Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net income	$ 40,758	$ 50,974
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	75,435	84,575
Accretion expense	8,263	8,266
Deferred income tax provision (benefit)	27,024	27,439
Settlement of asset retirement obligations	(14,880)	(2,980)
Non-cash stock compensation expense	2,296	1,750
Excess tax benefits	(104)	(847)
Non-cash derivative expense	1,385	1,342
Non-cash interest expense	4,041	1,573
Non-cash income from investment in subsidiaries
Change in current income taxes	(9,402)	(2,647)
Change in intercompany receivables/payables
(Increase) decrease in accounts receivable	19,952	(22,111)
Decrease in other current assets	40	35
Decrease in inventory	158
Increase (decrease) in accounts payable	2,004	(6,015)
Increase (decrease) in other current liabilities	(8,942)	(22,168)
Other	(1,262)	(254)
Net cash provided by (used in) operating activities	146,766	118,932
Cash flows from investing activities:
Investment in oil and gas properties	(160,968)	(106,469)
Proceeds from sale of oil and gas properties, net of expenses		403
Sale of fixed assets		134
Investment in fixed and other assets	(599)	(909)
Net cash (used in) provided by investing activities	(161,567)	(106,841)
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	104	847
Net payments for share based compensation	(3,465)	(3,033)
Net cash provided by financing activities	(3,372)	213,299
Net change in cash and cash equivalents	(18,173)	225,390
Cash and cash equivalents, beginning of period	279,526	38,451
Cash and cash equivalents, end of period	261,353	263,841
Parent
Cash flows from operating activities:
Net income	40,758	50,974
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	10,191	11,477
Accretion expense	93	148
Deferred income tax provision (benefit)	(4,941)	(9,763)
Settlement of asset retirement obligations
Non-cash stock compensation expense	2,296	1,750
Excess tax benefits	(104)	(847)
Non-cash interest expense	4,041	1,573
Non-cash income from investment in subsidiaries	(56,828)	(65,737)
Change in current income taxes	(9,402)	(2,647)
Change in intercompany receivables/payables	122,912	18,655
(Increase) decrease in accounts receivable	10,668	(983)
Decrease in other current assets	40	35
Decrease in inventory	158
Increase (decrease) in accounts payable	(1,890)	(25)
Increase (decrease) in other current liabilities	(20,011)	(23,155)
Other	(761)	(254)
Net cash provided by (used in) operating activities	97,220	(18,799)
Cash flows from investing activities:
Investment in oil and gas properties	(111,458)	(8,767)
Proceeds from sale of oil and gas properties, net of expenses		403
Sale of fixed assets		134
Investment in fixed and other assets	(599)	(909)
Net cash (used in) provided by investing activities	(112,057)	(9,139)
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	104	847
Net payments for share based compensation	(3,465)	(3,033)
Net cash provided by financing activities	(3,372)	213,299
Net change in cash and cash equivalents	(18,209)	185,361
Cash and cash equivalents, beginning of period	228,398	37,389
Cash and cash equivalents, end of period	210,189	222,750
Guarantor Subsidiary
Cash flows from operating activities:
Net income	56,828	66,138
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	65,244	73,000
Accretion expense	8,170	8,032
Deferred income tax provision (benefit)	31,965	37,202
Settlement of asset retirement obligations	(14,880)	(2,980)
Non-cash stock compensation expense
Excess tax benefits
Non-cash derivative expense	1,385	1,342
Non-cash interest expense
Non-cash income from investment in subsidiaries		(161)
Change in current income taxes
Change in intercompany receivables/payables	(122,912)	(18,445)
(Increase) decrease in accounts receivable	9,284	(21,226)
Decrease in other current assets
Increase (decrease) in accounts payable	3,894	(6,157)
Increase (decrease) in other current liabilities	11,069	987
Other	(501)
Net cash provided by (used in) operating activities	49,546	137,732
Cash flows from investing activities:
Investment in oil and gas properties	(49,510)	(97,703)
Proceeds from sale of oil and gas properties, net of expenses
Sale of fixed assets
Investment in fixed and other assets
Net cash (used in) provided by investing activities	(49,510)	(97,703)
Cash flows from financing activities:
Proceeds from bank borrowings
Deferred financing costs
Repayments of bank borrowings
Proceeds from issuance of senior convertible notes
Deferred financing costs of senior convertible notes
Proceeds from Sold Warrants
Payments for Purchased Call Options
Excess tax benefits
Net payments for share based compensation
Net cash provided by financing activities
Net change in cash and cash equivalents	36	40,029
Cash and cash equivalents, beginning of period	51,128	926
Cash and cash equivalents, end of period	51,164	40,955
Non-Guarantor Subsidiaries
Cash flows from operating activities:
Net income		(240)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization		98
Accretion expense		86
Deferred income tax provision (benefit)
Settlement of asset retirement obligations
Non-cash stock compensation expense
Excess tax benefits
Non-cash derivative expense
Non-cash interest expense
Non-cash income from investment in subsidiaries
Change in current income taxes
Change in intercompany receivables/payables		(210)
(Increase) decrease in accounts receivable		98
Decrease in other current assets
Decrease in inventory
Increase (decrease) in accounts payable		167
Increase (decrease) in other current liabilities
Other
Net cash provided by (used in) operating activities		(1)
Cash flows from investing activities:
Investment in oil and gas properties		1
Proceeds from sale of oil and gas properties, net of expenses
Sale of fixed assets
Investment in fixed and other assets
Net cash (used in) provided by investing activities		1
Cash flows from financing activities:
Proceeds from bank borrowings
Deferred financing costs
Repayments of bank borrowings
Proceeds from issuance of senior convertible notes
Deferred financing costs of senior convertible notes
Proceeds from Sold Warrants
Payments for Purchased Call Options
Excess tax benefits
Net payments for share based compensation
Net cash provided by financing activities
Net change in cash and cash equivalents
Cash and cash equivalents, beginning of period		136
Cash and cash equivalents, end of period		136
Eliminations
Cash flows from operating activities:
Net income	(56,828)	(65,898)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization
Accretion expense
Deferred income tax provision (benefit)
Settlement of asset retirement obligations
Non-cash stock compensation expense
Excess tax benefits
Non-cash derivative expense
Non-cash interest expense
Non-cash income from investment in subsidiaries	56,828	65,898
Change in current income taxes
Change in intercompany receivables/payables
(Increase) decrease in accounts receivable
Decrease in other current assets
Decrease in inventory
Increase (decrease) in accounts payable
Increase (decrease) in other current liabilities
Other
Net cash provided by (used in) operating activities
Cash flows from investing activities:
Investment in oil and gas properties
Proceeds from sale of oil and gas properties, net of expenses
Sale of fixed assets
Investment in fixed and other assets
Net cash (used in) provided by investing activities
Cash flows from financing activities:
Proceeds from bank borrowings
Deferred financing costs
Repayments of bank borrowings
Proceeds from issuance of senior convertible notes
Deferred financing costs of senior convertible notes
Proceeds from Sold Warrants
Payments for Purchased Call Options
Excess tax benefits
Net payments for share based compensation
Net cash provided by financing activities
Net change in cash and cash equivalents
Cash and cash equivalents, beginning of period
Cash and cash equivalents, end of period