Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 261,353		$ 279,526		$ 263,841	$ 38,451
Accounts receivable	147,336		167,288
Fair value of hedging contracts	18,712		39,655
Current income tax receivable	19,533		10,027
Deferred taxes	23,354	[1]	15,514	[1]
Inventory	4,027		4,207
Other current assets	3,587		3,626
Total current assets	477,902		519,843
Oil and gas properties, full cost method:
Proved	7,358,370		7,244,466
Less: accumulated DD&A	(5,584,698)		(5,510,166)
Net proved oil and gas properties	1,773,672		1,734,300
Unevaluated	471,457		447,795
Other property and equipment, net	22,059		22,115
Fair value of hedging contracts	6,792		9,199
Other assets, net	50,512		43,179
Investment in subsidiary
Total assets	2,802,394		2,776,431
Current liabilities:
Accounts payable to vendors	81,491		94,361
Undistributed oil and gas proceeds	28,904		23,414
Accrued interest	15,351		18,546
Fair value of hedging contracts	5,376		149
Asset retirement obligations	68,978		66,260
Other current liabilities	5,840		16,765
Total current liabilities	205,940		219,495
Long-term debt	917,267		914,126
Deferred taxes	336,108	[1]	310,830	[1]
Asset retirement obligations	412,706		422,042
Fair value of hedging contracts	2,997		1,530
Other long-term liabilities	33,853		36,275
Total liabilities	1,908,871		1,904,298
Commitments and contingencies
Stockholders' equity:
Common stock	487		484
Treasury stock	(860)		(860)
Additional paid-in capital	1,385,445		1,386,475
Accumulated deficit	(502,041)		(542,799)
Accumulated other comprehensive income	10,492		28,833
Total stockholders' equity	893,523		872,133
Total liabilities and stockholders' equity	2,802,394		2,776,431
Parent
Current assets:
Cash and cash equivalents	210,189		228,398		222,750	37,389
Accounts receivable	48,545		59,213
Fair value of hedging contracts
Current income tax receivable	19,533		10,027
Deferred taxes	3,387	[1]	5,947	[1]
Inventory	3,744		3,924
Other current assets	3,587		3,626
Total current assets	288,985		311,135
Oil and gas properties, full cost method:
Proved	1,078,367		1,004,808
Less: accumulated DD&A	(379,399)		(370,111)
Net proved oil and gas properties	698,968		634,697
Unevaluated	270,373		254,757
Other property and equipment, net	22,059		22,115
Fair value of hedging contracts
Other assets, net	48,951		41,679
Investment in subsidiary	774,817		736,331
Total assets	2,104,153		2,000,714
Current liabilities:
Accounts payable to vendors	182,210		74,503
Undistributed oil and gas proceeds	16,580		21,841
Accrued interest	15,351		18,546
Fair value of hedging contracts
Asset retirement obligations
Other current liabilities	5,522		16,765
Total current liabilities	219,663		131,655
Long-term debt	917,267		914,126
Deferred taxes	40,988	[1]	47,758	[1]
Asset retirement obligations	5,572		5,479
Fair value of hedging contracts
Other long-term liabilities	27,140		29,563
Total liabilities	1,210,630		1,128,581
Commitments and contingencies
Stockholders' equity:
Common stock	487		484
Treasury stock	(860)		(860)
Additional paid-in capital	1,385,445		1,386,475
Accumulated deficit	(502,041)		(542,799)
Accumulated other comprehensive income	10,492		28,833
Total stockholders' equity	893,523		872,133
Total liabilities and stockholders' equity	2,104,153		2,000,714
Guarantor Subsidiary
Current assets:
Cash and cash equivalents	51,164		51,128		40,955	926
Accounts receivable	221,703		108,075
Fair value of hedging contracts	18,712		39,655
Current income tax receivable
Deferred taxes	19,967	[1]	9,567	[1]
Inventory	283		283
Other current assets
Total current assets	311,829		208,708
Oil and gas properties, full cost method:
Proved	6,280,003		6,239,658
Less: accumulated DD&A	(5,205,299)		(5,140,055)
Net proved oil and gas properties	1,074,704		1,099,603
Unevaluated	201,084		193,038
Other property and equipment, net
Fair value of hedging contracts	6,792		9,199
Other assets, net	1,561		1,500
Investment in subsidiary
Total assets	1,595,970		1,512,048
Current liabilities:
Accounts payable to vendors	22,193		19,858
Undistributed oil and gas proceeds	12,324		1,573
Accrued interest
Fair value of hedging contracts	5,376		149
Asset retirement obligations	68,978		66,260
Other current liabilities	318
Total current liabilities	109,189		87,840
Long-term debt
Deferred taxes	295,120	[1]	263,072	[1]
Asset retirement obligations	407,134		416,563
Fair value of hedging contracts	2,997		1,530
Other long-term liabilities	6,713		6,712
Total liabilities	821,153		775,717
Commitments and contingencies
Stockholders' equity:
Common stock
Treasury stock
Additional paid-in capital	1,496,509		1,496,510
Accumulated deficit	(732,184)		(789,012)
Accumulated other comprehensive income	10,492		28,833
Total stockholders' equity	774,817		736,331
Total liabilities and stockholders' equity	1,595,970		1,512,048
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents					136	136
Accounts receivable
Fair value of hedging contracts
Current income tax receivable
Deferred taxes		[1]
Inventory
Other current assets
Total current assets
Oil and gas properties, full cost method:
Proved
Less: accumulated DD&A
Net proved oil and gas properties
Unevaluated
Other property and equipment, net
Fair value of hedging contracts
Other assets, net
Investment in subsidiary
Total assets
Current liabilities:
Accounts payable to vendors
Undistributed oil and gas proceeds
Accrued interest
Fair value of hedging contracts
Asset retirement obligations
Other current liabilities
Total current liabilities
Long-term debt
Deferred taxes		[1]		[1]
Asset retirement obligations
Fair value of hedging contracts
Other long-term liabilities
Total liabilities
Commitments and contingencies
Stockholders' equity:
Common stock
Treasury stock
Additional paid-in capital
Accumulated deficit
Accumulated other comprehensive income
Total stockholders' equity
Total liabilities and stockholders' equity
Eliminations
Current assets:
Cash and cash equivalents
Accounts receivable	(122,912)
Fair value of hedging contracts
Current income tax receivable
Deferred taxes		[1]
Inventory
Other current assets
Total current assets	(122,912)
Oil and gas properties, full cost method:
Proved
Less: accumulated DD&A
Net proved oil and gas properties
Unevaluated
Other property and equipment, net
Fair value of hedging contracts
Other assets, net
Investment in subsidiary	(774,817)		(736,331)
Total assets	(897,729)		(736,331)
Current liabilities:
Accounts payable to vendors	(122,912)
Undistributed oil and gas proceeds
Accrued interest
Fair value of hedging contracts
Asset retirement obligations
Other current liabilities
Total current liabilities	(122,912)
Long-term debt
Deferred taxes		[1]		[1]
Asset retirement obligations
Fair value of hedging contracts
Other long-term liabilities
Total liabilities	(122,912)
Commitments and contingencies
Stockholders' equity:
Common stock
Treasury stock
Additional paid-in capital	(1,496,509)		(1,496,510)
Accumulated deficit	732,184		789,012
Accumulated other comprehensive income	(10,492)		(28,833)
Total stockholders' equity	(774,817)		(736,331)
Total liabilities and stockholders' equity	$ (897,729)		$ (736,331)

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