Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash flows from operating activities:
Net income (loss)	$ 105,180	$ 148,809
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	260,982	204,777
Accretion expense	24,926	23,134
Deferred income tax provision (benefit)	59,573	97,926
Settlement of asset retirement obligations	(47,211)	(52,543)
Non-cash stock compensation expense	6,800	4,492
Excess tax benefits	(882)	(1,490)
Non-cash derivative income	(584)	(4,337)
Loss on early extinguishment of debt		607
Non-cash interest expense	9,068	1,412
Other non-cash (income) expense	16	(1,362)
Change in current income taxes	(3,240)	(21,710)
(Increase) decrease in accounts receivable	(15,832)	(17,117)
(Increase) decrease in other current assets	(2,456)	(185)
Decrease in inventory	136	1,606
Increase (decrease) in accounts payable	6,116	2,679
Decrease in other current liabilities	(24,301)	3,604
Other	(4,378)	(1,889)
Net cash provided by (used in) operating activities	373,913	388,413
Cash flows from investing activities:
Investment in oil and gas properties	(445,311)	(430,711)
Proceeds from sale of oil and gas properties, net of expenses	403	7,692
Sale of fixed assets	134
Investment in fixed and other assets	(3,998)	(1,788)
Net cash used in investing activities	(448,772)	(424,807)
Cash flows from financing activities:
Proceeds from bank borrowings	25,000
Deferred financing costs		(4,017)
Repayments of bank borrowings	(70,000)
Proceeds from issuance of senior convertible notes	300,000
Financing costs of senior convertible notes	(8,855)
Proceeds from Sold Warrants	40,170
Payments for Purchased Call Options	(70,830)
Excess tax benefits	882	1,490
Net payments for share based compensation	(3,773)	(2,581)
Net cash provided by financing activities	212,594	(5,108)
Net change in cash and cash equivalents	137,735	(41,502)
Cash and cash equivalents, beginning of period	38,451	106,956
Cash and cash equivalents, end of period	176,186	65,454
Parent
Cash flows from operating activities:
Net income (loss)	105,180	148,809
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	45,884	11,533
Accretion expense	423	11
Deferred income tax provision (benefit)	(30,793)	(561)
Non-cash stock compensation expense	6,800	4,492
Excess tax benefits	(882)	(1,490)
Loss on early extinguishment of debt		607
Non-cash (income) loss from investment in subsidiaries		(175,962)
Non-cash interest expense	9,068	1,412
Other non-cash (income) expense	16	(1,362)
Change in current income taxes	(3,240)	(19,357)
Change in intercompany receivables/payables	207,166	233,904
(Increase) decrease in accounts receivable	(9,229)	(16,878)
(Increase) decrease in other current assets	(2,456)	(200)
Decrease in inventory	136	1,592
Increase (decrease) in accounts payable	5,733	2,709
Decrease in other current liabilities	(20,441)	1,124
Other	(163,921)	(1,889)
Net cash provided by (used in) operating activities	149,444	188,494
Cash flows from investing activities:
Investment in oil and gas properties	(240,877)	(227,862)
Proceeds from sale of oil and gas properties, net of expenses	403	5,575
Sale of fixed assets	134
Investment in fixed and other assets	(3,998)	(1,788)
Net cash used in investing activities	(244,338)	(224,075)
Cash flows from financing activities:
Proceeds from bank borrowings	25,000
Deferred financing costs		(4,017)
Repayments of bank borrowings	(70,000)
Proceeds from issuance of senior convertible notes	300,000
Financing costs of senior convertible notes	(8,855)
Proceeds from Sold Warrants	40,170
Payments for Purchased Call Options	(70,830)
Excess tax benefits	882	1,490
Net payments for share based compensation	(3,773)	(2,581)
Net cash provided by financing activities	212,594	(5,108)
Net change in cash and cash equivalents	117,700	(40,689)
Cash and cash equivalents, beginning of period	37,389	105,115
Cash and cash equivalents, end of period	155,089	64,426
Guarantor Subsidiary
Cash flows from operating activities:
Net income (loss)	160,650	177,443
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	214,908	192,566
Accretion expense	24,246	22,849
Deferred income tax provision (benefit)	90,366	98,487
Settlement of asset retirement obligations	(47,211)	(52,543)
Non-cash derivative income	(584)	(4,337)
Non-cash (income) loss from investment in subsidiaries		(788)
Change in current income taxes		(2,353)
Change in intercompany receivables/payables	(206,669)	(233,577)
(Increase) decrease in accounts receivable	(6,631)	(242)
(Increase) decrease in other current assets		15
Decrease in inventory		14
Increase (decrease) in accounts payable	422	(30)
Decrease in other current liabilities	(3,860)	2,480
Other	(1,032)
Net cash provided by (used in) operating activities	224,605	199,984
Cash flows from investing activities:
Investment in oil and gas properties	(204,434)	(202,845)
Proceeds from sale of oil and gas properties, net of expenses		2,117
Net cash used in investing activities	(204,434)	(200,728)
Cash flows from financing activities:
Net change in cash and cash equivalents	20,171	(744)
Cash and cash equivalents, beginning of period	926	1,659
Cash and cash equivalents, end of period	21,097	915
Non-Guarantor Subsidiaries
Cash flows from operating activities:
Net income (loss)	(75)	(693)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	190	678
Accretion expense	257	274
Change in intercompany receivables/payables	(497)	(327)
(Increase) decrease in accounts receivable	28	3
Increase (decrease) in accounts payable	(39)
Net cash provided by (used in) operating activities	(136)	(65)
Cash flows from investing activities:
Investment in oil and gas properties		(4)
Net cash used in investing activities		(4)
Cash flows from financing activities:
Net change in cash and cash equivalents	(136)	(69)
Cash and cash equivalents, beginning of period	136	182
Eliminations
Cash flows from operating activities:
Net income (loss)	(160,575)	(176,750)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Non-cash (income) loss from investment in subsidiaries		176,750
Other	$ 160,575