Consolidated Statements of Cash Flows (USD $) In Thousands	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss)	$ 321,371	$ 426,509
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depletion of Oil and Gas Properties	238,773	167,795
Deferred Income Tax Expense (Benefit)	176,566	230,936
Unrealized Gain (Loss) on Derivatives	(33,658)	(268,535)
Excess Tax Benefit from Share-based Compensation, Operating Activities	(6,441)	(16,386)
Share-based Compensation	9,892	9,122
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities, Other	783	473
Increase (Decrease) in Operating Capital [Abstract]
Increase (Decrease) in Restricted Cash for Operating Activities	(11)	(82)
Increase (Decrease) in Accounts Receivable	(22,329)	(5,008)
Increase (Decrease) in Prepaid, Deferred Expense and Other Assets	(1,927)	(3,236)
Increase (Decrease) in Other Operating Assets	(135)	2,905
Increase (Decrease) in Accounts Payable and Accrued Liabilities	18,016	36,264
Other long-term obligations	14,432	20,562
Increase (Decrease) in Accrued Income Taxes Payable	4,460	2,825
Net Cash Provided by (Used in) Operating Activities	719,792	604,144
Net Cash Provided by (Used in) Investing Activities [Abstract]
Acquisition Costs	0	(400,993)
Oil and gas property expenditures	(1,081,450)	(831,423)
Gathering system expenditures	(35,179)	(61,343)
Restricted Cash Non Current	0	28,257
Change in capital cost accrual	72,568	59,928
Proceeds from Sale of Productive Assets	0	68,420
Inventory	1,212	(233)
Payments to Acquire Productive Assets	(939)	(769)
Net Cash Provided by (Used in) Investing Activities	(1,043,788)	(1,138,156)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from Issuance of Long-term Debt	896,000	986,000
Repayments of Long-term Debt	(618,000)	(955,000)
Proceeds from issuance of Senior Notes	0	500,000
Payments of Financing Costs	0	(2,265)
Payments for Repurchase of Equity	(28,625)	(23,707)
Excess Tax Benefit from Share-based Compensation, Financing Activities	6,441	16,386
Proceeds from Stock Options Exercised	9,655	5,562
Net Cash Provided by (Used in) Financing Activities	265,471	526,976
Cash and Cash Equivalents, Period Increase (Decrease)	(58,525)	(7,036)
Cash and Cash Equivalents, at Carrying Value	70,834	14,254
Cash and Cash Equivalents, at Carrying Value	$ 12,309	$ 7,218