Initial Public Offering (Tables)
 9 Months Ended
Sep. 30, 2015
Equity [Abstract]  
Change From Net Income Attributable to the Partnership and Transfers to Noncontrolling Interest
 
Three Months Ended
September 30,
 
Nine Months Ended September 30,
(in millions)
2015
 
2015
Net income attributable to the Partnership
$
22.0

 
$
51.6

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
$
22.0

 
$
(372.8
)