Initial Public Offering (Tables)
 3 Months Ended
Mar. 31, 2015
Equity [Abstract]  
Change From Net Income Attributable to the Partnership and Transfers to Noncontrolling Interest
 
Three Months Ended
March 31,
(in millions)
2015
Net income attributable to the Partnership
$
13.3

Decrease in partnership equity for the purchase of an additional 8.4 percent interest in Columbia OpCo
(424.4
)
Change from net income attributable to the Partnership and transfers to noncontrolling interest
$
(411.1
)