Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred Tax Assets, Gross [Abstract]
Net operating loss carryforwards	$ 122,393	$ 54,124
Fair value of derivatives	0	1,958
Statutory depletion carryforwards	8,159	7,359
Asset retirement obligations and other	21,814	21,165
Deferred tax assets, gross	152,366	84,606
Deferred Tax Liabilities, Gross [Abstract]
Fair value of derivatives	(4,208)	0
Property and equipment	(295,428)	(209,867)
Deferred tax liabilities, gross	(299,636)	(209,867)
Deferred tax liabilities, net	(147,270)	(125,261)
Components of Deferred Tax Assets and Liabilities [Abstract]
Current assets	8,560	8,948
Noncurrent liabilities	(155,830)	(134,209)
Deferred tax liabilities, net	$ (147,270)	$ (125,261)