Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Asset and liability derivatives
Derivatives designated as hedging instruments	$ 319,203	$ 512,161
Derivatives designated as hedging instruments	125,452	123,306
Commodity derivatives designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	263,141	412,626
Derivatives designated as hedging instruments	42,043	3,681
Commodity derivatives designated as hedging instruments | Interest rate derivatives
Asset and liability derivatives
Derivatives designated as hedging instruments	21,520	10,861
Commodity derivatives not designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	56,062	99,535
Derivatives designated as hedging instruments	$ 61,889	$ 108,764