Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 333,886			$ 331,224		$ 226,372		$ 279,526
Accounts receivable	197,495			171,971
Fair value of derivative contracts				4,549
Current income tax receivable	7,372			7,366
Deferred taxes	36,308	[1]		31,710	[1]
Inventory	3,723			3,723
Other current assets	1,955			1,874
Total current assets	580,739			552,417
Oil and gas properties, full cost method:
Proved	8,520,195			7,804,117
Less: accumulated DD&A	(6,144,815)			(5,908,760)
Net proved oil and gas properties	2,375,380			1,895,357
Unevaluated	554,201			724,339
Other property and equipment, net	28,302			26,178
Fair value of derivative contracts	415			1,378
Other assets, net	43,015			48,887
Total assets	3,582,052			3,248,556
Current liabilities:
Accounts payable to vendors	193,900			195,677
Undistributed oil and gas proceeds	62,927			37,029
Accrued interest	9,025			9,022
Fair value of derivative contracts	25,711			7,753
Asset retirement obligations	77,132			67,161
Other current liabilities	79,539			54,520
Total current liabilities	448,234			371,162
Long-term debt	1,033,931			1,027,084
Deferred taxes	404,188	[1]		390,693	[1]
Asset retirement obligations	431,060			435,352
Fair value of derivative contracts	5,251			470
Other long-term liabilities	47,843			53,509
Total liabilities	2,370,507			2,278,270
Commitments and contingencies
Stockholders' equity:
Common stock	549			488
Treasury stock	(860)			(860)
Additional paid-in capital	1,624,795			1,397,885
Accumulated deficit	(394,778)			(425,165)
Accumulated other comprehensive loss	(18,161)		(9,200)	(2,062)		24,500	10,500	28,800
Total stockholders' equity	1,211,545			970,286
Total liabilities and stockholders' equity	3,582,052			3,248,556
Parent [Member]
Current assets:
Cash and cash equivalents	244,254			246,294		174,530		228,398
Accounts receivable	153,316			74,887
Current income tax receivable	7,372			7,366
Deferred taxes	1,565	[1]		8,659	[1]
Inventory	3,440			3,440
Other current assets	1,955			1,874
Total current assets	411,902			342,520
Oil and gas properties, full cost method:
Proved	1,475,522			1,309,527
Less: accumulated DD&A	(527,605)			(459,932)
Net proved oil and gas properties	947,917			849,595
Unevaluated	302,396			325,113
Other property and equipment, net	28,302			26,178
Other assets, net	30,734			45,410
Investment in subsidiary	798,664			747,472
Total assets	2,519,915			2,336,288
Current liabilities:
Accounts payable to vendors	93,251			173,147
Undistributed oil and gas proceeds	60,724			34,386
Accrued interest	9,025			9,022
Other current liabilities	76,461			53,682
Total current liabilities	239,461			270,237
Long-term debt	1,033,931			1,027,084
Deferred taxes	(16,629)	[1]		10,227	[1]
Asset retirement obligations	3,764			4,945
Other long-term liabilities	47,843			53,509
Total liabilities	1,308,370			1,366,002
Commitments and contingencies
Stockholders' equity:
Common stock	549			488
Treasury stock	(860)			(860)
Additional paid-in capital	1,624,795			1,397,885
Accumulated deficit	(394,778)			(425,165)
Accumulated other comprehensive loss	(18,161)			(2,062)
Total stockholders' equity	1,211,545			970,286
Total liabilities and stockholders' equity	2,519,915			2,336,288
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	89,388			84,290		51,202		51,128
Accounts receivable	80,436			97,128
Fair value of derivative contracts				4,549
Deferred taxes	34,743	[1]		23,051	[1]
Inventory	283			283
Total current assets	204,850			209,301
Oil and gas properties, full cost method:
Proved	7,044,673			6,494,590
Less: accumulated DD&A	(5,617,210)			(5,448,828)
Net proved oil and gas properties	1,427,463			1,045,762
Unevaluated	237,146			388,643
Fair value of derivative contracts	415			1,378
Other assets, net	1,880			1,349
Total assets	1,871,754			1,646,433
Current liabilities:
Accounts payable to vendors	125,177			22,530
Undistributed oil and gas proceeds	2,203			2,643
Fair value of derivative contracts	25,711			7,753
Asset retirement obligations	77,132			67,161
Other current liabilities	3,078			838
Total current liabilities	233,301			100,925
Deferred taxes	420,817	[1]		380,466	[1]
Asset retirement obligations	427,296			430,407
Fair value of derivative contracts	5,251			470
Total liabilities	1,086,665			912,268
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,309,562			1,309,563
Accumulated deficit	(506,723)			(574,003)
Accumulated other comprehensive loss	(17,750)			(1,395)
Total stockholders' equity	785,089			734,165
Total liabilities and stockholders' equity	1,871,754			1,646,433
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	244			640		640
Total current assets	244			640
Oil and gas properties, full cost method:
Unevaluated	14,659			10,583
Other assets, net	10,401			2,128
Investment in subsidiary	25,156			12,711
Total assets	50,460			26,062
Current liabilities:
Accounts payable to vendors	11,729			44
Total current liabilities	11,729			44
Total liabilities	11,729			44
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	39,580			27,403
Accumulated deficit	(28)			(52)
Accumulated other comprehensive loss	(821)			(1,333)
Total stockholders' equity	38,731			26,018
Total liabilities and stockholders' equity	50,460			26,062
Consolidation, Eliminations [Member]
Current assets:
Accounts receivable	(36,257)			(44)
Total current assets	(36,257)			(44)
Oil and gas properties, full cost method:
Investment in subsidiary	(823,820)			(760,183)
Total assets	(860,077)			(760,227)
Current liabilities:
Accounts payable to vendors	(36,257)			(44)
Total current liabilities	(36,257)			(44)
Total liabilities	(36,257)			(44)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,349,142)			(1,336,966)
Accumulated deficit	506,751			574,055
Accumulated other comprehensive loss	18,571			2,728
Total stockholders' equity	(823,820)			(760,183)
Total liabilities and stockholders' equity	$ (860,077)			$ (760,227)

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