Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 176,186		$ 38,451		$ 65,454	$ 106,956
Accounts receivable	127,250		118,139
Fair value of hedging contracts	34,010		25,177
Current income tax receivable	24,066		19,946
Deferred taxes *	12,280	[1]	26,072	[1]
Inventory	4,506		4,643
Other current assets	3,246		791
Total current assets	381,544		233,219
Oil and gas properties, full cost method:
Proved	7,039,450		6,648,168
Less: accumulated DD&A	(5,427,668)		(5,174,729)
Net proved oil and gas properties	1,611,782		1,473,439
Unevaluated	454,639		401,609
Other property and equipment, net	13,399		11,172
Fair value of hedging contracts	18,155		22,543
Other assets, net	35,158		23,769
Total assets	2,514,677		2,165,751
Current liabilities:
Accounts payable to vendors	99,362		102,946
Undistributed oil and gas proceeds	24,277		27,328
Accrued interest	9,774		14,059
Fair value of hedging contracts	1,077		11,122
Asset retirement obligations	59,366		62,676
Other current liabilities	11,405		28,370
Total current liabilities	205,261		246,501
Long-term debt	811,024		620,000
Deferred taxes *	298,100		247,835
Asset retirement obligations	351,023		363,103
Fair value of hedging contracts	2,266		815
Other long-term liabilities	20,844		19,668
Total liabilities	1,688,518		1,497,922
Commitments and contingencies
Stockholders' equity:
Common stock	484		481
Treasury stock	(860)		(860)
Additional paid-in capital	1,383,741		1,338,565
Accumulated earnings (deficit)	(587,045)		(692,225)
Accumulated other comprehensive income (loss)	29,839		21,868
Total stockholders' equity	826,159		667,829
Total liabilities and stockholders' equity	2,514,677		2,165,751
Parent
Current assets:
Cash and cash equivalents	155,089		37,389		64,426	105,115
Accounts receivable	45,692		36,463
Current income tax receivable	24,066		19,946
Deferred taxes *	2,465	[1]	8,269	[1]
Inventory	4,223		4,360
Other current assets	3,246		791
Total current assets	234,781		107,218
Oil and gas properties, full cost method:
Proved	890,616		696,975
Less: accumulated DD&A	(353,857)		(309,421)
Net proved oil and gas properties	536,759		387,554
Unevaluated	287,645		246,269
Other property and equipment, net	13,399		11,172
Other assets, net	33,843		20,873
Investment in subsidiary	902,154		733,533
Total assets	2,008,581		1,506,619
Current liabilities:
Accounts payable to vendors	286,752		78,170
Undistributed oil and gas proceeds	22,822		26,036
Accrued interest	9,774		14,059
Other current liabilities	10,030		22,974
Total current liabilities	329,378		141,239
Long-term debt	811,024		620,000
Deferred taxes *	20,373		56,970
Asset retirement obligations	7,515		7,626
Other long-term liabilities	14,132		12,955
Total liabilities	1,182,422		838,790
Commitments and contingencies
Stockholders' equity:
Common stock	484		481
Treasury stock	(860)		(860)
Additional paid-in capital	1,383,741		1,338,565
Accumulated earnings (deficit)	(587,045)		(692,225)
Accumulated other comprehensive income (loss)	29,839		21,868
Total stockholders' equity	826,159		667,829
Total liabilities and stockholders' equity	2,008,581		1,506,619
Guarantor Subsidiary
Current assets:
Cash and cash equivalents	21,097		926		915	1,659
Accounts receivable	289,989		81,452
Fair value of hedging contracts	34,010		25,177
Deferred taxes *	9,815	[1]	17,803	[1]
Inventory	283		283
Total current assets	355,194		125,641
Oil and gas properties, full cost method:
Proved	6,148,834		5,946,141
Less: accumulated DD&A	(5,073,811)		(4,862,949)
Net proved oil and gas properties	1,075,023		1,083,192
Unevaluated	166,994		155,340
Fair value of hedging contracts	18,155		22,543
Other assets, net	1,315		2,896
Investment in subsidiary			(273)
Total assets	1,616,681		1,389,339
Current liabilities:
Accounts payable to vendors	21,041		25,866
Undistributed oil and gas proceeds	1,455		1,292
Fair value of hedging contracts	1,077		11,122
Asset retirement obligations	59,366		62,676
Other current liabilities	1,375		5,396
Total current liabilities	84,314		106,352
Deferred taxes *	277,727		190,865
Asset retirement obligations	343,508		351,061
Fair value of hedging contracts	2,266		815
Other long-term liabilities	6,712		6,713
Total liabilities	714,527		655,806
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,724,232		1,724,232
Accumulated earnings (deficit)	(851,917)		(1,012,567)
Accumulated other comprehensive income (loss)	29,839		21,868
Total stockholders' equity	902,154		733,533
Total liabilities and stockholders' equity	1,616,681		1,389,339
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents			136			182
Accounts receivable			1,353
Total current assets			1,489
Oil and gas properties, full cost method:
Proved			5,052
Less: accumulated DD&A			(2,359)
Net proved oil and gas properties			2,693
Total assets			4,182
Current liabilities:
Accounts payable to vendors			39
Total current liabilities			39
Asset retirement obligations			4,416
Total liabilities			4,455
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital			1,639
Accumulated earnings (deficit)			(1,912)
Total stockholders' equity			(273)
Total liabilities and stockholders' equity			4,182
Eliminations
Current assets:
Accounts receivable	(208,431)		(1,129)
Total current assets	(208,431)		(1,129)
Oil and gas properties, full cost method:
Investment in subsidiary	(902,154)		(733,260)
Total assets	(1,110,585)		(734,389)
Current liabilities:
Accounts payable to vendors	(208,431)		(1,129)
Total current liabilities	(208,431)		(1,129)
Total liabilities	(208,431)		(1,129)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,724,232)		(1,725,871)
Accumulated earnings (deficit)	851,917		1,014,479
Accumulated other comprehensive income (loss)	(29,839)		(21,868)
Total stockholders' equity	(902,154)		(733,260)
Total liabilities and stockholders' equity	$ (1,110,585)		$ (734,389)

[1] Deferred income taxes have been allocated to the guarantor subsidiary where related oil and gas properties reside.