Guarantor Financial Statements - Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Jun. 30, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 180,307			$ 331,224		$ 243,075		$ 279,526
Restricted cash	177,647
Accounts receivable	173,775			171,971
Fair value of derivative contracts	16,635			4,549
Current income tax receivable	7,373			7,366
Deferred taxes	24,036	[1]		31,710	[1]
Inventory	3,709			3,723
Other current assets	1,884			1,874
Total current assets	585,366			552,417
Oil and gas properties, full cost method:
Proved	8,692,017			7,804,117
Less: accumulated DD&A	(6,568,551)			(5,908,760)
Net proved oil and gas properties	2,123,466			1,895,357
Unevaluated	570,658			724,339
Other property and equipment, net	32,118			26,178
Fair value of derivative contracts	6,481			1,378
Other assets, net	40,860			48,887
Total assets	3,358,949			3,248,556
Current liabilities:
Accounts payable to vendors	121,485			195,677
Undistributed oil and gas proceeds	58,503			37,029
Accrued interest	22,240			9,022
Fair value of derivative contracts	156			7,753
Asset retirement obligations	73,451			67,161
Other current liabilities	57,630			54,520
Total current liabilities	333,465			371,162
Long-term debt	1,037,440			1,027,084
Deferred taxes	394,846	[1]		390,693	[1]
Asset retirement obligations	336,197			435,352
Fair value of derivative contracts	24			470
Other long-term liabilities	41,350			53,509
Total liabilities	2,143,322			2,278,270
Commitments and contingencies
Stockholders' equity:
Common stock	549			488
Treasury stock	(860)			(860)
Additional paid-in capital	1,628,942			1,397,885
Accumulated deficit	(424,193)			(425,165)
Accumulated other comprehensive income (loss)	11,189		(18,200)	(2,062)		6,400	24,500	28,800
Total stockholders' equity	1,215,627			970,286
Total liabilities and stockholders' equity	3,358,949			3,248,556
Parent [Member]
Current assets:
Cash and cash equivalents	125,740			246,294		191,193		228,398
Restricted cash	177,647
Accounts receivable	153,685			74,887
Current income tax receivable	7,373			7,366
Deferred taxes	3,445	[1]		8,659	[1]
Inventory	3,426			3,440
Other current assets	1,876			1,874
Total current assets	473,192			342,520
Oil and gas properties, full cost method:
Proved	1,563,596			1,309,527
Less: accumulated DD&A	(613,590)			(459,932)
Net proved oil and gas properties	950,006			849,595
Unevaluated	299,225			325,113
Other property and equipment, net	32,118			26,178
Other assets, net	21,464			45,410
Investment in subsidiary	857,909			747,472
Total assets	2,633,914			2,336,288
Current liabilities:
Accounts payable to vendors	245,870			173,147
Undistributed oil and gas proceeds	57,410			34,386
Accrued interest	22,240			9,022
Other current liabilities	57,131			53,682
Total current liabilities	382,651			270,237
Long-term debt	1,037,440			1,027,084
Deferred taxes	(46,057)	[1]		10,227	[1]
Asset retirement obligations	2,903			4,945
Other long-term liabilities	41,350			53,509
Total liabilities	1,418,287			1,366,002
Commitments and contingencies
Stockholders' equity:
Common stock	549			488
Treasury stock	(860)			(860)
Additional paid-in capital	1,628,942			1,397,885
Accumulated deficit	(424,193)			(425,165)
Accumulated other comprehensive income (loss)	11,189			(2,062)
Total stockholders' equity	1,215,627			970,286
Total liabilities and stockholders' equity	2,633,914			2,336,288
Guarantor Subsidiary [Member]
Current assets:
Cash and cash equivalents	54,440			84,290		51,242		51,128
Accounts receivable	224,491			97,128
Fair value of derivative contracts	16,635			4,549
Deferred taxes	20,591	[1]		23,051	[1]
Inventory	283			283
Total current assets	316,440			209,301
Oil and gas properties, full cost method:
Proved	7,128,421			6,494,590
Less: accumulated DD&A	(5,954,961)			(5,448,828)
Net proved oil and gas properties	1,173,460			1,045,762
Unevaluated	245,754			388,643
Fair value of derivative contracts	6,481			1,378
Other assets, net	1,906			1,349
Total assets	1,744,041			1,646,433
Current liabilities:
Accounts payable to vendors	48,661			22,530
Undistributed oil and gas proceeds	1,093			2,643
Fair value of derivative contracts	156			7,753
Asset retirement obligations	73,451			67,161
Other current liabilities	499			838
Total current liabilities	123,860			100,925
Deferred taxes	440,903	[1]		380,466	[1]
Asset retirement obligations	333,294			430,407
Fair value of derivative contracts	24			470
Total liabilities	898,081			912,268
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	1,309,562			1,309,563
Accumulated deficit	(476,828)			(574,003)
Accumulated other comprehensive income (loss)	13,226			(1,395)
Total stockholders' equity	845,960			734,165
Total liabilities and stockholders' equity	1,744,041			1,646,433
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	127			640		640
Accounts receivable	34
Other current assets	8
Total current assets	169			640
Oil and gas properties, full cost method:
Unevaluated	25,679			10,583
Other assets, net	17,490			2,128
Investment in subsidiary	40,605			12,711
Total assets	83,943			26,062
Current liabilities:
Accounts payable to vendors	31,389			44
Total current liabilities	31,389			44
Total liabilities	31,389			44
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	56,657			27,403
Accumulated deficit	(31)			(52)
Accumulated other comprehensive income (loss)	(4,072)			(1,333)
Total stockholders' equity	52,554			26,018
Total liabilities and stockholders' equity	83,943			26,062
Eliminations [Member]
Current assets:
Accounts receivable	(204,435)			(44)
Total current assets	(204,435)			(44)
Oil and gas properties, full cost method:
Investment in subsidiary	(898,514)			(760,183)
Total assets	(1,102,949)			(760,227)
Current liabilities:
Accounts payable to vendors	(204,435)			(44)
Total current liabilities	(204,435)			(44)
Total liabilities	(204,435)			(44)
Commitments and contingencies
Stockholders' equity:
Additional paid-in capital	(1,366,219)			(1,336,966)
Accumulated deficit	476,859			574,055
Accumulated other comprehensive income (loss)	(9,154)			2,728
Total stockholders' equity	(898,514)			(760,183)
Total liabilities and stockholders' equity	$ (1,102,949)			$ (760,227)

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