Guarantor Financial Statements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 263,841	$ 38,451	$ 45,906	$ 106,956
Accounts receivable	133,266	118,139
Fair value of hedging contracts	24,352	25,177
Current income tax receivable	23,140	19,946
Deferred taxes	30,206	26,072
Inventory	4,643	4,643
Other current assets	754	791
Total current assets	480,202	233,219
Oil and gas properties, full cost method
Proved	6,704,772	6,648,168
Less: accumulated depreciation, depletion and amortization	(5,252,894)	(5,174,729)
Net proved oil and gas properties	1,451,878	1,473,439
Unevaluated	443,889	401,609
Other property and equipment, net	11,444	11,172
Fair value of hedging contracts	10,937	22,543
Other assets, net	28,963	23,769
Total assets	2,427,313	2,165,751
Current liabilities:
Accounts payable to vendors	86,307	102,946
Undistributed oil and gas proceeds	26,359	27,328
Accrued interest	10,490	14,059
Fair value of hedging contracts	27,298	11,122
Asset retirement obligations	71,256	62,676
Other current liabilities	10,740	28,370
Total current liabilities	232,450	246,501
Long-term debt	805,140	620,000
Deferred taxes	266,996	247,835
Asset retirement obligations	359,274	363,103
Fair value of hedging contracts	8,025	815
Other long-term liabilities	20,523	19,668
Total liabilities	1,692,408	1,497,922		1,692,408
Commitments and contingencies
Stockholders' equity:
Common stock	483	481
Treasury stock (16,582 shares, at cost)	(860)	(860)
Additional paid-in capital	1,376,730	1,338,565
Accumulated earnings (deficit)	(641,251)	(692,225)
Accumulated other comprehensive income (loss)	(197)	21,868
Total stockholders' equity	734,905	667,829
Total liabilities and stockholders' equity	2,427,313	2,165,751
Parent [Member]
Current assets:
Cash and cash equivalents	222,750	37,389	45,746	105,115
Accounts receivable	37,849	36,463
Current income tax receivable	23,140	19,946
Deferred taxes	3,194	8,269
Inventory	4,360	4,360
Other current assets	754	791
Total current assets	292,047	107,218
Oil and gas properties, full cost method
Proved	672,697	696,975
Less: accumulated depreciation, depletion and amortization	(321,084)	(309,421)
Net proved oil and gas properties	351,613	387,554
Unevaluated	275,920	246,269
Other property and equipment, net	11,444	11,172
Other assets, net	27,494	20,873
Investment in subsidiary	777,606	733,533
Total assets	1,736,124	1,506,619
Current liabilities:
Accounts payable to vendors	92,986	78,170
Undistributed oil and gas proceeds	24,810	26,036
Accrued interest	10,490	14,059
Other current liabilities	4,613	22,974
Total current liabilities	132,899	141,239
Long-term debt	805,140	620,000
Deferred taxes	42,131	56,970
Asset retirement obligations	7,240	7,626
Other long-term liabilities	13,809	12,955
Total liabilities		838,790		1,001,219
Commitments and contingencies
Stockholders' equity:
Common stock	483	481
Treasury stock (16,582 shares, at cost)	(860)	(860)
Additional paid-in capital	1,376,730	1,338,565
Accumulated earnings (deficit)	(641,251)	(692,225)
Accumulated other comprehensive income (loss)	(197)	21,868
Total stockholders' equity	734,905	667,829
Total liabilities and stockholders' equity	1,736,124	1,506,619
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	40,955	926	108	1,659
Accounts receivable	113,735	81,452
Fair value of hedging contracts	24,352	25,177
Deferred taxes	27,012	17,803
Inventory	283	283
Total current assets	206,337	125,641
Oil and gas properties, full cost method
Proved	6,027,024	5,946,141
Less: accumulated depreciation, depletion and amortization	(4,929,353)	(4,862,949)
Net proved oil and gas properties	1,097,671	1,083,192
Unevaluated	167,969	155,340
Fair value of hedging contracts	10,937	22,543
Other assets, net	1,469	2,896
Investment in subsidiary	(512)	(273)
Total assets	1,483,871	1,389,339
Current liabilities:
Accounts payable to vendors	12,898	25,866
Undistributed oil and gas proceeds	1,549	1,292
Fair value of hedging contracts	27,298	11,122
Asset retirement obligations	71,256	62,676
Other current liabilities	6,127	5,396
Total current liabilities	119,128	106,352
Deferred taxes	224,865	190,865
Asset retirement obligations	347,533	351,061
Fair value of hedging contracts	8,025	815
Other long-term liabilities	6,714	6,713
Total liabilities		655,806		706,265
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,724,232	1,724,232
Accumulated earnings (deficit)	(946,429)	(1,012,567)
Accumulated other comprehensive income (loss)	(197)	21,868
Total stockholders' equity	777,606	733,533
Total liabilities and stockholders' equity	1,483,871	1,389,339
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	136	136	52	182
Accounts receivable	1,466	1,353
Total current assets	1,602	1,489
Oil and gas properties, full cost method
Proved	5,051	5,052
Less: accumulated depreciation, depletion and amortization	(2,457)	(2,359)
Net proved oil and gas properties	2,594	2,693
Total assets	4,196	4,182
Current liabilities:
Accounts payable to vendors	208	39
Total current liabilities	208	39
Asset retirement obligations	4,501	4,416
Total liabilities		4,455		4,709
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,639	1,639
Accumulated earnings (deficit)	(2,152)	(1,912)
Total stockholders' equity	(513)	(273)
Total liabilities and stockholders' equity	4,196	4,182
Eliminations [Member]
Current assets:
Accounts receivable	(19,784)	(1,129)
Total current assets	(19,784)	(1,129)
Oil and gas properties, full cost method
Investment in subsidiary	(777,094)	(733,260)
Total assets	(796,878)	(734,389)
Current liabilities:
Accounts payable to vendors	(19,785)	(1,129)
Total current liabilities	(19,785)	(1,129)
Total liabilities		(1,129)		(19,785)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,725,871)	(1,725,871)
Accumulated earnings (deficit)	948,581	1,014,479
Accumulated other comprehensive income (loss)	197	(21,868)
Total stockholders' equity	(777,093)	(733,260)
Total liabilities and stockholders' equity	$ (796,878)	$ (734,389)