Derivative Instruments - Non-Hedge Derivative Gains (Losses) in the Consolidated Statements of Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Non-Hedge Derivative Gains (Losses) [Line Items]
Receipts from (payments on) settlement of non-hedge derivative instruments			$ (13,604)	$ 19,906
Non-hedge derivative gains (losses)	72,528	47,296	62,065	(13,630)
Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	18,701	55,176	14,716	5,008
Receipts from (payments on) settlement of non-hedge derivative instruments	9,275	(13,060)	11,976	(16,342)
Costless Collars
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	43,682	5,934	34,574	(671)
Receipts from (payments on) settlement of non-hedge derivative instruments	2,599	156	2,632	490
Basis Protection Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(1,202)	798	(829)	1,939
Receipts from (payments on) settlement of non-hedge derivative instruments	$ (527)	$ (1,708)	$ (1,004)	$ (4,054)