Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified		3 Months Ended
	Dec. 31, 2012	Mar. 31, 2013 Oil and gas sales [Member]	Mar. 31, 2012 Oil and gas sales [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Loss Reclassified from Accumulated OCI into Income, Effective Portion		$ 387	$ 2,102
Commodity Derivatives, fair value	$ (233)