Derivative Instruments--Non-Hedge Derivative Gains (Losses) in the Consolidated Statements of Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Non-Hedge Derivative Gains (Losses) [Line Items]
Non-hedge derivative (losses) gains	$ (21,635)	$ 49,685	$ 34,408
Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(20,717)	(8,440)	50,004
Receipts from (payments on) settlement of non-hedge derivative instruments	5,122	28,716	(16,801)
Costless Collars
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(23,459)	21,182	3,540
Receipts from (payments on) settlement of non-hedge derivative instruments	14,548	10,229	1,250
Enhanced Swaps and Put Options
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	186	0	0
Basis Protection Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	3,242	(331)	4,355
Receipts from (payments on) settlement of non-hedge derivative instruments	$ (557)	$ (1,671)	$ (7,940)