Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011
Property and Equipment
Proved oil and gas properties	$ 4,913,556			$ 4,913,556			$ 4,343,867
Unproved oil and gas properties	91,415			91,415			84,146
Furniture, fixtures, and other equipment	41,297			41,297			38,832
Less - Accumulated depreciation, depletion, and amortization	(2,781,965)			(2,781,965)			(2,599,079)
Property and Equipment, Net	2,264,303			2,264,303			1,867,766
Accounts Payable and Accrued Liabilities
Trade accounts payable	60,785	[1]		60,785	[1]		42,080	[1]
Accrued operating expenses	11,328			11,328			15,833
Accrued payroll costs	10,984			10,984			14,345
Asset retirement obligation - current portion	6,088			6,088			9,279
Accrued taxes	8,618			8,618			7,604
Other payables	1,803			1,803			6,825
Total accounts payable and accrued liabilities	99,606			99,606			95,966
Accumulated other comprehensive income and related tax effects
Accumulated Other Comprehensive Income (Loss), Gross				(178)			0
Accumulated Other Comprehensive Income (Loss) Tax Effect				(65)			0
Accumulated Other Comprehensive Income (Loss), Net of Tax	(113)			(113)			0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	66		(72)	1,237		(405)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	24		(27)	450		(148)
Unrealized gains (losses) related to price risk management transactions, net of taxes	42		(45)	787		(257)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	(244)		(260)	(1,415)		623
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Tax	(89)		(95)	(515)		229
Reclassification of (gains) losses on price risk management transactions to net income, net of taxes	(155)		(165)	(900)		394
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1				76,393
Accretion expense	1,191		1,183	3,465		3,482
Liabilities incurred for new wells and facilities construction				1,148
Reductions due to sold and abandoned wells				(1,692)
Revisions in estimates				3,978
Asset Retirement Obligation	$ 83,292			$ 83,292			$ 76,393

[1] Included in “trade accounts payable” are liabilities of approximately $28.3 million and $18.7 million at September 30, 2012 and December 31, 2011, respectively, for outstanding checks.