Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ (4,905)	$ 1,536	$ (2,736)	$ 4,957
Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value of derivatives that do not qualify for hedge accounting	(7,095)	(825)	(5,913)	730
Realized losses on derivatives	2,213	2,368	3,238	4,128
Ineffective portion of derivatives qualifying for hedge accounting	(23)	(101)	(61)	(82)
Put option premium mark to market	$ 0	$ 94	$ 0	$ 181