Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Details)

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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset retirement obligations, Beginning of period $ 13,938  
Accretion expense 1,135  
Liabilities incurred 4  
Liabilities settled (512)  
Revisions to estimate (784)  
Asset retirement obligations, End of period 13,781  
Less: current asset retirement obligations 997 1,260
Long-term asset retirement obligations at June 30, 2012 12,784 12,678
Restricted Investments    
Restricted investments $ 3,792 $ 3,790