Derivative Instruments - Non-Hedge Derivative Gains (Losses) in the Consolidated Statements of Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Non-Hedge Derivative Gains (Losses) [Line Items]
Receipts from (payments on) settlement of non-hedge derivative instruments			$ (24,309)	$ 21,610
Non-hedge derivative gains (losses)	(25,030)	93,601	37,035	79,971
Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(22,167)	78,202	(7,451)	83,210
Receipts from (payments on) settlement of non-hedge derivative instruments	8,046	(245)	20,022	(16,587)
Costless Collars
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(14,094)	16,156	20,480	15,485
Receipts from (payments on) settlement of non-hedge derivative instruments	3,079	426	5,711	916
Basis Protection Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	526	947	(303)	2,886
Receipts from (payments on) settlement of non-hedge derivative instruments	$ (420)	$ (1,885)	$ (1,424)	$ (5,939)