Supplementary Information on Oil and Gas Operations—Unaudited - Costs Incurred (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / Mcfe	Dec. 31, 2016 USD ($) $ / Mcfe	Dec. 31, 2015 USD ($) $ / Mcfe
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Acquisition costs: Proved	$ 1,330	$ 3,346	$ 2,287
Acquisition costs: Unproved	12,762	2,197	2,550
Divestiture of proved leasehold	(4,795)	(7,000)	0
Exploration costs: Proved	9,466	715	29,322
Exploration costs: Unproved	(287)	603	7,677
Development costs	32,622	1,522	9,888
Capitalized general and administrative and interest costs	8,269	7,558	12,881
Total costs incurred	59,367	8,941	64,605
Accumulated depreciation, depletion and amortization (DD&A)
Balance, beginning of year	(1,243,286)	(1,157,455)	(1,648,060)
Provision for DD&A	(31,667)	(27,962)	(62,138)
Ceiling test writedown	0	(40,304)	(266,562)
Sale of proved properties and other	(10,707)	(17,565)	819,305
Balance, end of year	$ (1,285,660)	$ (1,243,286)	$ (1,157,455)
DD&A per Mcfe (usd per mcfe) | $ / Mcfe	1.15	1.19	1.82