Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Asset and liability derivatives
Derivatives designated as hedging instruments	$ 230,139	$ 304,237
Derivatives designated as hedging instruments	36,471	75,562
Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	230,139	304,237
Derivatives designated as hedging instruments	36,471	75,562
Commodity derivatives designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	206,776	259,459
Derivatives designated as hedging instruments	12,301	27,946
Commodity derivatives not designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Derivatives designated as hedging instruments	23,363	44,778
Derivatives designated as hedging instruments	$ 24,170	$ 47,616