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Note 3 - Rate and Regulatory Matters (Details Textual) - Otter Tail Power Company [Member] |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |
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Nov. 02, 2020 USD ($) | May 01, 2020 USD ($) | Jul. 16, 2019 USD ($) | Jul. 09, 2019 USD ($) | Jun. 11, 2019 USD ($) | May 30, 2019 USD ($) | Sep. 26, 2018 USD ($) | Apr. 20, 2018 USD ($) | Mar. 23, 2018 USD ($) | Nov. 16, 2016 USD ($) | Apr. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) | Nov. 02, 2017 USD ($) | May 01, 2017
EstimatedRecovery from MNCIP |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MISO ROE Incentive Subject to Recovery under Minnesota Transmission Cost Recovery Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MISO Incentive ROE |  |  |  |  |  |  |  |  |  |  |  | $ 2,600,000 |  | $ 2,600,000 |  |  |
Changes in Accrued Revenues Under Alternative Revenue Programs |  |  |  |  |  |  |  |  |  |  |  | 2,600,000 |  |  |  |  |
Minnesota Public Utilities Commission [Member] | The 2016 General Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Allowed Rate of Return on Rate Base |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.5056%
Percentage of Allowed Rate of Return on Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.41%
Minnesota Public Utilities Commission [Member] | The 2020 General Rate Case [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount | $ 14,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Percentage | 6.77% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of the Requested Allowed Rate of Return on Rate Base | 7.59% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Requested Allowed Rate of Return on Equity | 10.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity to Total Capitalization Ratio Basis for Return on Equity | 52.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
North Dakota Public Service Commission [Member] | The 2010 General Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Rate Revenue Increase Requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,100,000 |
North Dakota Public Service Commission [Member] | The 2017 General Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  | $ 4,600,000 |  | $ 7,100,000 |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Percentage |  |  |  |  |  |  | 3.10% |  | 4.80% |  |  |  |  |  |  |  |
Percentage of Requested Allowed Rate of Return on Equity |  |  |  |  |  |  | 9.77% |  |  |  |  |  |  |  |  |  |
Equity to Total Capitalization Ratio Basis for Return on Equity |  |  |  |  |  |  | 52.50% |  |  |  |  |  |  |  |  |  |
General Rate Revenue Increase Requested, Decrease |  |  |  |  |  |  |  |  | $ 6,000,000.0 |  |  |  |  |  |  |  |
General Rate Revenue Increase Requested, Decrease, Amount Related to Tax Reform |  |  |  |  |  |  |  |  | 4,800,000 |  |  |  |  |  |  |  |
General Rate Revenue Increase Requested, Decrease, Amount Related to Updates Other Than Tax Reform |  |  |  |  |  |  |  |  | $ 1,200,000 |  |  |  |  |  |  |  |
Refunds of Excess Revenues Collected, Amounts Collected Under Higher Interim Tax Rates |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |  |  |  |
South Dakota Public Utilities Commission [Member] | The 2018 General Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Allowed Rate of Return on Equity |  |  |  |  |  | 8.75% |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  | $ 3,300,000 |  |  |  |  |  |  |  |  |
Increase in Annual Non-fuel Rates Requested, Step One, Percentage |  |  |  |  |  |  |  | 10.10% |  |  |  |  |  |  |  |  |
Increase in Annual Non-fuel Rates Requested, Step Two, Percentage |  |  |  |  |  |  |  | 1.70% |  |  |  |  |  |  |  |  |
Contract with Customer, Liability, Revenue Recognized |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000.0 |  |  |  |
Annual Revenue Increase Approved |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |  |  |  |  |  |
Annual Revenue Requirement Shortfall Resulted from Understatement |  |  |  | $ 341,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Return of Equity |  |  |  | 8.75% |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Excess Weather-normalized Revenue, Refund to Customer |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum ROE, Input for Refund to Customer |  |  |  | 9.50% |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Any Earnings Above Maximum ROE |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Minnesota1 [Member] | Otter Tail Power Company's Minnesota Transmission Cost Recovery Rider [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred and Projected Transmission Costs Subject to Future Recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,400,000 |  |
Federal Energy Regulatory Commission [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contract with Customer, Liability, Total |  |  |  |  |  |  |  |  |  |  |  | 2,800,000 |  | 2,800,000 |  |  |
Federal Energy Regulatory Commission [Member] | First Complaint Period [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contract with Customer, Refund Liability, Total |  |  |  |  |  |  |  |  |  |  |  | 100,000 |  | $ 100,000 |  |  |
Current Return on Equity Used in Transmission Rates |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.32% |  |  |
Federal Energy Regulatory Commission [Member] | Second Complaint Period [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contract with Customer, Refund Liability, Total |  |  |  |  |  |  |  |  |  |  |  | 1,400,000 |  | $ 1,400,000 |  |  |
Current Return on Equity Used in Transmission Rates |  |  |  |  |  |  |  |  |  |  |  |  |  | 12.38% |  |  |
Federal Energy Regulatory Commission [Member] | Complaint Period Beginning September 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contract with Customer, Refund Liability, Total |  |  |  |  |  |  |  |  |  |  |  | $ 1,300,000 |  | $ 1,300,000 |  |  |
Current Return on Equity Used in Transmission Rates |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.82% |  |  |
Astoria Station Project [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current Project Capitalized Cost |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 131,600,000 |  |  |
Expanded Capacity of Projects (MW) |  |  |  |  |  |  |  |  |  |  |  |  |  | 245 |  |  |
EDF [Member] | Merricourt Project [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset Purchase Agreement, Purchase Price |  |  | $ 37,700,000 |  | $ 37,700,000 |  |  |  |  | $ 34,700,000 |  |  |  |  |  |  |
Turnkey Engineering, Procurement and Construction Services Agreement, Costs |  |  |  |  |  |  |  |  |  | $ 200,500,000 |  |  |  |  |  |  |
Current Project Capitalized Cost |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 231,100,000 |  |  |

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