Supplementary Information on Oil and Gas Operations—Unaudited - Costs Incurred (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) $ / Mcfe	Dec. 31, 2014 USD ($) $ / Mcfe	Dec. 31, 2013 USD ($) $ / Mcfe
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Acquisition costs: Proved	$ 2,287	$ 3,064	$ 177,880
Acquisition costs: Unproved	2,550	39,164	35,008
Exploration costs: Proved	29,322	67,297	34,344
Exploration costs: Unproved	7,677	13,515	20,112
Development costs	9,888	55,722	41,328
Capitalized general and administrative and interest costs	12,881	22,121	19,911
Total costs incurred	64,605	200,883	328,583
Movement in Accumulated Depreciation, Depletion and Amortization [Roll Forward]
Balance, beginning of year	(1,648,060)	(1,553,044)	(1,472,244)
Provision for DD&A	(62,138)	(86,406)	(69,357)
Ceiling test writedown	(266,562)	0	0
Sale of proved properties and other	819,305	(8,610)	(11,443)
Balance, end of year	$ (1,157,455)	$ (1,648,060)	$ (1,553,044)
DD&A per Mcfe (usd per mcfe) | $ / Mcfe	1.82	1.99	1.82