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Changes to Company's Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Asset Retirement Obligation Disclosure [Abstract]    
Balance at beginning of year $ 5,692 $ 5,301
Liabilities incurred 66 271
Liabilities settled (71) (65)
Accretion expense 84 86
Revision in estimated costs of plugging oil and natural gas wells 43  
Asset retirement obligation at end of period $ 5,814 $ 5,593