Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets
Loss carryforwards - federal	$ 0	$ 13,970
Loss carryforwards - state	35,007	41,960
Tax credit carryover	34,837	34,829
Derivative contracts	7,252	3,551
Enhanced oil recovery credit carryforwards	17,346	53,381
Stock based compensation	28,387	32,566
Other	37,226	35,279
Total deferred tax assets	160,055	215,536
Deferred tax liabilities
Property and equipment	(2,277,388)	(2,078,143)
Other	(6,963)	(5,813)
Total deferred tax liabilities	(2,284,351)	(2,083,956)
Total net deferred tax liability	$ (2,124,296)	$ (1,868,420)