Financial Instruments and Concentrations of Risk (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value Disclosures [Abstract]
Realized and unrealized gains (losses) from commodity derivatives	$ 149	$ 2,153	$ (11,969)

Net Increase or Decrease in the fair value of the derivative or group of derivatives included in earnings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959

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