Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Property and Equipment
Proved oil and gas properties	$ 5,201,142			$ 5,058,524
Unproved oil and gas properties	94,223			92,579
Furniture, fixtures, and other equipment	41,907			41,690
Less - Accumulated depreciation, depletion, and amortization	(2,908,302)			(2,847,773)
Property and Equipment, Net	2,428,970			2,345,020
Accounts Payable and Accrued Liabilities
Trade accounts payable	23,073	[1]		31,128	[1]
Accrued operating expenses	20,401			14,647
Accrued payroll costs	6,842			12,297
Asset retirement obligation - current portion	8,779			7,134
Accrued taxes	7,285			5,373
Other payables	3,276			4,799
Total accounts payable and accrued liabilities	69,656			75,378
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1	86,777
Revisions in estimates	71
Accretion expense	1,775		1,112
Liabilities incurred for new wells and facilities construction	563
Reductions due to sold and abandoned wells	(884)
Asset Retirement Obligation	$ 88,302

[1] Included in “trade accounts payable” are liabilities of approximately $11.3 million and $13.3 million at March 31, 2013 and December 31, 2012, respectively, for outstanding checks.