Initial Public Offering (Tables)
 6 Months Ended
Jun. 30, 2015
Equity [Abstract]  
Change From Net Income Attributable to the Partnership and Transfers to Noncontrolling Interest
 
Three Months Ended
June 30,
 
Six Months Ended June 30,
(in millions)
2015
 
2015
Net income attributable to the Partnership
$
16.3

 
$
29.6

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
$
16.3

 
$
(394.8
)