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Summary of Significant Accounting Policies - Results of Operations for Properties (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Oil And Gas Properties Sold [Abstract]        
Total oil and gas sales $ 4,535 $ 11,881 $ 17,747 $ 25,416
Total operating expenses [1] (1,743) (12,293) (6,134) (26,598)
Operating income (loss) $ 2,792 $ (412) $ 11,613 $ (1,182)
[1] Includes direct operating expenses, depreciation, depletion and amortization and exploration expense and excludes interest and general and administrative expense. No depreciation, depletion and amortization expense has been provided for subsequent to the date the assets were designated as held for sale.