Investment in Oil and Gas Properties - Unaudited (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2009
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Acquisition costs: Proved	$ 177,880	[1]	$ 352	[1]	$ 2,720	[1]
Acquisition costs: Unproved	35,008	[1]	15,677	[1]	43,207	[1]
Divestitures—unproved	(487)	[2]	(8,889)	[2]	(14,461)	[2]
Exploration costs: Proved	34,344		72,361		92,466
Exploration costs: Unproved	20,112		18,033		5,919
Unproved	41,328		18,740		34,400
Capitalized general and administrative and interest costs	19,911		18,961		18,210
Total costs incurred	328,096		135,235		182,461
Movement in Accumulated Depreciation, Depletion and Amortization [Roll Forward]
Balance, beginning of year	(1,472,244)		(1,265,603)				(1,175,553)
Provision for DD&A	(69,357)		(59,496)		(57,143)
Ceiling test writedown	0		(137,100)		(18,907)
Sale of proved properties and other	(11,443)	[3]	(10,045)	[3]	(14,000)	[3]
Balance, end of year	$ (1,553,044)		$ (1,472,244)		$ (1,265,603)		$ (1,175,553)
DD&A per Mcfe (usd per mcfe)	1.82		1.75		1.89

[1] During 2013, the Company closed on the Gulf of Mexico Acquisition for an aggregate cash purchase price of $188.8 million (see Note 2 - Acquisition). Additionally, the Company acquired 13,500 net unevaluated acres in Oklahoma targeting the Woodford Shale.

[2] During 2012, the Company sold an additional portion of its Mississippian Lime acreage for $6.1 million. During 2011, the Company sold a portion of its unproved Mississippian Lime acreage for $14.5 million.

[3] During 2012, the Company sold its non-operated Arkansas assets for a net cash purchase price of $8.5 million. During 2011, the Company received an additional $14 million payment associated with the achievement of certain production metrics stipulated under the joint development agreement.