Derivative Instruments (Effect of the Derivatives Not Designated as Hedges Results of Operations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Realized Gain Loss On Derivatives	$ 0	$ 0	$ (11,753)
[NondesignatedMember] | [OilAndGasCommodityDerivativesMember]
Derivative [Line Items]
Realized Gain Loss On Derivatives		0	(11,753)
Unrealized Gain (Loss) on Derivatives	(3,524)	(3,050)	10,943
Derivative, Nonmonetary Notional Amount	(3,524)	(3,050)	(810)
[NondesignatedMember] | [OilAndGasCommodityDerivativesMember] | [OilReservesMember]
Derivative [Line Items]
Realized Gain Loss On Derivatives	0
[NondesignatedMember] | [EmbeddedDerivativeFinancialInstrumentsMember]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 8,665	$ 11,428	$ 1,348