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Changes to the Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of year $ 3,063 $ 2,955
Liabilities incurred 152 142
Liabilities settled (80) (184)
Accretion expense 70 55
Revision in estimated costs of plugging oil and natural gas wells (2)  
Asset retirement obligation at end of period $ 3,203 $ 2,968