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, 2016
Sep. 30, 2015
Sep. 30, 2015
Sep. 30, 2016
Initial Public Offering [Line Items]        
Net Income (Loss) Allocated to Limited Partners $ 2.9 $ 22.0 $ 51.6 $ 48.3
Noncontrolling Interest, Increase from Subsidiary Equity Issuance [1]     0.0  
Change from net income attributable to the Partnership and transfers to noncontrolling interest 2.9 22.0 (372.8) 48.3
Noncontrolling Interest        
Initial Public Offering [Line Items]        
Noncontrolling Interest, Increase from Subsidiary Equity Issuance $ 0.0 $ 0.0 $ (424.4) [1] $ 0.0
[1] 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.