Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Property and Equipment
Proved oil and gas properties	$ 5,474,748			$ 5,474,748			$ 5,058,524
Unproved oil and gas properties	90,359			90,359			92,579
Furniture, fixtures, and other equipment	41,993			41,993			41,690
Less - Accumulated depreciation, depletion, and amortization	(3,034,614)			(3,034,614)			(2,847,773)
Property and Equipment, Net	2,572,486			2,572,486			2,345,020
Accounts Payable and Accrued Liabilities
Trade accounts payable	34,497	[1]		34,497	[1]		31,128	[1]
Accrued operating expenses	13,173			13,173			14,647
Accrued payroll costs	10,782			10,782			12,297
Asset retirement obligation - current portion	8,119			8,119			7,134
Accrued taxes	12,292			12,292			5,373
Other payables	2,745			2,745			4,799
Total accounts payable and accrued liabilities	81,608			81,608			75,378
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1				86,777
Accretion expense	1,478		1,191	4,732		3,465
Liabilities incurred for new wells and facilities construction				1,550
Reductions due to sold and abandoned wells				(14,010)
Revisions in estimates				(2,302)
Asset Retirement Obligation	$ 76,747			$ 76,747

[1] Included in “trade accounts payable” are liabilities of approximately $11.4 million and $13.3 million at September 30, 2013 and December 31, 2012, respectively, for outstanding checks.