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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Asset Retirement Obligation Disclosure [Abstract]    
Balance at beginning of year $ 4,837 $ 4,422
Liabilities incurred 411 276
Liabilities settled (68) (119)
Accretion expense 126 124
Revision in estimated costs of plugging oil and natural gas wells 19  
Asset retirement obligation at end of period $ 5,325 $ 4,703