Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 74,474		$ 74,488	$ 180,307		$ 331,224
Restricted cash	0		177,647
Accounts receivable	57,859		120,359
Fair value of derivative contracts	65,700		139,179
Current income tax receivable	0		7,212
Inventory	3,709		3,709
Other current assets	9,203		8,118
Total current assets	210,945		530,712
Oil and gas properties, full cost method:
Proved	9,204,693		8,817,268
Less: accumulated DD&A	(8,199,973)		(6,970,631)
Net proved oil and gas properties	1,004,720		1,846,637
Unevaluated	518,963		567,365
Other property and equipment, net	30,327		32,340
Fair value of derivative contracts	5,734		14,333
Other assets, net	25,623		27,224
Investment in subsidiary	0
Total assets	1,796,312		3,018,611
Current liabilities:
Accounts payable to vendors	66,054		132,629
Undistributed oil and gas proceeds	10,461		23,232
Accrued interest	22,241		9,022
Deferred taxes	0		20,119
Asset retirement obligations	42,624		69,400
Other current liabilities	42,134		49,505
Total current liabilities	183,514		303,907
Long-term debt	1,052,183		1,041,035
Deferred taxes	0		286,343
Asset retirement obligations	243,567		247,009
Fair value of derivative contracts	172		0
Other long-term liabilities	25,347		38,714
Total liabilities	$ 1,504,783		$ 1,917,008
Commitments and contingencies
Stockholders’ equity:
Common stock	$ 553		$ 549
Treasury stock	(860)		(860)
Additional paid-in capital	1,643,746		1,633,307
Accumulated deficit	(1,386,967)		(614,708)
Accumulated other comprehensive income (loss)	35,057	$ 40,700	83,315	11,200	$ (18,200)	(2,100)
Total stockholders’ equity	291,529		1,101,603
Total liabilities and stockholders’ equity	1,796,312		3,018,611
Parent [Member]
Current assets:
Cash and cash equivalents	63,322		72,886	125,740		246,294
Restricted cash			177,647
Accounts receivable	25,445		73,711
Fair value of derivative contracts	0
Current income tax receivable			7,212
Deferred taxes			4,095
Inventory	3,426		1,011
Other current assets	9,162		8,112
Total current assets	101,355		344,674
Oil and gas properties, full cost method:
Proved	1,821,870		1,689,802
Less: accumulated DD&A	(2,046,465)		(970,387)
Net proved oil and gas properties	(224,595)		719,415
Unevaluated	301,501		289,556
Other property and equipment, net	30,327		32,340
Fair value of derivative contracts	0
Other assets, net	24,217		20,857
Investment in subsidiary	1,280,184		1,050,546
Total assets	1,512,989		2,457,388
Current liabilities:
Accounts payable to vendors	66,623		74,756
Undistributed oil and gas proceeds	9,644		22,158
Accrued interest	22,241		9,022
Asset retirement obligations	0
Other current liabilities	41,575		49,306
Total current liabilities	140,083		155,242
Long-term debt	1,052,183		1,041,035
Deferred taxes			117,206
Asset retirement obligations	3,847		3,588
Fair value of derivative contracts	0
Other long-term liabilities	25,347		38,714
Total liabilities	$ 1,221,460		$ 1,355,785
Commitments and contingencies
Stockholders’ equity:
Common stock	$ 553		$ 549
Treasury stock	(860)		(860)
Additional paid-in capital	1,643,746		1,633,307
Accumulated deficit	(1,386,967)		(614,708)
Accumulated other comprehensive income (loss)	35,057		83,315
Total stockholders’ equity	291,529		1,101,603
Total liabilities and stockholders’ equity	1,512,989		2,457,388
Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	10,042		1,450	54,440		84,290
Accounts receivable	88,193		46,615
Fair value of derivative contracts	65,700		139,179
Inventory	283		2,698
Other current assets	0
Total current assets	164,218		189,942
Oil and gas properties, full cost method:
Proved	7,341,224		7,127,466
Less: accumulated DD&A	(6,111,909)		(6,000,244)
Net proved oil and gas properties	1,229,315		1,127,222
Unevaluated	215,085		241,230
Other property and equipment, net	0
Fair value of derivative contracts	5,734		14,333
Other assets, net	1,191		1,360
Investment in subsidiary	0
Total assets	1,615,543		1,574,087
Current liabilities:
Accounts payable to vendors	45,487		57,873
Undistributed oil and gas proceeds	817		1,074
Accrued interest	0
Deferred taxes			24,214
Asset retirement obligations	42,624		69,400
Other current liabilities	559		199
Total current liabilities	89,487		152,760
Long-term debt	0
Deferred taxes			169,137
Asset retirement obligations	239,720		243,421
Fair value of derivative contracts	172
Other long-term liabilities	0
Total liabilities	$ 329,379		$ 565,318
Commitments and contingencies
Stockholders’ equity:
Common stock	$ 0
Treasury stock	0
Additional paid-in capital	1,367,434		$ 1,362,684
Accumulated deficit	(122,362)		(440,699)
Accumulated other comprehensive income (loss)	41,092		86,784
Total stockholders’ equity	1,286,164		1,008,769
Total liabilities and stockholders’ equity	1,615,543		1,574,087
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	1,110		152	$ 127		$ 640
Accounts receivable	13		33
Fair value of derivative contracts	0
Inventory	0
Other current assets	41		6
Total current assets	1,164		191
Oil and gas properties, full cost method:
Proved	41,599
Less: accumulated DD&A	(41,599)
Net proved oil and gas properties	0
Unevaluated	2,377		36,579
Other property and equipment, net	0
Fair value of derivative contracts	0
Other assets, net	215		5,007
Investment in subsidiary	3,590		41,638
Total assets	7,346		$ 83,415
Current liabilities:
Accounts payable to vendors	9,736
Undistributed oil and gas proceeds	0
Accrued interest	0
Asset retirement obligations	0
Other current liabilities	0
Total current liabilities	9,736
Long-term debt	0
Asset retirement obligations	0
Fair value of derivative contracts	0
Other long-term liabilities	0
Total liabilities	$ 9,736
Commitments and contingencies
Stockholders’ equity:
Common stock	$ 0
Treasury stock	0
Additional paid-in capital	100,047		$ 90,339
Accumulated deficit	(90,368)		12
Accumulated other comprehensive income (loss)	(12,069)		(6,936)
Total stockholders’ equity	(2,390)		83,415
Total liabilities and stockholders’ equity	7,346		83,415
Eliminations [Member]
Current assets:
Cash and cash equivalents	0
Accounts receivable	(55,792)
Fair value of derivative contracts	0
Deferred taxes			(4,095)
Inventory	0
Other current assets	0
Total current assets	(55,792)		(4,095)
Oil and gas properties, full cost method:
Proved	0
Less: accumulated DD&A	0
Net proved oil and gas properties	0
Unevaluated	0
Other property and equipment, net	0
Fair value of derivative contracts	0
Other assets, net	0
Investment in subsidiary	(1,283,774)		(1,092,184)
Total assets	(1,339,566)		(1,096,279)
Current liabilities:
Accounts payable to vendors	(55,792)
Undistributed oil and gas proceeds	0
Accrued interest	0
Deferred taxes			(4,095)
Asset retirement obligations	0
Other current liabilities	0
Total current liabilities	(55,792)		(4,095)
Long-term debt	0
Asset retirement obligations	0
Fair value of derivative contracts	0
Other long-term liabilities	0
Total liabilities	$ (55,792)		$ (4,095)
Commitments and contingencies
Stockholders’ equity:
Common stock	$ 0
Treasury stock	0
Additional paid-in capital	(1,467,481)		$ (1,453,023)
Accumulated deficit	212,730		440,687
Accumulated other comprehensive income (loss)	(29,023)		(79,848)
Total stockholders’ equity	(1,283,774)		(1,092,184)
Total liabilities and stockholders’ equity	$ (1,339,566)		$ (1,096,279)