Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012		Sep. 30, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 243,075			$ 279,526		$ 176,186	$ 38,451
Accounts receivable	189,565			167,288
Fair value of hedging contracts	14,423			39,655
Current income tax receivable	10,893			10,027
Deferred taxes	26,350	[1]		15,514	[1]
Inventory	3,843			4,207
Other current assets	1,439			3,626
Total current assets	489,588			519,843
Oil and gas properties, full cost method: Proved	7,589,153			7,244,466
Less: accumulated DD&A	(5,762,937)			(5,510,166)
Net proved oil and gas properties	1,826,216			1,734,300
Unevaluated	576,395			447,795
Other property and equipment, net	23,965			22,115
Fair value of hedging contracts	5,114			9,199
Other assets, net	49,148			43,179
Total assets	2,970,426			2,776,431
Current liabilities:
Accounts payable to vendors	104,486			94,361
Undistributed oil and gas proceeds	42,913			23,414
Accrued interest	14,272			18,546
Fair value of hedging contracts	8,261			149
Asset retirement obligations	60,938			66,260
Other current liabilities	21,680			16,765
Total current liabilities	252,550			219,495
Long-term debt	923,745			914,126
Deferred taxes	387,033	[1]		310,830	[1]
Asset retirement obligations	407,712			422,042
Fair value of hedging contracts	360			1,530
Other long-term liabilities	26,463			36,275
Total liabilities	1,997,863			1,904,298
Commitments and contingencies
Stockholders' equity:
Common stock	488			484
Treasury stock	(860)			(860)
Additional paid-in capital	1,393,439			1,386,475
Accumulated deficit	(426,917)			(542,799)
Accumulated other comprehensive income	6,413		24,500	28,833
Total stockholders' equity	972,563			872,133
Total liabilities and stockholders' equity	2,970,426			2,776,431
Parent [Member]
Current assets:
Cash and cash equivalents	191,193			228,398		155,089	37,389
Accounts receivable	83,310			59,213
Current income tax receivable	10,893			10,027
Deferred taxes	6,631	[1]		5,947	[1]
Inventory	3,560			3,924
Other current assets	1,439			3,626
Total current assets	297,026			311,135
Oil and gas properties, full cost method: Proved	1,211,964			1,004,808
Less: accumulated DD&A	(429,380)			(370,111)
Net proved oil and gas properties	782,584			634,697
Unevaluated	274,986			254,757
Other property and equipment, net	23,965			22,115
Other assets, net	44,324			41,679
Investment in subsidiary	896,798			736,331
Total assets	2,319,683			2,000,714
Current liabilities:
Accounts payable to vendors	289,204			74,503
Undistributed oil and gas proceeds	39,141			21,841
Accrued interest	14,272			18,546
Other current liabilities	17,242			16,765
Total current liabilities	359,859			131,655
Long-term debt	923,745			914,126
Deferred taxes	31,295	[1]		47,758	[1]
Asset retirement obligations	5,758			5,479
Other long-term liabilities	26,463			29,563
Total liabilities	1,347,120			1,128,581
Commitments and contingencies
Stockholders' equity:
Common stock	488			484
Treasury stock	(860)			(860)
Additional paid-in capital	1,393,439			1,386,475
Accumulated deficit	(426,917)			(542,799)
Accumulated other comprehensive income	6,413			28,833
Total stockholders' equity	972,563			872,133
Total liabilities and stockholders' equity	2,319,683			2,000,714
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	51,242			51,128		21,097	926
Accounts receivable	313,610			108,075
Fair value of hedging contracts	14,423			39,655
Deferred taxes	19,719	[1]		9,567	[1]
Inventory	283			283
Total current assets	399,277			208,708
Oil and gas properties, full cost method: Proved	6,377,189			6,239,658
Less: accumulated DD&A	(5,333,557)			(5,140,055)
Net proved oil and gas properties	1,043,632			1,099,603
Unevaluated	291,750			193,038
Fair value of hedging contracts	5,114			9,199
Other assets, net	1,349			1,500
Total assets	1,741,122			1,512,048
Current liabilities:
Accounts payable to vendors	22,576			19,858
Undistributed oil and gas proceeds	3,772			1,573
Fair value of hedging contracts	8,261			149
Asset retirement obligations	60,938			66,260
Other current liabilities	4,438
Total current liabilities	99,985			87,840
Deferred taxes	355,738	[1]		263,072	[1]
Asset retirement obligations	401,954			416,563
Fair value of hedging contracts	360			1,530
Other long-term liabilities				6,712
Total liabilities	858,037			775,717
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,496,509			1,496,510
Accumulated deficit	(620,093)			(789,012)
Accumulated other comprehensive income	6,669			28,833
Total stockholders' equity	883,085			736,331
Total liabilities and stockholders' equity	1,741,122			1,512,048
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	640						136
Total current assets	640
Unevaluated	9,659
Other assets, net	3,475
Investment in subsidiary	13,116
Total assets	26,890
Current liabilities:
Accounts payable to vendors	61
Total current liabilities	61
Total liabilities	61
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	27,404
Accumulated deficit	(64)
Accumulated other comprehensive income	(511)
Total stockholders' equity	26,829
Total liabilities and stockholders' equity	26,890
Eliminations [Member]
Current assets:
Accounts receivable	(207,355)
Total current assets	(207,355)
Investment in subsidiary	(909,914)			(736,331)
Total assets	(1,117,269)			(736,331)
Current liabilities:
Accounts payable to vendors	(207,355)
Total current liabilities	(207,355)
Total liabilities	(207,355)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,523,913)			(1,496,510)
Accumulated deficit	620,157			789,012
Accumulated other comprehensive income	(6,158)			(28,833)
Total stockholders' equity	(909,914)			(736,331)
Total liabilities and stockholders' equity	$ (1,117,269)			$ (736,331)

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