Derivative Activities (Details 5) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ 67,697	$ 7,592	$ 70,878	$ 56,812
Gain Recognized in Income (Ineffective Portion)	(1,935)	2,389	7,089	2,048
Derivative fair value (loss) income	65,762	9,981	77,967	58,860
Collars [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	15,828	12,559	14,693	60,998
Gain Recognized in Income (Ineffective Portion)	(1,935)	2,389	7,089	2,048
Derivative fair value (loss) income	13,893	14,948	21,782	63,046
Swap [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	26,219	0	39,969	0
Gain Recognized in Income (Ineffective Portion)	0	0	0	0
Derivative fair value (loss) income	26,219	0	39,969	0
Basis Swap [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	0	(1,144)	(43)	(363)
Gain Recognized in Income (Ineffective Portion)	0	0	0	0
Derivative fair value (loss) income	0	(1,144)	(43)	(363)
Call Option [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	25,650	(3,823)	16,259	(3,823)
Gain Recognized in Income (Ineffective Portion)	0	0	0	0
Derivative fair value (loss) income	$ 25,650	$ (3,823)	$ 16,259	$ (3,823)