Derivatives (Summary of Derivative Income Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivatives	$ 472	$ 2,169
Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting	(631)	1,181
Realized losses on derivatives	1,115	1,026
Ineffective portion of derivatives qualifying for hedge accounting	(12)	(38)
Total Gain Loss on Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss on derivatives	$ 472	$ 2,169