Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable	$ 30,769	[1]	$ 31,128	[1]
Accrued operating expenses	16,016		16,479
Accrued payroll costs	10,938		12,297
Asset retirement obligation - current portion	15,859		7,134
Accrued taxes	5,845		5,473
Other payables	2,891		4,799
Total accounts payable and accrued liabilities	82,318		77,310
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation recorded as of January 1	86,777		76,393
Accretion expense	6,181		5,121		4,570
Liabilities incurred for new wells and facilities construction	1,588		2,195
Reductions due to sold and abandoned wells	(16,394)		(2,824)
Revisions in estimates	932		5,892
Asset Retirement Obligation recorded as of December 31	79,084		86,777		76,393
Property, Plant and Equipment [Abstract]
Proved oil and gas properties	5,600,279		5,058,524
Unproved oil and gas properties	71,452		92,579
Furniture, fixtures, and other equipment	42,368		41,690
Less - Accumulated depreciation, depletion, and amortization	(3,125,282)		(2,824,839)
Property and Equipment, Net	$ 2,588,817		$ 2,367,954
Crude Oil [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Portion of Future Oil and Gas Production Being Hedged	462,000
Derivative, Swap Type, Average Fixed Price	98.19
Crude Oil [Member] | Options Held [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Portion of Future Oil and Gas Production Being Hedged	87,000
Derivative, Average Cap Price	116.60
Crude Oil [Member] | Future [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Portion of Future Oil and Gas Production Being Hedged	87,000
Derivative, Average Cap Price	107.30
Derivative, Average Floor Price	95.00
Natural Gas excluding NGLs [Member] | Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.10
Future Gas Production Hedged in MMBTU (Energy Item Type)	7,460,000
Natural Gas excluding NGLs [Member] | Future [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Average Cap Price	4.44
Derivative, Average Floor Price	4.13
Future Gas Production Hedged in MMBTU (Energy Item Type)	3,075,000
Natural Gas excluding NGLs [Member] | Non-Current Swaps [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	4.42
Future Gas Production Hedged in MMBTU (Energy Item Type)	900,000
Natural Gas excluding NGLs [Member] | Forward Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Swap Type, Average Fixed Price	0.085
Future Gas Production Hedged in MMBTU (Energy Item Type)	2,030,000

[1] Included in “trade accounts payable” are liabilities of approximately $26.1 million and $13.3 million at December 31, 2013 and 2012, respectively, for outstanding checks.