Derivatives (Summary of Derivative Income Expense) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting			$ (22,405)
Realized losses on derivatives			26,542
Loss on derivatives	1,006	7,776	9,100
Commodity Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting	(3,473)	726	1,003
Realized losses on derivatives	4,514	7,015	7,955
Ineffective portion of derivatives qualifying for hedge accounting	(35)	(158)	142
Put Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Put option premium mark to market	0	193	0
Total Gain Loss on Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivatives	$ 1,006	$ 7,776	$ 9,100