Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative instruments carried at fair value in Condensed Consolidated Balance Sheet
Total carrying value of derivative assets in Condensed Consolidated Balance Sheet	$ 26,995	$ 30,382
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(46,998)	(59,419)
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	5,548	8,441
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(13,147)	(15,970)
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	14,969	15,795
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	(2,492)	(3,346)
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	6,478	6,146
Total carrying value of derivative liabilities in Condensed Consolidated Balance Sheet	$ (31,359)	$ (40,103)