Derivatives (Summary of Derivative Income/Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ 759	$ 563	$ (1,977)	$ 5,520
Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting	433	(619)	(5,481)	111
Realized losses on derivatives	308	1,227	3,547	5,355
Ineffective portion of derivatives qualifying for hedge accounting	18	(45)	(43)	(127)
Put option premium mark to market	$ 0	$ 0	$ 0	$ 181