Derivative Financial Instruments (Details 5) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Offsetting of derivative liabilities
Gross Amounts of Recognized Liabilities	$ 21,789	$ 23,603
Net Amounts of Liabilities Presented in the Balance Sheet	21,789	23,603
Gross Amounts Not Offset in the Balance Sheet	(21,789)	(23,603)
Commodity derivatives
Offsetting of derivative liabilities
Gross Amounts of Recognized Liabilities	15,877	17,664
Net Amounts of Liabilities Presented in the Balance Sheet	15,877	17,664
Gross Amounts Not Offset in the Balance Sheet	(15,877)	(17,664)
Interest rate derivatives
Offsetting of derivative liabilities
Gross Amounts of Recognized Liabilities	5,912	5,939
Net Amounts of Liabilities Presented in the Balance Sheet	5,912	5,939
Gross Amounts Not Offset in the Balance Sheet	$ (5,912)	$ (5,939)