Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 202,761		$ 331,224		$ 261,353	$ 279,526
Accounts receivable	190,573		171,971
Fair value of hedging contracts	745		4,549
Current income tax receivable	7,366		7,366
Deferred taxes	36,098	[1]	31,710	[1]
Inventory	4,651		3,723
Other current assets	1,774		1,874
Total current assets	443,968		552,417
Oil and gas properties, full cost method:
Proved	7,898,668		7,804,117
Less: accumulated DD&A	(6,052,894)		(5,908,760)
Net proved oil and gas properties	1,845,774		1,895,357
Unevaluated	906,043		724,339
Other property and equipment, net	26,975		26,178
Fair value of hedging contracts	1,867		1,378
Other assets, net	37,154		48,887
Investment in subsidiary
Total assets	3,261,781		3,248,556
Current liabilities:
Accounts payable to vendors	165,973		195,677
Undistributed oil and gas proceeds	55,676		37,029
Accrued interest	22,247		9,022
Fair value of hedging contracts	15,277		7,753
Asset retirement obligations	87,927		67,161
Other current liabilities	28,467		54,520
Total current liabilities	375,567		371,162
Long-term debt	1,030,466		1,027,084
Deferred taxes	406,477	[1]	390,693	[1]
Asset retirement obligations	416,171		435,352
Fair value of hedging contracts	376		470
Other long-term liabilities	46,772		53,509
Total liabilities	2,275,829		2,278,270
Commitments and contingencies
Stockholders' equity:
Common stock	491		488
Treasury stock	(860)		(860)
Additional paid-in capital	1,394,694		1,397,885
Accumulated deficit	(399,222)		(425,165)
Accumulated other comprehensive loss	(9,151)		(2,062)
Total stockholders' equity	985,952		970,286
Total liabilities and stockholders' equity	3,261,781		3,248,556
Parent [Member]
Current assets:
Cash and cash equivalents	148,239		246,294		210,189	228,398
Accounts receivable	103,049		74,887
Current income tax receivable	7,366		7,366
Deferred taxes	1,493	[1]	8,659	[1]
Inventory	4,368		3,440
Other current assets	1,774		1,874
Total current assets	266,289		342,520
Oil and gas properties, full cost method:
Proved	1,370,697		1,309,527
Less: accumulated DD&A	(498,058)		(459,932)
Net proved oil and gas properties	872,639		849,595
Unevaluated	318,865		325,113
Other property and equipment, net	26,975		26,178
Other assets, net	31,058		45,410
Investment in subsidiary	777,728		747,472
Total assets	2,293,554		2,336,288
Current liabilities:
Accounts payable to vendors	127,122		173,147
Undistributed oil and gas proceeds	53,140		34,386
Accrued interest	22,247		9,022
Other current liabilities	26,915		53,682
Total current liabilities	229,424		270,237
Long-term debt	1,030,466		1,027,084
Deferred taxes	(2,845)	[1]	10,227	[1]
Asset retirement obligations	3,785		4,945
Other long-term liabilities	46,772		53,509
Total liabilities	1,307,602		1,366,002
Commitments and contingencies
Stockholders' equity:
Common stock	491		488
Treasury stock	(860)		(860)
Additional paid-in capital	1,394,694		1,397,885
Accumulated deficit	(399,222)		(425,165)
Accumulated other comprehensive loss	(9,151)		(2,062)
Total stockholders' equity	985,952		970,286
Total liabilities and stockholders' equity	2,293,554		2,336,288
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	54,347		84,290		51,164	51,128
Accounts receivable	133,644		97,128
Fair value of hedging contracts	745		4,549
Deferred taxes	34,605	[1]	23,051	[1]
Inventory	283		283
Total current assets	223,624		209,301
Oil and gas properties, full cost method:
Proved	6,527,971		6,494,590
Less: accumulated DD&A	(5,554,836)		(5,448,828)
Net proved oil and gas properties	973,135		1,045,762
Unevaluated	575,845		388,643
Fair value of hedging contracts	1,867		1,378
Other assets, net	1,348		1,349
Total assets	1,775,819		1,646,433
Current liabilities:
Accounts payable to vendors	81,527		22,530
Undistributed oil and gas proceeds	2,536		2,643
Fair value of hedging contracts	15,277		7,753
Asset retirement obligations	87,927		67,161
Other current liabilities	1,552		838
Total current liabilities	188,819		100,925
Deferred taxes	409,322	[1]	380,466	[1]
Asset retirement obligations	412,386		430,407
Fair value of hedging contracts	376		470
Total liabilities	1,010,903		912,268
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,309,562		1,309,563
Accumulated deficit	(536,662)		(574,003)
Accumulated other comprehensive loss	(7,984)		(1,395)
Total stockholders' equity	764,916		734,165
Total liabilities and stockholders' equity	1,775,819		1,646,433
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	175		640
Total current assets	175		640
Oil and gas properties, full cost method:
Unevaluated	11,333		10,583
Other assets, net	4,748		2,128
Investment in subsidiary	16,171		12,711
Total assets	32,427		26,062
Current liabilities:
Accounts payable to vendors	3,444		44
Total current liabilities	3,444		44
Total liabilities	3,444		44
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	31,359		27,403
Accumulated deficit	(44)		(52)
Accumulated other comprehensive loss	(2,332)		(1,333)
Total stockholders' equity	28,983		26,018
Total liabilities and stockholders' equity	32,427		26,062
Consolidation, Eliminations [Member]
Current assets:
Cash and cash equivalents
Accounts receivable	(46,120)		(44)
Total current assets	(46,120)		(44)
Oil and gas properties, full cost method:
Investment in subsidiary	(793,899)		(760,183)
Total assets	(840,019)		(760,227)
Current liabilities:
Accounts payable to vendors	(46,120)		(44)
Total current liabilities	(46,120)		(44)
Total liabilities	(46,120)		(44)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,340,921)		(1,336,966)
Accumulated deficit	536,706		574,055
Accumulated other comprehensive loss	10,316		2,728
Total stockholders' equity	(793,899)		(760,183)
Total liabilities and stockholders' equity	$ (840,019)		$ (760,227)

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