Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Property and Equipment
Proved oil and gas properties	$ 4,521,651		$ 4,343,867
Unproved oil and gas properties	87,327		84,146
Furniture, fixtures, and other equipment	39,709		38,832
Less - Accumulated depreciation, depletion, and amortization	(2,660,839)		(2,599,079)
Property and Equipment, Net	1,987,848		1,867,766
Accounts Payable and Accrued Liabilities
Trade accounts payable	39,219		42,080
Accrued operating expenses	15,540		15,833
Accrued payroll costs	4,957		14,345
Asset retirement obligation - current portion	9,303		9,279
Accrued taxes	7,446		7,604
Other payables	2,333		6,825
Total accounts payable and accrued liabilities	78,798		95,966
Accumulated other comprehensive income and related tax effects
Other comprehensive gain (loss) at Gross Value
Other comprehensive gain (loss) at Tax Effect
Other comprehensive gain (loss) at Net of Tax Value
Change in fair value of cash flow hedges, Gross Value	212
Change in fair value of cash flow hedges, Tax Effect	(77)
Change in fair value of cash flow hedges, Net of Tax Value	135
Effect of cash flow hedges settled during the period, Gross Value	198
Effect of cash flow hedges settled during the period, Tax Effect	(72)
Effect of cash flow hedges settled during the period, Net of Tax Value	126
Other comprehensive gain (loss) at Gross Value	410
Other comprehensive gain (loss) at Tax Effect	(149)
Other comprehensive gain (loss) at Net of Tax Value	261
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1	76,393	78,879
Accretion expense	1,112	1,136
Liabilities incurred for new wells and facilities construction	282	139
Reductions due to sold and abandoned wells	(79)	(32)
Revisions in estimates	1,147	(1,385)
Asset Retirement Obligation	$ 78,855	$ 78,737