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Property and Equipment (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Oil and gas properties, on the basis of full-cost accounting    
Proved properties $ 818,013 $ 809,272
Unproved properties not being amortized 1,869 4,050
Other property and equipment 6,340 6,345
Less accumulated depreciation, depletion, amortization and impairment (287,544) (266,198)
Net property and equipment $ 538,678 $ 553,469