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Jointly owned facilities (Details) - Jointly owned electricity generation plant - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Big Stone Station [Member] |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Utility plant in service | $ 158,084 | $ 157,144
Less accumulated depreciation | 51,740 | 49,568
Utility plant in services net | 106,344 | 107,576
Coyote Station [Member] |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Utility plant in service | 155,287 | 156,334
Less accumulated depreciation | 103,897 | 105,928
Utility plant in services net | 51,390 | 50,406
Wygen III [Member] |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Utility plant in service | 65,065 | 66,251
Less accumulated depreciation | 7,652 | 7,550
Utility plant in services net | $ 57,413 | $ 58,701

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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 + Details
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Balance Type: | debit
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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 + 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 + Details
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- Details
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- Details
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