Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Property and Equipment
Proved oil and gas properties	$ 5,058,524		$ 4,343,867
Unproved oil and gas properties	92,579		84,146
Furniture, fixtures, and other equipment	41,690		38,832
Less - Accumulated depreciation, depletion, and amortization	(2,847,773)		(2,599,079)
Property and Equipment, Net	2,345,020		1,867,766
Accounts Payable and Accrued Liabilities
Trade accounts payable	31,128	[1]	42,080	[1]
Accrued operating expenses	14,647		15,833
Accrued payroll costs	12,297		14,345
Asset retirement obligation - current portion	7,134		9,279
Accrued taxes	5,373		7,604
Other payables	4,799		6,825
Total accounts payable and accrued liabilities	75,378		95,966
Accumulated other comprehensive income and related tax effects
Accumulated Other Comprehensive Income (Loss), Gross			0
Accumulated Other Comprehensive Income (Loss) Tax Effect			0
Accumulated Other Comprehensive Income (Loss), Net of Tax	0		0
Change in fair value of cash flow hedges, Gross Value	1,210		(520)		868
Change in fair value of cash flow hedges, Tax Effect	440		(190)		319
Change in fair value of cash flow hedges, Net of Tax Value	770		(330)		549
Effect of cash flow hedges settled during the period, Gross Value	(1,210)		738		(732)
Effect of cash flow hedges settled during the period, Tax Effect	(440)		270		(269)
Effect of cash flow hedges settled during the period, Net of Tax Value	(770)		468		(463)
Other Comprehensive Income (Loss), before Tax	0		218		136
Other Comprehensive Income (Loss), Tax	0		80		51
Other comprehensive loss, Ending Balance, Net of Tax	0		138		85
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1	76,393		78,879		64,236
Accretion expense	5,121		4,570		3,956
Liabilities incurred for new wells and facilities construction	2,195		590		1,287
Reductions due to sold and abandoned wells	(2,824)		(28,194)		(749)
Revisions in estimates	5,892		20,548		10,149
Asset Retirement Obligation recorded as of December 31	$ 86,777		$ 76,393		$ 78,879

[1] Included in “trade accounts payable” are liabilities of approximately $13.3 million and $18.7 million at December 31, 2012 and 2011, respectively, for outstanding checks.