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Jointly owned facilities (Details) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Big Stone Station: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | $ 157,761 | $ 64,283
Less accumulated depreciation | 48,242 | 43,043
Utility plant in services net | 109,519 | 21,240
Coyote Station: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | 140,895 | 138,810
Less accumulated depreciation | 94,755 | 94,443
Utility plant in services net | 46,140 | 44,367
Wygen III: |  |
Jointly owned facilities [Line Items] |  |
Utility plant in service | 65,023 | 65,597
Less accumulated depreciation | 6,788 | 5,928
Utility plant in services net | $ 58,235 | $ 59,669

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- Definition The period end amount owned by the reporting utility of a jointly owned utility plant in service. + 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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- Definition The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation 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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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation 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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