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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of year $ 3,455 $ 3,063
Liabilities incurred 418 361
Liabilities settled (150) (110)
Accretion expense 163 143
Revision in estimated costs of plugging oil and natural gas wells 536 (2)
Asset retirement obligation at end of year $ 4,422 $ 3,455