Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 162,128		$ 74,488		$ 202,761	$ 331,224
Restricted cash			177,647
Accounts receivable	112,150		120,359
Fair value of derivative contracts	124,192		139,179
Current income tax receivable	24		7,212
Inventory	3,709		3,709
Other current assets	6,234		8,118
Total current assets	408,437		530,712
Oil and gas properties, full cost method:
Proved	8,887,832		8,817,268
Less: accumulated DD&A	(7,546,222)		(6,970,631)
Net proved oil and gas properties	1,341,610		1,846,637
Unevaluated	602,467		567,365
Other property and equipment, net	31,828		32,340
Fair value of derivative contracts	13,966		14,333
Other assets, net	24,672		27,224
Total assets	2,422,980		3,018,611
Current liabilities:
Accounts payable to vendors	68,146		132,629
Undistributed oil and gas proceeds	26,202		23,232
Accrued interest	22,243		9,022
Deferred taxes	23,378	[1]	20,119	[1]
Asset retirement obligations	60,837		69,400
Other current liabilities	40,286		49,505
Total current liabilities	241,092		303,907
Long-term debt	1,044,675		1,041,035
Deferred taxes	98,194	[1]	286,343	[1]
Asset retirement obligations	244,835		247,009
Other long-term liabilities	31,604		38,714
Total liabilities	1,660,400		1,917,008
Commitments and contingencies
Stockholders' equity:
Common stock	553		549
Treasury stock	(860)		(860)
Additional paid-in capital	1,634,171		1,633,307
Accumulated earnings (deficit)	(942,096)		(614,708)
Accumulated other comprehensive income (loss)	70,812		83,315		(9,200)	(2,100)
Total stockholders' equity	762,580		1,101,603
Total liabilities and stockholders' equity	2,422,980		3,018,611
Parent [Member]
Current assets:
Cash and cash equivalents	131,936		72,886		148,239	246,294
Restricted cash			177,647
Accounts receivable	102,556		73,711
Current income tax receivable	24		7,212
Deferred taxes			4,095	[1]
Inventory	3,426		1,011
Other current assets	6,230		8,112
Total current assets	244,172		344,674
Oil and gas properties, full cost method:
Proved	1,716,567		1,689,802
Less: accumulated DD&A	(1,502,385)		(970,387)
Net proved oil and gas properties	214,182		719,415
Unevaluated	297,075		289,556
Other property and equipment, net	31,828		32,340
Other assets, net	21,692		20,857
Investment in subsidiary	1,068,365		1,050,546
Total assets	1,877,314		2,457,388
Current liabilities:
Accounts payable to vendors	30,986		74,756
Undistributed oil and gas proceeds	25,188		22,158
Accrued interest	22,243		9,022
Deferred taxes	335	[1]
Other current liabilities	39,483		49,306
Total current liabilities	118,235		155,242
Long-term debt	1,044,675		1,041,035
Deferred taxes	(83,458)	[1]	117,206	[1]
Asset retirement obligations	3,678		3,588
Other long-term liabilities	31,604		38,714
Total liabilities	1,114,734		1,355,785
Commitments and contingencies
Stockholders' equity:
Common stock	553		549
Treasury stock	(860)		(860)
Additional paid-in capital	1,634,171		1,633,307
Accumulated earnings (deficit)	(942,096)		(614,708)
Accumulated other comprehensive income (loss)	70,812		83,315
Total stockholders' equity	762,580		1,101,603
Total liabilities and stockholders' equity	1,877,314		2,457,388
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	30,015		1,450		54,347	84,290
Accounts receivable	42,014		46,615
Fair value of derivative contracts	124,192		139,179
Inventory	283		2,698
Total current assets	196,504		189,942
Oil and gas properties, full cost method:
Proved	7,171,265		7,127,466
Less: accumulated DD&A	(6,043,837)		(6,000,244)
Net proved oil and gas properties	1,127,428		1,127,222
Unevaluated	261,172		241,230
Fair value of derivative contracts	13,966		14,333
Other assets, net	1,080		1,360
Total assets	1,600,150		1,574,087
Current liabilities:
Accounts payable to vendors	61,410		57,873
Undistributed oil and gas proceeds	1,014		1,074
Deferred taxes	23,043	[1]	24,214	[1]
Asset retirement obligations	60,837		69,400
Other current liabilities	803		199
Total current liabilities	147,107		152,760
Deferred taxes	181,652	[1]	169,137	[1]
Asset retirement obligations	241,157		243,421
Total liabilities	569,916		565,318
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,363,981		1,362,684
Accumulated earnings (deficit)	(411,672)		(440,699)
Accumulated other comprehensive income (loss)	77,925		86,784
Total stockholders' equity	1,030,234		1,008,769
Total liabilities and stockholders' equity	1,600,150		1,574,087
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	177		152		175	640
Accounts receivable	30		33
Other current assets	4		6
Total current assets	211		191
Oil and gas properties, full cost method:
Unevaluated	44,220		36,579
Other assets, net	1,900		5,007
Investment in subsidiary	46,161		41,638
Total assets	92,492		83,415
Current liabilities:
Accounts payable to vendors	8,200
Total current liabilities	8,200
Total liabilities	8,200
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	98,507		90,339
Accumulated earnings (deficit)	12		12
Accumulated other comprehensive income (loss)			(6,936)
Total stockholders' equity	84,292		83,415
Total liabilities and stockholders' equity	92,492		83,415
Eliminations [Member]
Current assets:
Accounts receivable	(32,450)
Deferred taxes			(4,095)	[1]
Total current assets	(32,450)		(4,095)
Oil and gas properties, full cost method:
Investment in subsidiary	(1,114,526)		(1,092,184)
Total assets	(1,146,976)		(1,096,279)
Current liabilities:
Accounts payable to vendors	(32,450)
Deferred taxes			(4,095)	[1]
Total current liabilities	(32,450)		(4,095)
Total liabilities	(32,450)		(4,095)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,462,488)		(1,453,023)
Accumulated earnings (deficit)	411,660		440,687
Accumulated other comprehensive income (loss)	(63,698)		(79,848)
Total stockholders' equity	(1,114,526)		(1,092,184)
Total liabilities and stockholders' equity	$ (1,146,976)		$ (1,096,279)

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