Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:
Net income (loss)	$ 115,882	$ 105,180
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	255,497	260,982
Accretion expense	25,012	24,926
Deferred income tax provision (benefit)	77,001	59,573
Settlement of asset retirement obligations	(61,178)	(47,211)
Non-cash stock compensation expense	7,583	6,800
Excess tax benefits	(156)	(882)
Non-cash derivative expense	1,626	(584)
Non-cash interest expense	12,384	9,068
Other non-cash expense		16
Change in current income taxes	(704)	(3,240)
(Increase) decrease in accounts receivable	(22,277)	(15,832)
(Increase) decrease in other current assets	2,187	(2,456)
Decrease in inventory	158	136
Increase (decrease) in accounts payable	8,035	6,116
Increase (decrease) in other current liabilities	20,251	(24,301)
Other	(1,791)	(4,378)
Net cash provided by (used in) operating activities	439,510	373,913
Cash flows from investing activities:
Investment in oil and gas properties	(472,304)	(445,311)
Proceeds from sale of oil and gas properties, net of expenses	6,300	403
Sale of fixed assets		134
Investment in fixed and other assets	(3,830)	(3,998)
Change in restricted funds	(2,394)
Net cash used in investing activities	(472,228)	(448,772)
Cash flows from financing activities:
Proceeds from bank borrowings		25,000
Deferred financing costs	(11)
Repayments of bank borrowings		(70,000)
Proceeds from issuance of senior convertible notes		300,000
Deferred financing costs of senior convertible notes		(8,855)
Proceeds from sold warrants		40,170
Payments for purchased call options		(70,830)
Excess tax benefits	156	882
Net payments for share-based compensation	(3,733)	(3,773)
Net cash (used in) provided by financing activities	(3,588)	212,594
Effect of exchange rate changes on cash	(145)
Net change in cash and cash equivalents	(36,451)	137,735
Cash and cash equivalents, beginning of period	279,526	38,451
Cash and cash equivalents, end of period	243,075	176,186
Parent [Member]
Cash flows from operating activities:
Net income (loss)	115,882	105,180
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	62,007	45,884
Accretion expense	279	423
Deferred income tax provision (benefit)	(17,980)	(30,793)
Non-cash stock compensation expense	7,583	6,800
Excess tax benefits	(156)	(882)
Non-cash interest expense	12,384	9,068
Other non-cash expense		16
Non-cash (income) loss from investment in subsidiaries	(168,887)
Change in current income taxes	(704)	(3,240)
Change in intercompany receivables/payables	207,267	207,166
(Increase) decrease in accounts receivable	(24,053)	(9,229)
(Increase) decrease in other current assets	2,187	(2,456)
Decrease in inventory	158	136
Increase (decrease) in accounts payable	2,610	5,733
Increase (decrease) in other current liabilities	13,613	(20,441)
Other	499	(163,921)
Net cash provided by (used in) operating activities	212,689	149,444
Cash flows from investing activities:
Investment in oil and gas properties	(234,776)	(240,877)
Proceeds from sale of oil and gas properties, net of expenses	6,300	403
Sale of fixed assets		134
Investment in fixed and other assets	(3,830)	(3,998)
Investment in subsidiaries	(14,000)
Net cash used in investing activities	(246,306)	(244,338)
Cash flows from financing activities:
Proceeds from bank borrowings		25,000
Deferred financing costs	(11)
Repayments of bank borrowings		(70,000)
Proceeds from issuance of senior convertible notes		300,000
Deferred financing costs of senior convertible notes		(8,855)
Proceeds from sold warrants		40,170
Payments for purchased call options		(70,830)
Excess tax benefits	156	882
Net payments for share-based compensation	(3,733)	(3,773)
Net cash (used in) provided by financing activities	(3,588)	212,594
Net change in cash and cash equivalents	(37,205)	117,700
Cash and cash equivalents, beginning of period	228,398	37,389
Cash and cash equivalents, end of period	191,193	155,089
Guarantor Subsidiary [Member]
Cash flows from operating activities:
Net income (loss)	168,919	160,650
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	193,490	214,908
Accretion expense	24,733	24,246
Deferred income tax provision (benefit)	94,981	90,366
Settlement of asset retirement obligations	(61,178)	(47,211)
Non-cash derivative expense	1,626	(584)
Change in intercompany receivables/payables	(207,311)	(206,669)
(Increase) decrease in accounts receivable	1,776	(6,631)
Increase (decrease) in accounts payable	5,425	422
Increase (decrease) in other current liabilities	6,638	(3,860)
Other	(2,290)	(1,032)
Net cash provided by (used in) operating activities	226,809	224,605
Cash flows from investing activities:
Investment in oil and gas properties	(226,695)	(204,434)
Net cash used in investing activities	(226,695)	(204,434)
Cash flows from financing activities:
Net change in cash and cash equivalents	114	20,171
Cash and cash equivalents, beginning of period	51,128	926
Cash and cash equivalents, end of period	51,242	21,097
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net income (loss)	(64)	(75)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization		190
Accretion expense		257
Non-cash (income) loss from investment in subsidiaries	32
Change in intercompany receivables/payables	44	(497)
(Increase) decrease in accounts receivable		28
Increase (decrease) in accounts payable		(39)
Net cash provided by (used in) operating activities	12	(136)
Cash flows from investing activities:
Investment in oil and gas properties	(10,833)
Change in restricted funds	(2,394)
Investment in subsidiaries	(13,404)
Net cash used in investing activities	(26,631)
Cash flows from financing activities:
Equity proceeds from parent	27,404
Net cash (used in) provided by financing activities	27,404
Effect of exchange rate changes on cash	(145)
Net change in cash and cash equivalents	640	(136)
Cash and cash equivalents, beginning of period		136
Cash and cash equivalents, end of period	640
Eliminations [Member]
Cash flows from operating activities:
Net income (loss)	(168,855)	(160,575)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Non-cash (income) loss from investment in subsidiaries	168,855
Other		160,575
Cash flows from investing activities:
Investment in subsidiaries	27,404
Net cash used in investing activities	27,404
Cash flows from financing activities:
Equity proceeds from parent	(27,404)
Net cash (used in) provided by financing activities	$ (27,404)