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Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
8. Asset Retirement Obligations
The following table sets forth asset retirement obligations for the years ended December 31, 2012 and 2011:
 
 
Year Ended December 31,
 
 
2012
 
2011
 
 
(In thousands)
Asset retirement obligations at beginning of period
 
$
8,324

 
$
5,250

Liabilities incurred
 
1,573

 
757

Liabilities settled
 
(1,666
)
 
(162
)
Reduction due to property sales
 
(3,272
)
 
(368
)
Accretion expense
 
372

 
235

Revisions of previous estimates
 
828

 
2,612

Asset retirement obligations at end of period
 
6,159

 
8,324

Asset retirement obligations due within one year included in "Other accrued liabilities"
 
(1,670
)
 
(925
)
Long-term asset retirement obligations
 
$
4,489

 
$
7,399