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Note 3 - Rate and Regulatory Matters - Summary of Revenues Recorded Under Rate Riders (Details) - Otter Tail Power Company [Member] - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Revenues recorded under rate riders | $ 26,092 | $ 35,962 | $ 47,850
Conservation Improvement Program Costs and Incentives [Member] | Minnesota1 [Member] |  |  |
Revenues recorded under rate riders | 8,271 | 8,127 | 6,008
Conservation Improvement Program Costs and Incentives [Member] | South Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 851 | 628 | 598
Renewable Resource Adjustment [Member] | Minnesota1 [Member] |  |  |
Revenues recorded under rate riders | 5,513 | 3,067 | (196)
Renewable Resource Adjustment [Member] | North Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 230 | 8,529 | 7,620
Transmission Cost Recovery Rider [Member] | Minnesota1 [Member] |  |  |
Revenues recorded under rate riders | 2,497 | (2,039) | 2,973
Transmission Cost Recovery Rider [Member] | North Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 5,292 | 7,016 | 8,729
Transmission Cost Recovery Rider [Member] | South Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 2,165 | 1,664 | 1,843
Environmental Cost Recovery Rider [Member] | Minnesota1 [Member] |  |  |
Revenues recorded under rate riders | (1) | (24) | 8,148
Environmental Cost Recovery Rider [Member] | North Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 550 | 7,318 | 9,782
Environmental Cost Recovery Rider [Member] | South Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | (29) | 1,676 | 2,345
Generation Cost Recovery [Member] | North Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | 878 | 0 | 0
Phase-In Rate Plan [Member] | South Dakota 1 [Member] |  |  |
Revenues recorded under rate riders | $ (125) | $ 0 | $ 0

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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(1)) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 + Details
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