Supplementary Information on Oil and Gas Operations—Unaudited - Costs Incurred (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Acquisition costs: Proved	$ 3,064	$ 177,880	$ 352
Acquisition costs: Unproved	39,164	35,008	15,677
Divestitures—unproved	(3,298)	(487)	(8,889)
Exploration costs: Proved	67,297	34,344	72,361
Exploration costs: Unproved	13,515	20,112	18,033
Development costs	55,722	41,328	18,740
Capitalized general and administrative and interest costs	22,121	19,911	18,961
Total costs incurred	197,585	328,096	135,235
Movement in Accumulated Depreciation, Depletion and Amortization [Roll Forward]
Balance, beginning of year	(1,553,044)	(1,472,244)	(1,265,603)
Provision for DD&A	(86,406)	(69,357)	(59,496)
Ceiling test writedown	0	0	(137,100)
Sale of proved properties and other	(8,610)	(11,443)	(10,045)
Balance, end of year	$ (1,648,060)	$ (1,553,044)	$ (1,472,244)
DD&A per Mcfe (usd per mcfe)	1.99	1.82	1.75