Derivatives (Summary of Derivative Income Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss on derivatives	$ (445)	$ (4,905)	$ 27	$ (2,736)
Commodity Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting	388	(7,095)	(244)	(5,913)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(802)	2,213	314	3,238
Ineffective portion of derivatives qualifying for hedge accounting	$ (31)	$ (23)	$ (43)	$ (61)