Guarantor Financial Statements - Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities:
Net income (loss)	$ 972	$ 115,882
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	255,772	255,497
Write-down of oil and gas properties	47,130
Accretion expense	21,827	25,012
Deferred income tax (benefit) provision	3,599	77,001
Settlement of asset retirement obligations	(47,217)	(61,178)
Non-cash stock compensation expense	8,409	7,583
Excess tax benefits		(156)
Non-cash derivative expense (income)	(2,386)	1,626
Non-cash interest expense	12,393	12,384
Change in current income taxes	(6)	(704)
(Increase) decrease in accounts receivable	(1,805)	(22,277)
(Increase) decrease in other current assets	(10)	2,187
Decrease in inventory		158
Increase (decrease) in accounts payable	(3,547)	8,035
Increase (decrease) in other current liabilities	37,441	20,251
Other	(172)	(1,791)
Net cash provided by operating activities	332,400	439,510
Cash flows from investing activities:
Investment in oil and gas properties	(727,488)	(472,304)
Proceeds from sale of oil and gas properties, net of expenses	223,299	6,300
Investment in fixed and other assets	(8,790)	(3,830)
Change in restricted funds	(185,752)	(2,394)
Net cash used in investing activities	(698,731)	(472,228)
Cash flows from financing activities:
Net proceeds from issuance of common stock	225,999
Deferred financing costs	(3,329)	(11)
Excess tax benefits		156
Net payments for share-based compensation	(7,161)	(3,733)
Net cash provided by (used in) financing activities	215,509	(3,588)
Effect of exchange rate changes on cash	(95)	(145)
Net change in cash and cash equivalents	(150,917)	(36,451)
Cash and cash equivalents, beginning of period	331,224	279,526
Cash and cash equivalents, end of period	180,307	243,075
Eliminations [Member]
Cash flows from operating activities:
Net income (loss)	(97,196)	(168,855)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Non-cash (income) loss from investment in subsidiaries	97,196	168,855
Cash flows from investing activities:
Investment in subsidiaries	29,253	27,404
Net cash used in investing activities	29,253	27,404
Cash flows from financing activities:
Equity proceeds from parent	(29,253)	(27,404)
Net cash provided by (used in) financing activities	(29,253)	(27,404)
Parent [Member]
Cash flows from operating activities:
Net income (loss)	972	115,882
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	95,038	62,007
Write-down of oil and gas properties	47,130
Accretion expense	185	279
Deferred income tax (benefit) provision	(51,074)	(17,980)
Settlement of asset retirement obligations	(84)
Non-cash stock compensation expense	8,409	7,583
Excess tax benefits		(156)
Non-cash interest expense	12,393	12,384
Non-cash (income) loss from investment in subsidiaries	(97,185)	(168,887)
Change in current income taxes	(6)	(704)
Change in intercompany receivables/payables	(119,004)	207,267
(Increase) decrease in accounts receivable	125,593	(24,053)
(Increase) decrease in other current assets	(2)	2,187
Decrease in inventory		158
Increase (decrease) in accounts payable	900	2,610
Increase (decrease) in other current liabilities	39,329	13,613
Other	1,414	499
Net cash provided by operating activities	64,008	212,689
Cash flows from investing activities:
Investment in oil and gas properties	(225,831)	(234,776)
Proceeds from sale of oil and gas properties, net of expenses	12,197	6,300
Investment in fixed and other assets	(8,790)	(3,830)
Change in restricted funds	(177,647)
Investment in subsidiaries		(14,000)
Net cash used in investing activities	(400,071)	(246,306)
Cash flows from financing activities:
Net proceeds from issuance of common stock	225,999
Deferred financing costs	(3,329)	(11)
Excess tax benefits		156
Net payments for share-based compensation	(7,161)	(3,733)
Net cash provided by (used in) financing activities	215,509	(3,588)
Net change in cash and cash equivalents	(120,554)	(37,205)
Cash and cash equivalents, beginning of period	246,294	228,398
Cash and cash equivalents, end of period	125,740	191,193
Guarantor Subsidiary [Member]
Cash flows from operating activities:
Net income (loss)	97,175	168,919
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	160,734	193,490
Accretion expense	21,642	24,733
Deferred income tax (benefit) provision	54,673	94,981
Settlement of asset retirement obligations	(47,133)	(61,178)
Non-cash derivative expense (income)	(2,386)	1,626
Change in intercompany receivables/payables	90,313	(207,311)
(Increase) decrease in accounts receivable	(127,363)	1,776
Increase (decrease) in accounts payable	(4,447)	5,425
Increase (decrease) in other current liabilities	(1,888)	6,638
Other	(1,586)	(2,290)
Net cash provided by operating activities	239,734	226,809
Cash flows from investing activities:
Investment in oil and gas properties	(480,686)	(226,695)
Proceeds from sale of oil and gas properties, net of expenses	211,102
Net cash used in investing activities	(269,584)	(226,695)
Cash flows from financing activities:
Net change in cash and cash equivalents	(29,850)	114
Cash and cash equivalents, beginning of period	84,290	51,128
Cash and cash equivalents, end of period	54,440	51,242
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net income (loss)	21	(64)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Non-cash (income) loss from investment in subsidiaries	(11)	32
Change in intercompany receivables/payables	28,691	44
(Increase) decrease in accounts receivable	(35)
(Increase) decrease in other current assets	(8)
Net cash provided by operating activities	28,658	12
Cash flows from investing activities:
Investment in oil and gas properties	(20,971)	(10,833)
Change in restricted funds	(8,105)	(2,394)
Investment in subsidiaries	(29,253)	(13,404)
Net cash used in investing activities	(58,329)	(26,631)
Cash flows from financing activities:
Equity proceeds from parent	29,253	27,404
Net cash provided by (used in) financing activities	29,253	27,404
Effect of exchange rate changes on cash	(95)	(145)
Net change in cash and cash equivalents	(513)	640
Cash and cash equivalents, beginning of period	640
Cash and cash equivalents, end of period	$ 127	$ 640