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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2012
Asset Retirement Obligation [Abstract]  
Schedule of asset retirement obligations
The following table summarizes the Company's asset retirement obligation activity during the three and six months ended June 30, 2012 and 2011: 

 
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
 
2012
 
2011
 
2012
 
2011
 
 
(in thousands)
Beginning asset retirement obligations
 
$
134,314

 
$
154,689

 
$
136,742

 
$
152,291

Liabilities assumed in acquisitions
 
8,515

 
—

 
8,515

 
6

New wells placed on production
 
1,581

 
1,404

 
2,411

 
2,075

Changes in estimates
 
1,603

 
(179
)
 
1,603

 
121

Disposition of wells
 
—

 
(367
)
 
—

 
(448
)
Liabilities settled
 
(3,054
)
 
(6,235
)
 
(8,742
)
 
(7,469
)
Accretion of discount from continuing operations
 
2,444

 
2,048

 
4,874

 
4,092

Accretion of discount from integrated services (a)
 
54

 
—

 
54

 
—

Accretion of discount from discontinued operations
 
—

 
610

 
—

 
1,302

Ending asset retirement obligations
 
$
145,457

 
$
151,970

 
$
145,457

 
$
151,970