Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Property and Equipment
Proved oil and gas properties	$ 5,817,395			$ 5,817,395			$ 5,600,279
Unproved oil and gas properties	63,662			63,662			71,452
Furniture, fixtures, and other equipment	42,400			42,400			42,368
Less - Accumulated depreciation, depletion, and amortization	(3,308,992)			(3,308,992)			(3,174,453)
Property and Equipment, Net	2,614,465			2,614,465			2,539,646
Accounts Payable and Accrued Liabilities
Trade accounts payable	39,240	[1]		39,240	[1]		30,769	[1]
Accrued operating expenses	15,988			15,988			17,059
Accrued payroll costs	8,655			8,655			10,938
Asset retirement obligation - current portion	13,717			13,717			15,859
Accrued taxes	7,533			7,533			5,845
Deferred liability	12,500	[2]		12,500	[2]		0	[2]
Other payables	6,890			6,890			2,891
Total accounts payable and accrued liabilities	104,523			104,523			83,361
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1				79,084
Accretion expense	1,415		1,479	2,801		3,254
Liabilities incurred for new wells and facilities construction				180
Reductions due to sold and abandoned wells				(2,643)
Revisions in estimates				258
Asset Retirement Obligation	$ 79,680			$ 79,680
Crude Oil [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Portion of Future Oil and Gas Production Being Hedged	197,500			197,500
Derivative, Swap Type, Average Fixed Price	101.76			101.76
Natural Gas excluding NGLs [Member] | Basis Swap [Member] | Year 2014 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	(0.09)			(0.09)
Future Gas Production Hedged in MMBTU (Energy Item Type)	8,850,000			8,850,000
Natural Gas excluding NGLs [Member] | Basis Swap [Member] | Year 2015 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	(0.02)			(0.02)
Future Gas Production Hedged in MMBTU (Energy Item Type)	8,200,000			8,200,000
Natural Gas excluding NGLs [Member] | Swap [Member] | Year 2014 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.29			4.29
Future Gas Production Hedged in MMBTU (Energy Item Type)	7,020,000			7,020,000
Natural Gas excluding NGLs [Member] | Swap [Member] | Year 2015 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.42			4.42
Future Gas Production Hedged in MMBTU (Energy Item Type)	900,000			900,000
Natural Gas excluding NGLs [Member] | Collar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Average Cap Price	4.46			4.46
Derivative, Average Floor Price	4.13			4.13
Future Gas Production Hedged in MMBTU (Energy Item Type)	2,100,000			2,100,000

[1] Included in “trade accounts payable” are liabilities of approximately $7.5 million and $26.1 million at June 30, 2014 and December 31, 2013, respectively, for outstanding checks.

[2] This amount includes the liability related to the Fasken joint venture for the restricted cash, held in escrow, at June 30, 2014.