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Asset Retirement Obligations
 9 Months Ended
Sep. 30, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations

The following table is a reconciliation of the ARO liability as of and for the nine months ended September 30, 2015 and the twelve months ended December 31, 2014.
 
 
September 30,
2015
 
December 31, 2014
Asset retirement obligation at beginning of period
$
44,214

 
$
51,954

Liabilities incurred on properties acquired and developed
2,505

 
3,721

Revisions to previous estimates
—

 
(14,821
)
Liabilities on properties sold or settled
(3,721
)
 
—

Accretion expense
4,593

 
3,360

Asset retirement obligation at end of period
$
47,591

 
$
44,214