Fair Value (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	$ 6,184	$ 4,762
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(159,466)	(131,779)
Commodity contracts (net) | Recurring | Significant other observable inputs (Level 2)
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	333	52
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(81,878)	(77,776)
Commodity contracts (net) | Recurring | Significant unobservable inputs (Level 3)
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	4,731	3,674
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(27,021)	(18,031)
Recurring | Significant unobservable inputs (Level 3) | Embedded derivatives in commodity contracts (net)
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	1,120	1,036
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	$ (50,567)	$ (35,972)