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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
The following table provides a summary of our asset retirement obligation activity during the nine months ended September 30, 2012 and 2011. 
 
 
For the nine months ended September 30,
 
 
2012
 
2011
Beginning balance
 
$
46,492

 
$
41,695

Liabilities incurred in current period
 
551

 
396

Liabilities settled in current period
 
(1,484
)
 
(756
)
Accretion expense
 
2,962

 
2,683

 
 
48,521

 
44,018

Less current portion
 
2,900

 
1,350

 
 
$
45,621

 
$
42,668