Guarantor Financial Statements - Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net income	$ 30,387	$ 79,780
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	175,481	162,644
Accretion expense	15,288	16,581
Deferred income tax provision (benefit)	18,094	56,422
Settlement of asset retirement obligations	(24,915)	(37,335)
Non-cash stock compensation expense	5,358	4,866
Excess tax benefits		(104)
Non-cash derivative expense	2,697	311
Non-cash interest expense	8,229	8,181
Change in current income taxes	(6)	(16,399)
(Increase) decrease in accounts receivable	(25,524)	1,138
Increase in other current assets	(82)	(245)
(Increase) decrease in inventory		158
Increase (decrease) in accounts payable	1,843	6,593
Increase (decrease) in other current liabilities	50,785	(3,170)
Other	(675)	(3,448)
Net cash (used in) provided by operating activities	256,960	275,973
Cash flows from investing activities:
Investment in oil and gas properties	(517,904)	(320,218)
Proceeds from sale of oil and gas properties, net of expenses	51,955
Investment in fixed and other assets	(3,896)	(1,711)
Change in restricted funds	(356)	(3,515)
Net cash used in investing activities	(470,201)	(325,444)
Cash flows from financing activities:
Net proceeds from issuance of common stock	226,036
Deferred financing costs	(3,167)	(11)
Excess tax benefits		104
Net payments for share-based compensation	(6,948)	(3,590)
Net cash provided by (used in) financing activities	215,921	(3,497)
Effect of exchange rate changes on cash	(18)	(186)
Net change in cash and cash equivalents	2,662	(53,154)
Cash and cash equivalents, beginning of period	331,224	279,526
Cash and cash equivalents, end of period	333,886	226,372
Parent [Member]
Cash flows from operating activities:
Net income	30,387	79,780
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	58,440	35,279
Accretion expense	129	186
Deferred income tax provision (benefit)	(19,765)	(6,961)
Settlement of asset retirement obligations	(82)
Non-cash stock compensation expense	5,358	4,866
Excess tax benefits		(104)
Non-cash interest expense	8,229	8,181
Non-cash income (loss) from investment in subsidiaries	(67,292)	(112,722)
Change in current income taxes	(6)	(16,399)
Change in intercompany receivables/payables	(126,526)	170,815
(Increase) decrease in accounts receivable	(42,216)	(17,365)
Increase in other current assets	(82)	(245)
(Increase) decrease in inventory		158
Increase (decrease) in accounts payable	2,052	4,507
Increase (decrease) in other current liabilities	48,987	(5,258)
Other	381	(2,482)
Net cash (used in) provided by operating activities	(102,006)	142,236
Cash flows from investing activities:
Investment in oil and gas properties	(121,836)	(176,896)
Proceeds from sale of oil and gas properties, net of expenses	9,777
Investment in fixed and other assets	(3,896)	(1,711)
Investment in subsidiaries		(14,000)
Net cash used in investing activities	(115,955)	(192,607)
Cash flows from financing activities:
Net proceeds from issuance of common stock	226,036
Deferred financing costs	(3,167)	(11)
Excess tax benefits		104
Net payments for share-based compensation	(6,948)	(3,590)
Net cash provided by (used in) financing activities	215,921	(3,497)
Net change in cash and cash equivalents	(2,040)	(53,868)
Cash and cash equivalents, beginning of period	246,294	228,398
Cash and cash equivalents, end of period	244,254	174,530
Guarantor Subsidiary [Member]
Cash flows from operating activities:
Net income	67,280	112,762
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	117,041	127,365
Accretion expense	15,159	16,395
Deferred income tax provision (benefit)	37,859	63,383
Settlement of asset retirement obligations	(24,833)	(37,335)
Non-cash derivative expense	2,697	311
Change in intercompany receivables/payables	114,841	(170,859)
(Increase) decrease in accounts receivable	16,692	18,503
Increase (decrease) in accounts payable	(209)	2,086
Increase (decrease) in other current liabilities	1,798	2,088
Other	(1,056)	(966)
Net cash (used in) provided by operating activities	347,269	133,733
Cash flows from investing activities:
Investment in oil and gas properties	(384,349)	(133,659)
Proceeds from sale of oil and gas properties, net of expenses	42,178
Net cash used in investing activities	(342,171)	(133,659)
Cash flows from financing activities:
Net change in cash and cash equivalents	5,098	74
Cash and cash equivalents, beginning of period	84,290	51,128
Cash and cash equivalents, end of period	89,388	51,202
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net income	24	(80)
Adjustments to reconcile net income to net cash provided by operating activities:
Non-cash income (loss) from investment in subsidiaries	(12)	40
Change in intercompany receivables/payables	11,685	44
Net cash (used in) provided by operating activities	11,697	4
Cash flows from investing activities:
Investment in oil and gas properties	(11,719)	(9,663)
Change in restricted funds	(356)	(3,515)
Investment in subsidiaries	(12,176)	(13,404)
Net cash used in investing activities	(24,251)	(26,582)
Cash flows from financing activities:
Equity proceeds from parent	12,176	27,404
Net cash provided by (used in) financing activities	12,176	27,404
Effect of exchange rate changes on cash	(18)	(186)
Net change in cash and cash equivalents	(396)	640
Cash and cash equivalents, beginning of period	640
Cash and cash equivalents, end of period	244	640
Consolidation, Eliminations [Member]
Cash flows from operating activities:
Net income	(67,304)	(112,682)
Adjustments to reconcile net income to net cash provided by operating activities:
Non-cash income (loss) from investment in subsidiaries	67,304	112,682
Cash flows from investing activities:
Investment in subsidiaries	12,176	27,404
Net cash used in investing activities	12,176	27,404
Cash flows from financing activities:
Equity proceeds from parent	(12,176)	(27,404)
Net cash provided by (used in) financing activities	$ (12,176)	$ (27,404)