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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of period $ 4,422 $ 3,455
Liabilities incurred 177 182
Liabilities settled (31) (83)
Accretion expense 82 79
Revision in estimated costs of plugging oil and natural gas wells   536
Asset retirement obligation at end of period $ 4,650 $ 4,169