Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Mar. 31, 2013	Dec. 31, 2012		Jun. 30, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 226,372			$ 279,526		$ 223,849	$ 38,451
Accounts receivable	166,150			167,288
Fair value of hedging contracts	30,053			39,655
Current income tax receivable	26,530			10,027
Deferred taxes	15,653	[1]		15,514	[1]
Inventory	4,006			4,207
Other current assets	3,872			3,626
Total current assets	472,636			519,843
Oil and gas properties, full cost method:
Proved	7,478,481			7,244,466
Less: accumulated DD&A	(5,671,005)			(5,510,166)
Net proved oil and gas properties	1,807,476			1,734,300
Unevaluated	534,979			447,795
Other property and equipment, net	22,517			22,115
Fair value of hedging contracts	13,221			9,199
Other assets, net	55,066			43,179
Total assets	2,905,895			2,776,431
Current liabilities:
Accounts payable to vendors	93,965			94,361
Undistributed oil and gas proceeds	27,280			23,414
Accrued interest	18,059			18,546
Fair value of hedging contracts	1,880			149
Asset retirement obligations	63,646			66,260
Other current liabilities	9,705			16,765
Total current liabilities	214,535			219,495
Long-term debt	920,485			914,126
Deferred taxes	366,077	[1]		310,830	[1]
Asset retirement obligations	420,417			422,042
Fair value of hedging contracts	436			1,530
Other long-term liabilities	32,909			36,275
Total liabilities	1,954,859			1,904,298
Commitments and contingencies
Stockholders' equity:
Common stock	487			484
Treasury stock	(860)			(860)
Additional paid-in capital	1,389,898			1,386,475
Accumulated deficit	(463,019)			(542,799)
Accumulated other comprehensive income	24,530		10,500	28,833
Total stockholders' equity	951,036			872,133
Total liabilities and stockholders' equity	2,905,895			2,776,431
Parent
Current assets:
Cash and cash equivalents	174,530			228,398		192,755	37,389
Accounts receivable	76,622			59,213
Current income tax receivable	26,530			10,027
Deferred taxes	2,947	[1]		5,947	[1]
Inventory	3,723			3,924
Other current assets	3,872			3,626
Total current assets	288,224			311,135
Oil and gas properties, full cost method:
Proved	1,132,623			1,004,808
Less: accumulated DD&A	(403,585)			(370,111)
Net proved oil and gas properties	729,038			634,697
Unevaluated	290,344			254,757
Other property and equipment, net	22,517			22,115
Other assets, net	50,056			41,679
Investment in subsidiary	858,749			736,331
Total assets	2,238,928			2,000,714
Current liabilities:
Accounts payable to vendors	245,554			74,503
Undistributed oil and gas proceeds	24,378			21,841
Accrued interest	18,059			18,546
Other current liabilities	8,945			16,765
Total current liabilities	296,936			131,655
Long-term debt	920,485			914,126
Deferred taxes	38,609	[1]		47,758	[1]
Asset retirement obligations	5,665			5,479
Other long-term liabilities	26,197			29,563
Total liabilities	1,287,892			1,128,581
Commitments and contingencies
Stockholders' equity:
Common stock	487			484
Treasury stock	(860)			(860)
Additional paid-in capital	1,389,898			1,386,475
Accumulated deficit	(463,019)			(542,799)
Accumulated other comprehensive income	24,530			28,833
Total stockholders' equity	951,036			872,133
Total liabilities and stockholders' equity	2,238,928			2,000,714
Guarantor Subsidiary
Current assets:
Cash and cash equivalents	51,202			51,128		30,958	926
Accounts receivable	260,432			108,075
Fair value of hedging contracts	30,053			39,655
Deferred taxes	12,706	[1]		9,567	[1]
Inventory	283			283
Total current assets	354,676			208,708
Oil and gas properties, full cost method:
Proved	6,345,858			6,239,658
Less: accumulated DD&A	(5,267,420)			(5,140,055)
Net proved oil and gas properties	1,078,438			1,099,603
Unevaluated	235,187			193,038
Fair value of hedging contracts	13,221			9,199
Other assets, net	1,584			1,500
Total assets	1,683,106			1,512,048
Current liabilities:
Accounts payable to vendors	19,238			19,858
Undistributed oil and gas proceeds	2,902			1,573
Fair value of hedging contracts	1,880			149
Asset retirement obligations	63,646			66,260
Other current liabilities	760
Total current liabilities	88,426			87,840
Deferred taxes	327,468	[1]		263,072	[1]
Asset retirement obligations	414,752			416,563
Fair value of hedging contracts	436			1,530
Other long-term liabilities	6,712			6,712
Total liabilities	837,794			775,717
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,496,509			1,496,510
Accumulated deficit	(676,250)			(789,012)
Accumulated other comprehensive income	25,053			28,833
Total stockholders' equity	845,312			736,331
Total liabilities and stockholders' equity	1,683,106			1,512,048
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	640					136	136
Total current assets	640
Oil and gas properties, full cost method:
Unevaluated	9,448
Other assets, net	3,426
Investment in subsidiary	12,841
Total assets	26,355
Current liabilities:
Accounts payable to vendors	77
Total current liabilities	77
Total liabilities	77
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	27,404
Accumulated deficit	(80)
Accumulated other comprehensive income	(1,046)
Total stockholders' equity	26,278
Total liabilities and stockholders' equity	26,355
Eliminations
Current assets:
Accounts receivable	(170,904)
Total current assets	(170,904)
Oil and gas properties, full cost method:
Investment in subsidiary	(871,590)			(736,331)
Total assets	(1,042,494)			(736,331)
Current liabilities:
Accounts payable to vendors	(170,904)
Total current liabilities	(170,904)
Total liabilities	(170,904)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,523,913)			(1,496,510)
Accumulated deficit	676,330			789,012
Accumulated other comprehensive income	(24,007)			(28,833)
Total stockholders' equity	(871,590)			(736,331)
Total liabilities and stockholders' equity	$ (1,042,494)			$ (736,331)

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