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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2013
Changes to Asset Retirement Obligation
The following table describes changes to the asset retirement obligation liability (dollars in thousands):
 
Year Ended December 31
 
2013
 
2012
Asset retirement obligation at beginning of period
$
5,145

 
$
5,095

Acquisition
23,841

 
—

Liabilities incurred
3,164

 
—

Accretion expense
275

 
50

Revisions in estimated cash flows
(378
)
 
—

Asset retirement obligation at end of period
$
32,047

 
$
5,145