Provision (Benefit) for Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred tax assets:
Federal net operating losses ("NOLs")	$ 93,600	$ 54,954	$ 30,700
NOLs for excess stock-based compensation	(9,676)	(9,450)	7,600
State NOL carryovers	13,686	11,377
Alternative minimum tax credits	2,092	3,451
Other Carryover Items	1,378	1,141
Unrealized share-based compensation	9,096	7,151
Valuation allowance	(6,318)	(3,901)
Other	5,909	7,580
Total deferred tax assets	109,767	72,303
Deferred tax liabilities:
Oil and gas exploration and development costs	(324,031)	(270,158)
Other	(3,300)	(2,109)
Total deferred tax liabilities	(327,331)	(272,267)
Net deferred tax liabilities	(217,564)	(199,964)
Net current deferred tax assets	5,679	6,603
Net non-current deferred tax liabilities	$ (223,243)	$ (206,567)