Guarantor Financial Statements (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Cash flows from operating activities:
Net income	$ 50,974	$ 39,792
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	84,575	67,669
Accretion expense	8,266	7,717
Deferred income tax provision	27,439	22,383
Settlement of asset retirement obligations	(2,980)	(19,034)
Non-cash stock compensation expense	1,750	1,680
Excess tax benefits	(847)	(649)
Non-cash derivative expense	1,342	1,804
Other non-cash expenses	1,173	(104)
Change in current income taxes	(2,647)	(3,681)
Increase in accounts receivable	(21,708)	(21,555)
Decrease in other current assets	35	25
Decrease in inventory		365
Increase (decrease) in accounts payable	(6,015)	2,690
Increase (decrease) in other current liabilities	(22,168)	(10,059)
Other	146	272
Net cash provided by operating activities	119,335	89,315
Cash flows from investing activities:
Investment in oil and gas properties	(106,469)	(156,535)
Proceeds from sale of oil and gas properties		7,692
Sale of fixed assets	134
Investment in fixed and other assets	(909)	(262)
Net cash used in investing activities	(107,244)	(149,105)
Cash flows from financing activities:
Proceeds from bank borrowings	25,000
Repayments of bank borrowings	(70,000)
Proceeds from issuance of senior convertible notes	300,000
Financing costs of senior convertible notes	(8,855)	(66)
Proceeds from Sold Warrants	40,170
Payments for Purchased Call Options	(70,830)
Excess tax benefits	847	649
Net payments for share based compensation	(3,033)	(1,843)
Net cash provided by (used in) financing activities	213,299	(1,260)
Net change in cash and cash equivalents	225,390	(61,050)
Cash and cash equivalents, beginning of period	38,451	106,956
Cash and cash equivalents, end of period	263,841	45,906
Parent [Member]
Cash flows from operating activities:
Net income	50,974	39,792
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	11,477	5,942
Accretion expense	148	4
Deferred income tax provision	(9,763)	(7,611)
Non-cash stock compensation expense	1,750	1,680
Excess tax benefits	(847)	(649)
Non-cash (income) loss from investment in subsidiary	(65,737)	(52,756)
Other non-cash expenses	1,173	(104)
Change in current income taxes	(2,647)	(3,681)
Change in intercompany receivable/payables	18,655	44,230
Increase in accounts receivable	983	6,833
Decrease in other current assets	(35)	25
Decrease in inventory		365
Increase (decrease) in accounts payable	(25)	2,609
Increase (decrease) in other current liabilities	(23,155)	(10,929)
Other	(146)	272
Net cash provided by operating activities	(18,799)	26,022
Cash flows from investing activities:
Investment in oil and gas properties	8,364	(89,444)
Proceeds from sale of oil and gas properties		5,575
Sale of fixed assets	134
Investment in fixed and other assets	909	(262)
Net cash used in investing activities	(9,139)	(84,131)
Cash flows from financing activities:
Proceeds from bank borrowings	25,000
Repayments of bank borrowings	(70,000)
Proceeds from issuance of senior convertible notes	300,000
Financing costs of senior convertible notes	(8,855)	(66)
Proceeds from Sold Warrants	40,170
Payments for Purchased Call Options	70,830
Excess tax benefits	847	649
Net payments for share based compensation	3,033	(1,843)
Net cash provided by (used in) financing activities	213,299	(1,260)
Net change in cash and cash equivalents	185,361	(59,369)
Cash and cash equivalents, beginning of period	37,389	105,115
Cash and cash equivalents, end of period	222,750	45,746
Guarantor Subsidiary [Member]
Cash flows from operating activities:
Net income	66,138	53,323
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	73,000	61,510
Accretion expense	8,032	7,622
Deferred income tax provision	37,202	29,994
Settlement of asset retirement obligations	(2,980)	(19,034)
Non-cash derivative expense	1,342	1,804
Non-cash (income) loss from investment in subsidiary		(438)
Change in intercompany receivable/payables	(18,445)	(43,804)
Increase in accounts receivable	20,823	(28,509)
Increase (decrease) in accounts payable	(6,157)	81
Increase (decrease) in other current liabilities	987	870
Net cash provided by operating activities	138,135	63,419
Cash flows from investing activities:
Investment in oil and gas properties	98,106	(67,087)
Proceeds from sale of oil and gas properties		2,117
Net cash used in investing activities	(98,106)	(64,970)
Cash flows from financing activities:
Net change in cash and cash equivalents	40,029	(1,551)
Cash and cash equivalents, beginning of period	926	1,659
Cash and cash equivalents, end of period	40,955	108
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net income	(240)	(129)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	98	217
Accretion expense	86	91
Change in intercompany receivable/payables	(210)	(426)
Increase in accounts receivable	(98)	121
Increase (decrease) in accounts payable	167
Net cash provided by operating activities	(1)	(126)
Cash flows from investing activities:
Investment in oil and gas properties	(1)	(4)
Net cash used in investing activities	1	(4)
Cash flows from financing activities:
Net change in cash and cash equivalents		(130)
Cash and cash equivalents, beginning of period	136	182
Cash and cash equivalents, end of period	136	52
Eliminations [Member]
Cash flows from operating activities:
Net income	(65,898)	(53,194)
Adjustments to reconcile net income to net cash provided by operating activities:
Non-cash (income) loss from investment in subsidiary		$ 53,194