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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
	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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No definition available.

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No definition available.

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- Definition

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No definition available.

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No definition available.

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- Definition

Federally approved transmission tariff return on equity premium previously credited to customers through the Minnesota Transmission Cost Recovery Rider that is not eligible for inclusion in the rider and is now subject to recovery through the rider.

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No definition available.

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Represents the amount of costs associated with the Turnkey Engineering, Procurement and Construction Services Agreement.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

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