Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax assets:
Federal benefit of state income taxes	$ (2,479)	$ 6,862
Credit carryforwards	28,783	28,808
Equity and deferred compensation	12,620	11,229
Derivatives	8,669	26,838
Net operating loss	412	14,545
Other, net	29	1,293
Deferred Tax Assets, Net	48,034	89,575
Deferred tax liabilities:
Depreciation and depletion	(219,705)	(386,440)
Deferred Tax Liabilities	(219,705)	(386,440)
Net deferred income tax liabilities	$ (171,671)	$ (296,865)