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Note 1 - Summary of Significant Accounting Policies - OTP's Ownership Interests in Jointly Owned Facilities (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Big Stone Plant [Member] |  |  |  |
Electric Plant in Service |  | $ 337,197 | $ 336,051 |
Construction Work in Progress |  | 384 | 361 |
Accumulated Depreciation |  | (98,654) | (92,007) |
Net Plant |  | $ 238,927 | $ 244,405 |
Big Stone Plant [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 53.90% | 53.90% |
Coyote Station [Member] |  |  |  |
Electric Plant in Service |  | $ 184,493 | $ 177,713 |
Construction Work in Progress |  | 83 | 2,588 |
Accumulated Depreciation |  | (108,248) | (100,997) |
Net Plant |  | $ 76,328 | $ 79,304 |
Coyote Station [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 35.00% | 35.00% |
Big Stone South Ellendale Multi Value Project [Member] |  |  |  |
Electric Plant in Service | [1] | $ 106,343 | $ 0 |
Construction Work in Progress | [1] | 0 | 106,490 |
Accumulated Depreciation | [1] | (819) | 0 |
Net Plant | [1] | $ 105,524 | $ 106,490 |
Big Stone South Ellendale Multi Value Project [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage | [1] | 50.00% | 50.00% |
Fargo Project [Member] |  |  |  |
Electric Plant in Service |  | $ 78,184 | $ 78,184 |
Construction Work in Progress |  | 0 | 0 |
Accumulated Depreciation |  | (7,011) | (5,891) | [1]
Net Plant |  | $ 71,173 | $ 72,293 |
Fargo Project [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 14.20% | 14.20% |
Big Stone South Brookings Multi Value Project [Member] |  |  |  |
Electric Plant in Service |  | $ 53,036 | $ 53,235 |
Construction Work in Progress |  | 0 | (150) |
Accumulated Depreciation |  | (2,016) | (1,264) |
Net Plant |  | $ 51,020 | $ 51,821 |
Big Stone South Brookings Multi Value Project [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 50.00% | 50.00% |
Brookings Project [Member] |  |  |  |
Electric Plant in Service |  | $ 26,286 | $ 26,281 |
Construction Work in Progress |  | 0 | 0 |
Accumulated Depreciation |  | (2,086) | (1,713) |
Net Plant |  | $ 24,200 | $ 24,568 |
Brookings Project [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 4.80% | 4.80% |
Bemidji Project [Member] |  |  |  |
Electric Plant in Service |  | $ 16,331 | $ 16,331 |
Construction Work in Progress |  | 0 | 0 |
Accumulated Depreciation |  | (233) | (2,091) |
Net Plant |  | $ 16,098 | $ 14,240 |
Bemidji Project [Member] | Otter Tail Power Company [Member] |  |  |  |
Ownership Percentage |  | 14.80% | 14.80% |
[1] | Midcontinent Independent System Operator, Inc. (MISO) Multi-Value Project (MVP) designation provides for a return on invested funds while under construction under the MISO Open Access Transmission, Energy and Operating Reserve Markets Tariff (MISO Tariff).

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- Definition The period end amount of construction work in progress, or the deficit in the construction work in progress, at a jointly owned electricity generation plant. + References No definition available. + Details
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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 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 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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