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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes in asset retirement obligations
Changes in asset retirement obligations (“ARO”) for 2012 and 2011 are as follows:
 
 
2012
 
2011
 
(In thousands)
Beginning of year
$
40,794

 
$
40,444

Additional ARO from new properties
7,868

 
1,526

Sales or abandonments of properties
(2,184
)
 
(4,425
)
Accretion expense
3,696

 
2,757

Revisions of previous estimates
1,303

 
492

End of year
$
51,477

 
$
40,794