Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss)	$ 84,262	$ 68,720
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depletion of Oil and Gas Properties	112,702	73,759
Deferred Income Tax Expense (Benefit)	44,073	41,330
Unrealized Gain (Loss) on Derivatives	(57,746)	29,405
(Excess) Reduction in Tax Benefits from Share-based Compensation, Operating Activities	3,739	(1,106)
Share-based Compensation	2,456	3,122
Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities, Other	537	251
Increase (Decrease) in Operating Capital [Abstract]
Increase (Decrease) in Accounts Receivable	49,060	(6,159)
Increase (Decrease) in Prepaid, Deferred Expense and Other Assets	(5,698)	(4,791)
Increase (Decrease) in Other Operating Assets	0	45
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(28,421)	(42,672)
Increase (Decrease) in Production Taxes Payable	(2,586)	(7,439)
Increase (Decrease) in Interest Payable	(21,529)	14,867
Other long-term obligations	10,100	12,565
Increase (Decrease) in Accrued Income Taxes Payable	(1,646)	2,725
Net Cash Provided by (Used in) Operating Activities	189,303	184,622
Net Cash Provided by (Used in) Investing Activities [Abstract]
Oil and gas property expenditures	(272,241)	(305,700)
Gathering system expenditures	(16,391)	(13,545)
Change in capital cost accrual	(14,139)	(818)
Inventory	(671)	(430)
Purchase of Capital Assets	(1,571)	(175)
Net Cash Provided by (Used in) Investing Activities	(305,013)	(320,668)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from Issuance of Long-term Debt	323,000	201,000
Repayments of Long-term Debt	(207,000)	(113,000)
Payments for Repurchase of Equity	(5,957)	(2,967)
Excess (Reduction in) Tax Benefits from Share-based Compensation, Financing Activities	(3,739)	1,106
Proceeds from Stock Options Exercised	632	1,965
Net Cash Provided by (Used in) Financing Activities	106,936	88,104
Cash and Cash Equivalents, Period Increase (Decrease)	(8,774)	(47,942)
Cash and Cash Equivalents, at Carrying Value	11,307	70,834
Cash and Cash Equivalents, at Carrying Value	$ 2,533	$ 22,892