Income Taxes (Deferred Tax) (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred Tax Assets [Abstract]
Deferred compensation	$ 2,268	$ 5,859
Unrealized loss on commodity derivative instruments	0	1,260
Asset retirement obligation	87,787	29,122
Net operating loss and capital loss carryforward	334,121	157,570
Unrealized loss on embedded derivative instruments	1,084	4,005
Property, plant and equipment	19,596	12,427
Inventory / other	8,089	0
Total deferred tax assets	452,945	210,243
Less valuation allowance	(84,691)	(58,532)
Total deferred tax assets after valuation allowance	368,254	151,711
Deferred Tax Liability [Abstract]
Property, plant and equipment	(478,719)	(258,779)
Asset retirement obligation	0	0
Unrealized gain on derivative instruments	0	0
Petroleum revenue tax, net of tax benefit	(22,703)	(229)
Debt discount	(630)	(752)
Other	0	(7,710)
Total deferred tax liabilities	(502,052)	(267,470)
Net deferred tax liability	$ (133,798)	$ (115,759)