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Oil and Gas Reserve Information - Unaudited - Standardized measure (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows [Abstract]
Future cash flows	$ 748,914	$ 1,080,392	$ 810,131
Future production costs	(220,750)	(264,219)	(223,175)
Future development costs	(121,346)	(180,846)	(144,451)
Future income taxes	(10,205)	(86,612)	(41,156)
Future net cash flows	396,613	548,715	401,349
10% annual discount	(164,218)	(244,834)	(164,974)
Standardized measure of discounted future net cash flows	$ 232,395	$ 303,881	$ 236,375

This is the amount that is derived from using a discount rate of ten percent a year to reflect the timing of the future net cash flows relating to proved oil and gas reserves.

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- Definition

The estimated future income tax expenses that will be incurred based on the future income less future expenses derived from production and sale of proved oil and gas reserves in this geographic region.

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- Definition

These amounts are the result of subtracting future development and production costs and future income tax expenses from future cash inflows for each geographic region where proved oil and gas reserves exist.

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