Guarantor Financial Statements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 223,849	$ 38,451	$ 82,021	$ 106,956
Accounts receivable	148,934	118,139
Fair value of hedging contracts	61,203	25,177
Current income tax receivable	24,353	19,946
Deferred taxes	7,474	26,072
Inventory	4,607	4,643
Other current assets	914	791
Total current assets	471,334	233,219
Oil and gas properties, full cost method
Proved	6,927,917	6,648,168
Less: accumulated depreciation, depletion and amortization	(5,339,251)	(5,174,729)
Net proved oil and gas properties	1,588,666	1,473,439
Unevaluated	435,384	401,609
Other property and equipment, net	11,910	11,172
Fair value of hedging contracts	41,981	22,543
Other assets, net	33,955	23,769
Total assets	2,583,230	2,165,751
Current liabilities:
Accounts payable to vendors	143,389	102,946
Undistributed oil and gas proceeds	27,248	27,328
Accrued interest	15,735	14,059
Fair value of hedging contracts		11,122
Asset retirement obligations	75,493	62,676
Other current liabilities	9,470	28,370
Total current liabilities	271,335	246,501
Long-term debt	808,050	620,000
Deferred taxes	298,551	247,835
Asset retirement obligations	351,785	363,103
Fair value of hedging contracts	366	815
Other long-term liabilities	20,651	19,668
Total liabilities	1,750,738	1,497,922
Commitments and contingencies
Stockholders' equity:
Common stock	483	481
Treasury stock (16,582 shares, at cost)	(860)	(860)
Additional paid-in capital	1,380,568	1,338,565
Accumulated earnings (deficit)	(610,704)	(692,225)
Accumulated other comprehensive income	63,005	21,868
Total stockholders' equity	832,492	667,829
Total liabilities and stockholders' equity	2,583,230	2,165,751
Parent [Member]
Current assets:
Cash and cash equivalents	192,755	37,389	81,589	105,115
Accounts receivable	73,168	36,463
Current income tax receivable	24,353	19,946
Deferred taxes	2,031	8,269
Inventory	4,324	4,360
Other current assets	914	791
Total current assets	297,545	107,218
Oil and gas properties, full cost method
Proved	815,886	696,975
Less: accumulated depreciation, depletion and amortization	(335,378)	(309,421)
Net proved oil and gas properties	480,508	387,554
Unevaluated	290,751	246,269
Other property and equipment, net	11,910	11,172
Other assets, net	32,387	20,873
Investment in subsidiary	893,122	733,533
Total assets	2,006,223	1,506,619
Current liabilities:
Accounts payable to vendors	263,942	78,170
Undistributed oil and gas proceeds	25,566	26,036
Accrued interest	15,735	14,059
Other current liabilities	8,844	22,974
Total current liabilities	314,087	141,239
Long-term debt	808,050	620,000
Deferred taxes	30,278	56,970
Asset retirement obligations	7,377	7,626
Other long-term liabilities	13,939	12,955
Total liabilities	1,173,731	838,790
Commitments and contingencies
Stockholders' equity:
Common stock	483	481
Treasury stock (16,582 shares, at cost)	(860)	(860)
Additional paid-in capital	1,380,568	1,338,565
Accumulated earnings (deficit)	(610,704)	(692,225)
Accumulated other comprehensive income	63,005	21,868
Total stockholders' equity	832,492	667,829
Total liabilities and stockholders' equity	2,006,223	1,506,619
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	30,958	926	319	1,659
Accounts receivable	218,282	81,452
Fair value of hedging contracts	61,203	25,177
Deferred taxes	5,443	17,803
Inventory	283	283
Total current assets	316,169	125,641
Oil and gas properties, full cost method
Proved	6,106,980	5,946,141
Less: accumulated depreciation, depletion and amortization	(5,001,323)	(4,862,949)
Net proved oil and gas properties	1,105,657	1,083,192
Unevaluated	144,633	155,340
Fair value of hedging contracts	41,981	22,543
Other assets, net	1,568	2,896
Investment in subsidiary	(374)	(273)
Total assets	1,609,634	1,389,339
Current liabilities:
Accounts payable to vendors	23,541	25,866
Undistributed oil and gas proceeds	1,682	1,292
Fair value of hedging contracts		11,122
Asset retirement obligations	75,493	62,676
Other current liabilities	626	5,396
Total current liabilities	101,342	106,352
Deferred taxes	268,273	190,865
Asset retirement obligations	339,821	351,061
Fair value of hedging contracts	366	815
Other long-term liabilities	6,712	6,713
Total liabilities	716,514	655,806
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,724,232	1,724,232
Accumulated earnings (deficit)	(894,117)	(1,012,567)
Accumulated other comprehensive income	63,005	21,868
Total stockholders' equity	893,120	733,533
Total liabilities and stockholders' equity	1,609,634	1,389,339
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	136	136	113	182
Accounts receivable	1,578	1,353
Total current assets	1,714	1,489
Oil and gas properties, full cost method
Proved	5,051	5,052
Less: accumulated depreciation, depletion and amortization	(2,550)	(2,359)
Net proved oil and gas properties	2,501	2,693
Total assets	4,215	4,182
Current liabilities:
Accounts payable to vendors		39
Total current liabilities		39
Asset retirement obligations	4,587	4,416
Total liabilities	4,587	4,455
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,639	1,639
Accumulated earnings (deficit)	(2,011)	(1,912)
Total stockholders' equity	(372)	(273)
Total liabilities and stockholders' equity	4,215	4,182
Eliminations [Member]
Current assets:
Cash and cash equivalents
Accounts receivable	(144,094)	(1,129)
Total current assets	(144,094)	(1,129)
Oil and gas properties, full cost method
Investment in subsidiary	(892,748)	(733,260)
Total assets	(1,036,842)	(734,389)
Current liabilities:
Accounts payable to vendors	(144,094)	(1,129)
Total current liabilities	(144,094)	(1,129)
Total liabilities	(144,094)	(1,129)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,725,871)	(1,725,871)
Accumulated earnings (deficit)	896,128	1,014,479
Accumulated other comprehensive income	(63,005)	(21,868)
Total stockholders' equity	(892,748)	(733,260)
Total liabilities and stockholders' equity	$ (1,036,842)	$ (734,389)