Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011	Dec. 31, 2011
Property and Equipment
Proved oil and gas properties	$ 4,715,417,000			$ 4,715,417,000			$ 4,343,867,000
Unproved oil and gas properties	89,005,000			89,005,000			84,146,000
Furniture, fixtures, and other equipment	41,062,000			41,062,000			38,832,000
Less - Accumulated depreciation, depletion, and amortization	(2,722,546,000)			(2,722,546,000)			(2,599,079,000)
Property and Equipment, Net	2,122,938,000			2,122,938,000			1,867,766,000
Accounts Payable and Accrued Liabilities
Trade accounts payable	45,183,000	[1]		45,183,000	[1]		42,080,000	[1]
Accrued operating expenses	15,606,000			15,606,000			15,833,000
Accrued payroll costs	8,111,000			8,111,000			14,345,000
Asset retirement obligation - current portion	9,656,000			9,656,000			9,279,000
Accrued taxes	8,610,000			8,610,000			7,604,000
Other payables	2,237,000			2,237,000			6,825,000
Total accounts payable and accrued liabilities	89,403,000			89,403,000			95,966,000
Accumulated other comprehensive income and related tax effects
Other Comprehensive Income (Loss), before Tax	(411,000)		698,000	0		550,000
Other Comprehensive Income (Loss), Tax	(150,000)		256,000	0		202,000
Other Comprehensive Income (Loss), Net of Tax	(261,000)		442,000	0	[2]	348,000	138,000
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	958,000		(352,000)	1,171,000		(333,000)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	349,000		(128,000)	426,000		(121,000)
Unrealized gains (losses) related to price risk management transactions, net of taxes	609,000		(224,000)	745,000		(212,000)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	(1,369,000)		1,050,000	(1,171,000)		883,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Tax	(498,000)		384,000	(426,000)		324,000
Reclassification of (gains) losses on price risk management transactions to net income, net of taxes	(871,000)		666,000	(745,000)		559,000
Other Comprehensive Income (Loss), before Tax	(411,000)		698,000	0		550,000
Other Comprehensive Income (Loss), Tax	(150,000)		256,000	0		202,000
Other Comprehensive Income (Loss), Net of Tax	(261,000)		442,000	0	[2]	348,000	138,000
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1				76,393,000
Accretion expense	1,162,000		1,163,000	2,274,000		2,299,000
Liabilities incurred for new wells and facilities construction				566,000
Reductions due to sold and abandoned wells				(80,000)
Revisions in estimates				1,475,000
Asset Retirement Obligation	$ 80,628,000			$ 80,628,000			$ 76,393,000

[1] Included in “trade accounts payable” are liabilities of approximately $23.7 million and $18.7 million at June 30, 2012 and December 31, 2011, respectively, for outstanding checks.

[2] Unaudited