Investment in Oil and Gas Properties (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Acquisition costs: Proved	$ 352		$ 2,720		$ 10,421
Acquisition costs: Unproved	15,677		43,207		11,310
Divestitures—unproved	(8,889)		(14,461)		(36,139)
Exploration costs: Proved	72,361		92,466		34,310
Exploration costs: Unproved	18,033		5,919		10,384
Unproved	18,740		34,400		34,286
Capitalized general and administrative and interest costs	18,961		18,210		19,665
Total costs incurred	135,235		182,461		84,237
Movement in Accumulated Depreciation, Depletion and Amortization [Roll Forward]
Balance, beginning of year	(1,265,603)		(1,175,553)		(1,082,381)
Provision for DD&A	(59,496)		(57,143)		(58,172)
Ceiling test writedown	(137,100)		(18,907)		0
Sale of proved properties and other	(10,045)	[1]	(14,000)	[1]	(35,000)	[1]
Balance, end of year	$ (1,472,244)		$ (1,265,603)		$ (1,175,553)
DD&A per Mcfe (usd per mcfe)	1.75		1.89		1.88

[1] During 2012, the Company sold its non-operated Arkansas assets for a net cash purchase price of $9.2 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 (See Note 3). During 2010, the Company recorded $35 million in consideration from the sale of a portion of its evaluated properties in the Woodford as part of its Woodford joint development agreement.