Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Current assets:
Cash and cash equivalents	$ 74,488		$ 331,224		$ 279,526	$ 38,451
Restricted cash	177,647
Accounts receivable	120,359		171,971
Fair value of derivative contracts	139,179		4,549
Current income tax receivable	7,212		7,366
Deferred taxes			31,710	[1]
Inventory	3,709		3,723
Other current assets	8,118		1,874
Total current assets	530,712		552,417
Oil and gas properties, full cost method:
Proved	8,817,268		7,804,117
Less: accumulated DD&A	(6,970,631)		(5,908,760)
Net proved oil and gas properties	1,846,637		1,895,357
Unevaluated	567,365		724,339
Other property and equipment, net	32,340		26,178
Fair value of derivative contracts	14,333		1,378
Other assets, net	27,224		48,887
Total assets	3,018,611		3,248,556
Current liabilities:
Accounts payable to vendors	132,629		195,677
Undistributed oil and gas proceeds	23,232		37,029
Accrued interest	9,022		9,022
Deferred taxes	20,119	[1]
Fair value of derivative contracts			7,753
Asset retirement obligations	69,400		67,161
Other current liabilities	49,505		54,520
Total current liabilities	303,907		371,162
Long-term debt	1,041,035		1,027,084
Deferred taxes	286,343	[1]	390,693	[1]
Asset retirement obligations	247,009		435,352
Fair value of derivative contracts			470
Other long-term liabilities	38,714		53,509
Total liabilities	1,917,008		2,278,270
Commitments and contingencies
Stockholders' equity:
Common stock	549		488
Treasury stock	(860)		(860)
Additional paid-in capital	1,633,307		1,397,885
Accumulated deficit	(614,708)		(425,165)
Accumulated other comprehensive income (loss)	83,315		(2,062)		28,833
Total stockholders' equity	1,101,603		970,286
Total liabilities and stockholders' equity	3,018,611		3,248,556
Parent [Member]
Current assets:
Cash and cash equivalents	72,886		246,294		228,398	37,389
Restricted cash	177,647
Accounts receivable	73,711		74,887
Current income tax receivable	7,212		7,366
Deferred taxes	4,095		8,659	[1]
Inventory	1,011		3,440
Other current assets	8,112		1,874
Total current assets	344,674		342,520
Oil and gas properties, full cost method:
Proved	1,689,802		1,309,527
Less: accumulated DD&A	(970,387)		(459,932)
Net proved oil and gas properties	719,415		849,595
Unevaluated	289,556		325,113
Other property and equipment, net	32,340		26,178
Other assets, net	20,857		45,410
Investment in subsidiary	1,050,546		747,472
Total assets	2,457,388		2,336,288
Current liabilities:
Accounts payable to vendors	74,756		173,147
Undistributed oil and gas proceeds	22,158		34,386
Accrued interest	9,022		9,022
Other current liabilities	49,306		53,682
Total current liabilities	155,242		270,237
Long-term debt	1,041,035		1,027,084
Deferred taxes	117,206	[1]	10,227	[1]
Asset retirement obligations	3,588		4,945
Other long-term liabilities	38,714		53,509
Total liabilities	1,355,785		1,366,002
Commitments and contingencies
Stockholders' equity:
Common stock	549		488
Treasury stock	(860)		(860)
Additional paid-in capital	1,633,307		1,397,885
Accumulated deficit	(614,708)		(425,165)
Accumulated other comprehensive income (loss)	83,315		(2,062)
Total stockholders' equity	1,101,603		970,286
Total liabilities and stockholders' equity	2,457,388		2,336,288
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	1,450		84,290		51,128	926
Accounts receivable	46,615		97,128
Fair value of derivative contracts	139,179		4,549
Deferred taxes			23,051	[1]
Inventory	2,698		283
Total current assets	189,942		209,301
Oil and gas properties, full cost method:
Proved	7,127,466		6,494,590
Less: accumulated DD&A	(6,000,244)		(5,448,828)
Net proved oil and gas properties	1,127,222		1,045,762
Unevaluated	241,230		388,643
Fair value of derivative contracts	14,333		1,378
Other assets, net	1,360		1,349
Total assets	1,574,087		1,646,433
Current liabilities:
Accounts payable to vendors	57,873		22,530
Undistributed oil and gas proceeds	1,074		2,643
Deferred taxes	24,214	[1]
Fair value of derivative contracts			7,753
Asset retirement obligations	69,400		67,161
Other current liabilities	199		838
Total current liabilities	152,760		100,925
Deferred taxes	169,137	[1]	380,466	[1]
Asset retirement obligations	243,421		430,407
Fair value of derivative contracts			470
Total liabilities	565,318		912,268
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,362,684		1,309,563
Accumulated deficit	(440,699)		(574,003)
Accumulated other comprehensive income (loss)	86,784		(1,395)
Total stockholders' equity	1,008,769		734,165
Total liabilities and stockholders' equity	1,574,087		1,646,433
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	152		640			136
Accounts receivable	33
Other current assets	6
Total current assets	191		640
Oil and gas properties, full cost method:
Unevaluated	36,579		10,583
Other assets, net	5,007		2,128
Investment in subsidiary	41,638		12,711
Total assets	83,415		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	90,339		27,403
Accumulated deficit	12		(52)
Accumulated other comprehensive income (loss)	(6,936)		(1,333)
Total stockholders' equity	83,415		26,018
Total liabilities and stockholders' equity	83,415		26,062
Eliminations [Member]
Current assets:
Accounts receivable			(44)
Deferred taxes	(4,095)
Total current assets	(4,095)		(44)
Oil and gas properties, full cost method:
Investment in subsidiary	(1,092,184)		(760,183)
Total assets	(1,096,279)		(760,227)
Current liabilities:
Accounts payable to vendors			(44)
Deferred taxes	(4,095)	[1]
Total current liabilities	(4,095)		(44)
Total liabilities	(4,095)		(44)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,453,023)		(1,336,966)
Accumulated deficit	440,687		574,055
Accumulated other comprehensive income (loss)	(79,848)		2,728
Total stockholders' equity	(1,092,184)		(760,183)
Total liabilities and stockholders' equity	$ (1,096,279)		$ (760,227)

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