Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Current assets:
Cash and cash equivalents	$ 279,526		$ 38,451		$ 106,956	$ 69,293
Accounts receivable	167,288		118,139
Fair value of hedging contracts	39,655		25,177
Current income tax receivable	10,027		19,946
Deferred taxes	15,514	[1]	26,072	[1]
Inventory	4,207		4,643
Other current assets	3,626		791
Total current assets	519,843		233,219
Oil and gas properties, full cost method:
Proved, net	1,734,300		1,473,439
Unevaluated	447,795		401,609
Other property and equipment, net	22,115		11,172
Fair value of hedging contracts	9,199		22,543
Other assets, net	43,179		23,769
Total assets	2,776,431		2,165,751
Current liabilities:
Accounts payable to vendors	94,361		102,946
Undistributed oil and gas proceeds	23,414		27,328
Accrued interest	18,546		14,059
Fair value of hedging contracts	149		11,122
Asset retirement obligations	66,260		62,676
Other current liabilities	16,765		28,370
Total current liabilities	219,495		246,501
Long-term debt	914,126		620,000
Deferred taxes	310,830	[1]	247,835	[1]
Asset retirement obligations	422,042		363,103
Fair value of hedging contracts	1,530		815
Other long-term liabilities	36,275		19,668
Total liabilities	1,904,298		1,497,922
Commitments and contingencies
Stockholders' equity:
Common stock	484		481
Treasury stock	(860)		(860)
Additional paid-in capital	1,386,475		1,338,565
Accumulated earnings (deficit)	(542,799)		(692,225)
Accumulated other comprehensive income (loss)	28,833		21,868
Total stockholders' equity	872,133		667,829
Total liabilities and stockholders' equity	2,776,431		2,165,751
Parent
Current assets:
Cash and cash equivalents	228,398		37,389		105,115	64,830
Accounts receivable	59,213		36,463
Current income tax receivable	10,027		19,946
Deferred taxes	5,947	[1]	8,269	[1]
Inventory	3,924		4,360
Other current assets	3,626		791
Total current assets	311,135		107,218
Oil and gas properties, full cost method:
Proved, net	634,697		387,554
Unevaluated	254,757		246,269
Other property and equipment, net	22,115		11,172
Other assets, net	41,679		20,873
Investment in subsidiary	736,331		733,533
Total assets	2,000,714		1,506,619
Current liabilities:
Accounts payable to vendors	74,503		78,170
Undistributed oil and gas proceeds	21,841		26,036
Accrued interest	18,546		14,059
Other current liabilities	16,765		22,974
Total current liabilities	131,655		141,239
Long-term debt	914,126		620,000
Deferred taxes	47,758	[1]	56,970	[1]
Asset retirement obligations	5,479		7,626
Other long-term liabilities	29,563		12,955
Total liabilities	1,128,581		838,790
Commitments and contingencies
Stockholders' equity:
Common stock	484		481
Treasury stock	(860)		(860)
Additional paid-in capital	1,386,475		1,338,565
Accumulated earnings (deficit)	(542,799)		(692,225)
Accumulated other comprehensive income (loss)	28,833		21,868
Total stockholders' equity	872,133		667,829
Total liabilities and stockholders' equity	2,000,714		1,506,619
Guarantor Subsidiary
Current assets:
Cash and cash equivalents	51,128		926		1,659	3,963
Accounts receivable	108,075		81,452
Fair value of hedging contracts	39,655		25,177
Deferred taxes	9,567	[1]	17,803	[1]
Inventory	283		283
Total current assets	208,708		125,641
Oil and gas properties, full cost method:
Proved, net	1,099,603		1,083,192
Unevaluated	193,038		155,340
Fair value of hedging contracts	9,199		22,543
Other assets, net	1,500		2,896
Investment in subsidiary			(273)
Total assets	1,512,048		1,389,339
Current liabilities:
Accounts payable to vendors	19,858		25,866
Undistributed oil and gas proceeds	1,573		1,292
Fair value of hedging contracts	149		11,122
Asset retirement obligations	66,260		62,676
Other current liabilities			5,396
Total current liabilities	87,840		106,352
Deferred taxes	263,072	[1]	190,865	[1]
Asset retirement obligations	416,563		351,061
Fair value of hedging contracts	1,530		815
Other long-term liabilities	6,712		6,713
Total liabilities	775,717		655,806
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,496,510		1,724,232
Accumulated earnings (deficit)	(789,012)		(1,012,567)
Accumulated other comprehensive income (loss)	28,833		21,868
Total stockholders' equity	736,331		733,533
Total liabilities and stockholders' equity	1,512,048		1,389,339
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents			136		182	500
Accounts receivable			1,353
Total current assets			1,489
Oil and gas properties, full cost method:
Proved, net			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			(1,129)
Total current assets			(1,129)
Oil and gas properties, full cost method:
Investment in subsidiary	(736,331)		(733,260)
Total assets	(736,331)		(734,389)
Current liabilities:
Accounts payable to vendors			(1,129)
Total current liabilities			(1,129)
Total liabilities			(1,129)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,496,510)		(1,725,871)
Accumulated earnings (deficit)	789,012		1,014,479
Accumulated other comprehensive income (loss)	(28,833)		(21,868)
Total stockholders' equity	(736,331)		(733,260)
Total liabilities and stockholders' equity	$ (736,331)		$ (734,389)

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