Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013
Property and Equipment
Proved oil and gas properties	$ 5,704,702			$ 5,600,279
Unproved oil and gas properties	66,246			71,452
Furniture, fixtures, and other equipment	42,357			42,368
Less - Accumulated depreciation, depletion, and amortization	(3,236,443)			(3,174,453)
Property and Equipment, Net	2,576,862			2,539,646
Accounts Payable and Accrued Liabilities
Trade accounts payable	24,760	[1]		30,769	[1]
Accrued operating expenses	18,126			17,059
Accrued payroll costs	10,550			10,938
Asset retirement obligation - current portion	14,586			15,859
Accrued taxes	6,874			5,845
Other payables	6,286			2,891
Total accounts payable and accrued liabilities	81,182			83,361
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1	79,084
Accretion expense	1,386		1,775
Liabilities incurred for new wells and facilities construction	40
Reductions due to sold and abandoned wells	(1,685)
Revisions in estimates	264
Asset Retirement Obligation	$ 79,089
Crude Oil [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Portion of Future Oil and Gas Production Being Hedged	297,500
Derivative, Swap Type, Average Fixed Price	98.15
Natural Gas excluding NGLs [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.26
Future Gas Production Hedged in MMBTU (Energy Item Type)	8,880,000
Natural Gas excluding NGLs [Member] | Future [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Average Cap Price	4.43
Derivative, Average Floor Price	4.12
Future Gas Production Hedged in MMBTU (Energy Item Type)	3,340,000
Natural Gas excluding NGLs [Member] | Non-Current Swaps [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.42
Future Gas Production Hedged in MMBTU (Energy Item Type)	900,000
Natural Gas excluding NGLs [Member] | Forward Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	0.09
Future Gas Production Hedged in MMBTU (Energy Item Type)	9,600,000

[1] Included in “trade accounts payable” are liabilities of approximately $10.9 million and $26.1 million at March 31, 2014 and December 31, 2013, respectively, for outstanding checks.