Supplemental Information on Oil and Gas Producing Activities (Unaudited) (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Capitalized Costs Relating to Oil and Gas Producing Activities:
Properties not being amortized	$ 188,208	$ 178,653	$ 25,504
Properties being amortized(1)	195,329	182,820	170,457
Total capitalized costs	262,123	228,867	195,961
Less accumulated depreciation, depletion, and amortization	(155,681)	(129,166)	(99,277)
Net capitalized costs	$ 106,442	$ 99,701	$ 96,684