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Note 3 - Rate and Regulatory Matters (Details Textual) - Otter Tail Power Company [Member]										1 Months Ended	3 Months Ended	6 Months Ended
	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 ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	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
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%
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%
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
Percentage of Requested Allowed Rate of Return on Equity						9.77%
Equity to Total Capitalization Ratio Basis for Return on Equity						52.50%
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												2,900,000
Federal Energy Regulatory Commission [Member] | First Complaint Period [member]
Contract with Customer, Refund Liability, Total												$ 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												$ 1,500,000
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
Current Return on Equity Used in Transmission Rates												10.82%
Astoria Station Project [Member]
Current Project Capitalized Cost												$ 108,000,000.0
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												$ 131,700,000

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- Definition

Annual revenue increase approved by state regulatory body in a rate proceeding.

+ References

No definition available.

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- Definition

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- Definition

The percentage of authorized return of equity.

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No definition available.

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- Definition

The capitalized cost of current projects.

+ References

No definition available.

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- Definition

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+ References

No definition available.

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- Definition

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+ References

No definition available.

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- Definition

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+ References

No definition available.

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- Definition

The estimated amount recovery of accrued financial incentives and program costs from Minnesota Conservation Improvement Programs.

+ References

No definition available.

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- Definition

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- Definition

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+ References

No definition available.

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- Definition

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+ References

No definition available.

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- Definition

Represents the amount by which the general rate revenue increase request has decreased as a result of tax reform.

+ References

No definition available.

+ Details

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- Definition

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+ References

No definition available.

+ Details

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- Definition

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+ References

No definition available.

+ Details

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- Definition

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+ References

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.

+ References

No definition available.

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- Definition

The maximum percentage of ROE used as input to determine the refund to customers.

+ References

No definition available.

+ Details

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- Definition

This element represents percentage of allowed rate of return on equity.

+ References

No definition available.

+ Details

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- Definition

This element represents the percentage of allowed rate of return on rate base.

+ References

No definition available.

+ Details

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- Definition

The percentage of any earnings above maximum ROE will refund to customer.

+ References

No definition available.

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- Definition

The percentage of excess weather-normalized revenue will refund to customer.

+ References

No definition available.

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- Definition

This element represents percentage of a requested allowed rate of return on equity in a pending rate case.

+ References

No definition available.

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- Definition

The amount of refunds of excess revenues collected, including the amounts collected under higher interim tax rates.

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No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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Amount of liability for consideration received or receivable from customer which is not included in transaction price, when consideration is expected to be refunded to customer.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested rate increase (decrease) with regulatory agency.

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