Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 331,224		$ 279,526		$ 38,451	$ 106,956
Accounts receivable	171,971		167,288
Fair value of hedging contracts	4,549		39,655
Current income tax receivable	7,366		10,027
Deferred taxes	31,710	[1]	15,514	[1]
Inventory	3,723		4,207
Other current assets	1,874		3,626
Total current assets	552,417		519,843
Oil and gas properties, full cost method:
Proved	7,804,117		7,244,466
Less: accumulated DD&A	(5,908,760)		(5,510,166)
Net proved oil and gas properties	1,895,357		1,734,300
Unevaluated	724,339		447,795
Other property and equipment, net	26,178		22,115
Fair value of hedging contracts	1,378		9,199
Other assets, net	48,887		43,179
Total assets	3,248,556		2,776,431
Current liabilities:
Accounts payable to vendors	195,677		94,361
Undistributed oil and gas proceeds	37,029		23,414
Accrued interest	9,022		18,546
Fair value of hedging contracts	7,753		149
Asset retirement obligations	67,161		66,260
Other current liabilities	54,520		16,765
Total current liabilities	371,162		219,495
Long-term debt	1,027,084		914,126
Deferred taxes	390,693	[1]	310,830	[1]
Asset retirement obligations	435,352		422,042
Fair value of hedging contracts	470		1,530
Other long-term liabilities	53,509		36,275
Total liabilities	2,278,270		1,904,298
Commitments and contingencies
Stockholders' equity:
Common stock	488		484
Treasury stock	(860)		(860)
Additional paid-in capital	1,397,885		1,386,475
Accumulated deficit	(425,165)		(542,799)
Accumulated other comprehensive income	(2,062)		28,833
Total stockholders' equity	970,286		872,133		667,829	430,357
Total liabilities and stockholders' equity	3,248,556		2,776,431
Parent [Member]
Current assets:
Cash and cash equivalents	246,294		228,398		37,389	105,115
Accounts receivable	74,887		59,213
Current income tax receivable	7,366		10,027
Deferred taxes	8,659	[1]	5,947	[1]
Inventory	3,440		3,924
Other current assets	1,874		3,626
Total current assets	342,520		311,135
Oil and gas properties, full cost method:
Proved	1,309,527		1,004,808
Less: accumulated DD&A	(459,932)		(370,111)
Net proved oil and gas properties	849,595		634,697
Unevaluated	325,113		254,757
Other property and equipment, net	26,178		22,115
Other assets, net	45,410		41,679
Investment in subsidiary	747,472		736,331
Total assets	2,336,288		2,000,714
Current liabilities:
Accounts payable to vendors	173,147		74,503
Undistributed oil and gas proceeds	34,386		21,841
Accrued interest	9,022		18,546
Other current liabilities	53,682		16,765
Total current liabilities	270,237		131,655
Long-term debt	1,027,084		914,126
Deferred taxes	10,227	[1]	47,758	[1]
Asset retirement obligations	4,945		5,479
Other long-term liabilities	53,509		29,563
Total liabilities	1,366,002		1,128,581
Commitments and contingencies
Stockholders' equity:
Common stock	488		484
Treasury stock	(860)		(860)
Additional paid-in capital	1,397,885		1,386,475
Accumulated deficit	(425,165)		(542,799)
Accumulated other comprehensive income	(2,062)		28,833
Total stockholders' equity	970,286		872,133
Total liabilities and stockholders' equity	2,336,288		2,000,714
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	84,290		51,128		926	1,659
Accounts receivable	97,128		108,075
Fair value of hedging contracts	4,549		39,655
Deferred taxes	23,051	[1]	9,567	[1]
Inventory	283		283
Total current assets	209,301		208,708
Oil and gas properties, full cost method:
Proved	6,494,590		6,239,658
Less: accumulated DD&A	(5,448,828)		(5,140,055)
Net proved oil and gas properties	1,045,762		1,099,603
Unevaluated	388,643		193,038
Fair value of hedging contracts	1,378		9,199
Other assets, net	1,349		1,500
Total assets	1,646,433		1,512,048
Current liabilities:
Accounts payable to vendors	22,530		19,858
Undistributed oil and gas proceeds	2,643		1,573
Fair value of hedging contracts	7,753		149
Asset retirement obligations	67,161		66,260
Other current liabilities	838
Total current liabilities	100,925		87,840
Deferred taxes	380,466	[1]	263,072	[1]
Asset retirement obligations	430,407		416,563
Fair value of hedging contracts	470		1,530
Other long-term liabilities			6,712
Total liabilities	912,268		775,717
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,309,563		1,496,510
Accumulated deficit	(574,003)		(789,012)
Accumulated other comprehensive income	(1,395)		28,833
Total stockholders' equity	734,165		736,331
Total liabilities and stockholders' equity	1,646,433		1,512,048
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	640				136	182
Total current assets	640
Oil and gas properties, full cost method:
Unevaluated	10,583
Other assets, net	2,128
Investment in subsidiary	12,711
Total assets	26,062
Current liabilities:
Accounts payable to vendors	44
Total current liabilities	44
Total liabilities	44
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	27,403
Accumulated deficit	(52)
Accumulated other comprehensive income	(1,333)
Total stockholders' equity	26,018
Total liabilities and stockholders' equity	26,062
Consolidation, Eliminations [Member]
Current assets:
Accounts receivable	(44)
Total current assets	(44)
Oil and gas properties, full cost method:
Investment in subsidiary	(760,183)		(736,331)
Total assets	(760,227)		(736,331)
Current liabilities:
Accounts payable to vendors	(44)
Total current liabilities	(44)
Total liabilities	(44)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,336,966)		(1,496,510)
Accumulated deficit	574,055		789,012
Accumulated other comprehensive income	2,728		(28,833)
Total stockholders' equity	(760,183)		(736,331)
Total liabilities and stockholders' equity	$ (760,227)		$ (736,331)

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