Fair Value Measurements (Narrative) (Details)	3 Months Ended
Mar. 31, 2013
Fair Value Disclosures [Abstract]
Fair Value Measurements, Significant Assumptions	Volatilities used in these valuations ranged from 9% to 25% depending on the maturity dates, underlying commodities and strike prices of the option contracts.

Description of significant assumptions used by management to measure fair value.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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