   $ 51.6
Noncontrolling Interest, Increase from Subsidiary Equity Issuance 0.0 $ 0.0 [2]  
Change from net income attributable to the Partnership and transfers to noncontrolling interest $ 22.0 (372.8)  
Noncontrolling Interest      
Initial Public Offering [Line Items]      
Noncontrolling Interest, Increase from Subsidiary Equity Issuance [2]   $ (424.4)  
[1] Net income attributable to limited partners and in excess of distribution is for the period subsequent to the IPO.
[2] Represents the purchase of an additional 8.4% limited partner interest in Columbia OpCo, recorded at the historical carrying value of Columbia OpCo’s net assets after giving effect to the $1,168.4 million equity contribution. This decreases common unitholders and subordinated unitholders equity by the same amount it increases noncontrolling interest because the Partnership's purchase price for its additional 8.4% interest in Columbia OpCo exceeded book value.