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Changes to Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]    
Balance at beginning of year $ 5,692 $ 5,301
Liabilities incurred 164 340
Liabilities settled (124) (120)
Accretion expense 166 171
Revision in estimated costs of plugging oil and natural gas wells 42  
Asset retirement obligation at end of year $ 5,940 $ 5,692