Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Federal benefit of state income taxes	$ 239	$ 2,479
Credit carryforwards	36,028	29,186
Equity and deferred compensation	15,139	12,620
Derivatives	0	8,669
Net operating loss	412	412
Bad debt expense	163	0
Other, net	610	980
Valuation allowance	(1,201)	(403)
Deferred Tax Assets, Net	50,912	48,985
Deferred tax liabilities:
Depreciation and depletion	(298,659)	(220,656)
Derivatives	(8,477)	0
Deferred Tax Liabilities	(256,224)	(171,671)
Deferred Tax Liabilities, Gross	$ (307,136)	$ (220,656)