Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012
Property and Equipment
Proved oil and gas properties	$ 5,354,887			$ 5,354,887			$ 5,058,524
Unproved oil and gas properties	93,990			93,990			92,579
Furniture, fixtures, and other equipment	42,375			42,375			41,690
Less - Accumulated depreciation, depletion, and amortization	(2,968,161)			(2,968,161)			(2,847,773)
Property and Equipment, Net	2,523,091			2,523,091			2,345,020
Accounts Payable and Accrued Liabilities
Trade accounts payable	41,145	[1]		41,145	[1]		31,128	[1]
Accrued operating expenses	17,970			17,970			14,647
Accrued payroll costs	8,238			8,238			12,297
Asset retirement obligation - current portion	9,520			9,520			7,134
Accrued taxes	8,652			8,652			5,373
Other payables	2,894			2,894			4,799
Total accounts payable and accrued liabilities	88,419			88,419			75,378
Roll-forward of our asset retirement obligation
Asset Retirement Obligation recorded as of January 1				86,777
Accretion expense	1,479		1,162	3,254		2,274
Liabilities incurred for new wells and facilities construction				1,214
Reductions due to sold and abandoned wells				(12,780)
Revisions in estimates				(113)
Asset Retirement Obligation	$ 78,352			$ 78,352

[1] Included in “trade accounts payable” are liabilities of approximately $27.9 million and $13.3 million at June 30, 2013 and December 31, 2012, respectively, for outstanding checks.