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Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Summary of Significant Accounting Policies |  |  |
Realization of brokered natural gas margin | $ 5.5 | $ 7.4 | $ 8.8
Total sales |  |  |
Credit Risk |  |  |
Threshold percentage which the entity uses for disclosure |  | 10.00% |
Customer |  |  |
Credit Risk |  |  |
Number of customers | 3 |  |
Customer one |  |  |
Credit Risk |  |  |
Percentage of total sales | 18.00% |  | 11.00%
Number of customers |  |  | 1
Customer two |  |  |
Credit Risk |  |  |
Percentage of total sales | 12.00% |  |
Customer three |  |  |
Credit Risk |  |  |
Percentage of total sales | 10.00% |  |

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- Definition Margin related to brokering natural gas, a result of buying and selling natural gas in back-to-back transactions, during the reporting period. + References No definition available. + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13537-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13531-108611 + Details
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