Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Asset and liability derivatives
Derivatives designated as hedging instruments	$ 203,002	$ 304,237
Derivatives designated as hedging instruments	18,353	75,562
Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	203,002	304,237
Derivatives designated as hedging instruments	18,353	75,562
Commodity derivatives designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	190,053	259,459
Derivatives designated as hedging instruments	6,655	27,946
Commodity derivatives not designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	12,949	44,778
Derivatives designated as hedging instruments	$ 11,698	$ 47,616