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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation [Abstract]  
Changes to Asset Retirement Obligation
The following table describes changes to the asset retirement obligation liability (dollars in millions):
 
Year Ended December 31
 
2014
 
2013
Asset retirement obligation at beginning of period
$
32.0

 
$
5.1

Acquisition
4.1

 
23.8

Liabilities incurred
—

 
3.2

Accretion expense
1.0

 
0.3

Payments
(0.1
)
 
—

Revisions in estimated cash flows
—

 
(0.4
)
Asset retirement obligation at end of period
$
37.0

 
$
32.0