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Percentage of ownership interests (Details) | Dec. 31, 2012
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Big Stone Station: |
Jointly owned utility plant interests [Line Items] |
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates. | 22.70%
Coyote Station: |
Jointly owned utility plant interests [Line Items] |
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates. | 25.00%
Wygen III: |
Jointly owned utility plant interests [Line Items] |
Company's percentage ownership interests in the assets, liabilities and expenses of the joint ventures or affiliates. | 25.00%

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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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