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Jointly owned facilities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Jointly owned electricity generation plant | Big Stone Station |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Proportionate ownership share | 22.70% |
Utility plant in service | $ 155,967 | $ 152,836
Construction work in progress | 104 | 518
Less accumulated depreciation | 45,435 | 46,266
Utility plant in services net | $ 110,636 | 107,088
Jointly owned electricity generation plant | Coyote Station |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Proportionate ownership share | 25.00% |
Utility plant in service | $ 159,784 | 160,235
Construction work in progress | 323 | 21
Less accumulated depreciation | 108,852 | 107,638
Utility plant in services net | $ 51,255 | 52,618
Jointly owned electricity generation plant | Wygen III |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Proportionate ownership share | 25.00% |
Utility plant in service | $ 66,101 | 67,869
Construction work in progress | 232 | 112
Less accumulated depreciation | 10,038 | 10,482
Utility plant in services net | $ 56,295 | 57,499
Jointly Owned Electricity Transmission and Distribution System | BSSE |  |
Jointly Owned Utility Plant Interests [Line Items] |  |
Proportionate ownership share | 50.00% |
Utility plant in service | $ 107,442 | 105,767
Construction work in progress | 0 | 0
Less accumulated depreciation | 2,682 | 1,232
Utility plant in services net | $ 104,760 | $ 104,535

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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://fasb.org/us-gaap/role/ref/legacyRef -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=115995737&loc=d3e659983-123033 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=115995737&loc=d3e659983-123033 + Details
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- Definition The period end amount of construction work in progress at a jointly owned electricity generation plant. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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=115995737&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://fasb.org/us-gaap/role/ref/legacyRef -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=115995737&loc=d3e659983-123033 + Details
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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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=115995737&loc=d3e659983-123033 + Details
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