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Changes to the Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of year $ 3,063 $ 2,955
Liabilities incurred 361 335
Liabilities settled (110) (339)
Accretion expense 143 112
Revision in estimated costs of plugging oil and natural gas wells (2)  
Asset retirement obligation at end of year $ 3,455 $ 3,063