Derivative Activities (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	$ 146,697	$ 45,529	$ 85,627	$ 3,181
Gain (Loss) Recognized in Income (Ineffective Portion)	1,872	7,510	2,109	9,024
Derivative fair value income	148,569	53,039	87,736	12,205
Swap [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	129,313	15,613	76,328	13,750
Gain (Loss) Recognized in Income (Ineffective Portion)	562		666
Derivative fair value income	129,875	15,613	76,994	13,750
Collars [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	14,894	6,452	12,392	(1,135)
Gain (Loss) Recognized in Income (Ineffective Portion)	1,310	7,510	1,443	9,024
Derivative fair value income	16,204	13,962	13,835	7,889
Call Options [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	11,234	23,464	5,651	(9,391)
Derivative fair value income	11,234	23,464	5,651	(9,391)
Basis Swap [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)				(43)
Derivative fair value income				(43)
Purchased Swap [Member]
Effects of non-hedge derivatives on consolidated statement of operations
Gain (Loss) Recognized in Income (Non-hedge Derivatives)	(8,744)		(8,744)
Derivative fair value income	$ (8,744)		$ (8,744)