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Supplementary Information on Oil and Gas Operations—Unaudited - Costs Incurred (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
$ / Mcfe
Dec. 31, 2017
USD ($)
$ / Mcfe
Dec. 31, 2016
USD ($)
$ / Mcfe
Oil and Gas Exploration and Production Industries Disclosures [Abstract]      
Acquisition costs: Proved $ 241 $ 1,330 $ 3,346
Acquisition costs: Unproved 6,190 12,762 2,197
Divestiture of proved leasehold (2,604) (4,795) (7,000)
Exploration costs: Proved (51) 9,466 715
Exploration costs: Unproved 233 (287) 603
Development costs (2) (18,928) 32,622 1,522
Capitalized general and administrative and interest costs 8,070 8,269 7,558
Total costs incurred (6,849) 59,367 8,941
Accumulated depreciation, depletion and amortization (DD&A)      
Balance, beginning of year (1,285,660) (1,243,286) (1,157,455)
Provision for DD&A (22,410) (31,667) (27,962)
Ceiling test writedown 0 0 (40,304)
Sale of proved properties and other 6,478 (10,707) (17,565)
Balance, end of year $ (1,301,592) $ (1,285,660) $ (1,243,286)
DD&A per Mcfe (usd per mcfe) | $ / Mcfe 1.05 1.15 1.19