Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Commodity contracts
Asset and liability derivatives
Commodity derivatives designated as hedging instruments	$ 512,161	$ 225,339
Commodity derivatives designated as hedging instruments	123,306	106,721
Commodity derivatives designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Commodity derivatives designated as hedging instruments	412,626	141,834
Commodity derivatives designated as hedging instruments	3,681	12,097
Commodity derivatives designated as hedging instruments | Interest rate derivatives
Asset and liability derivatives
Commodity derivatives designated as hedging instruments	10,861
Commodity derivatives not designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Commodity derivatives designated as hedging instruments	99,535	83,505
Commodity derivatives designated as hedging instruments	$ 108,764	$ 94,624