Fair Value Measurement (Recurring Fair Value Measurements) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Interest Rate Swaps [Member] | Swap [Member]
Financial assets (liabilities)		$ (1,614)
Interest Rate Swaps [Member] | Swap [Member] | Level 2 [Member]
Financial assets (liabilities)		0
Interest Rate Swaps [Member] | Swap [Member] | Level 3 [Member]
Financial assets (liabilities)		(1,614)
Commodity Derivative [Member]
Financial assets (liabilities)	(3,602)	(9,086)
Commodity Derivative [Member] | Level 2 [Member]
Financial assets (liabilities)	(15,351)	(19,954)
Commodity Derivative [Member] | Level 3 [Member]
Financial assets (liabilities)	$ 11,749	$ 10,868