Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	$ 24,790	$ 4,762	$ 24,631
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(155,954)	(131,779)
Recurring | Significant other observable inputs (Level 2) | 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	5,063	52
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(79,358)	(77,776)
Recurring | Significant unobservable inputs (Level 3) | 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	12,210	3,674
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(15,175)	(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	7,517	1,036
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	$ (61,421)	$ (35,972)