Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Proved oil and gas properties	$ 5,826,995		$ 5,600,279
Unproved oil and gas properties	64,903		71,452
Furniture, fixtures, and other equipment	42,257		42,368
Less - Accumulated depreciation, depletion, and amortization	(3,839,118)		(3,125,282)
Property and Equipment, Net	2,095,037		2,588,817
Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable	31,153	[1]	30,769	[1]
Accrued operating expenses	10,784		16,016
Accrued payroll costs	8,100		10,938
Asset retirement obligation - current portion	10,709		15,859
Accrued taxes	2,957		5,845
Other payables	4,541		2,891
Total accounts payable and accrued liabilities	68,244		82,318
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation recorded as of January 1	79,084		86,777
Accretion expense	5,712		6,181		5,121
Liabilities incurred for new wells and facilities construction	469		1,588
Reductions due to sold and abandoned wells	(8,253)		(16,394)
Revisions in estimates	(4,181)		932
Asset Retirement Obligation recorded as of December 31	$ 72,831		$ 79,084		$ 86,777
Year 2015 [Member] | Energy Related Derivative [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Future Gas Production Hedged in MMBTU (Energy Item Type)	600,000
Derivative, Swap Type, Average Fixed Price	4.42
Year 2015 [Member] | Energy Related Derivative [Member] | Future [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Future Gas Production Hedged in MMBTU (Energy Item Type)	1,280,000
Derivative, Average Floor Price	4.05
Derivative, Average Cap Price	4.88
Year 2015 [Member] | Other Contract [Member] | Forward Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Future Gas Production Hedged in MMBTU (Energy Item Type)	7,280,000
Derivative, Swap Type, Average Fixed Price	(0.02)

[1] Included in “trade accounts payable” are liabilities of approximately $13.7 million and $26.1 million at December 31, 2014 and 2013, respectively, for outstanding checks.