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Note 3 - Rate and Regulatory Matters (Details Textual)																1 Months Ended			3 Months Ended		12 Months Ended
	May 01, 2020 USD ($)	Nov. 21, 2019	Jul. 16, 2019 USD ($)	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 ($)	Sep. 28, 2016	Feb. 12, 2015	Jan. 05, 2015	Nov. 06, 2014	Nov. 12, 2013	Apr. 30, 2019 USD ($)	Jun. 30, 2016	Dec. 22, 2015	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Nov. 02, 2017 USD ($)	May 01, 2017
Federal Energy Regulatory Commission [Member]
Additional Refund from Reducing ROE																			$ 200,000
Otter Tail Power Company [Member] | Litigation Case with Minnesota Customers [Member]
Loss Contingency, Estimate of Possible Loss																					$ 2,600,000
Otter Tail Power Company [Member] | Subsequent Event [Member]
EstimatedRecovery from MNCIP	$ 2,700,000
Otter Tail Power Company [Member] | 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%
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2010 General Rate Case [Member]
General Rate Revenue Increase Requested																						$ 13,100,000
Otter Tail Power Company [Member] | 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
Otter Tail Power Company [Member] | 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%
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Current Return on Equity Used in Transmission Rates											10.32%				12.38%			10.32%
Proposed Reduced Return on Equity Used in Transmission Rates											9.88%	8.67%			9.15%		9.70%
Additional Incentive Basis Point														0.50%
Expected Percentage of Return on Equity											10.82%
Contract with Customer, Refund Liability, Total																			3,000,000.0
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | First Complaint Period [member]
Current Return on Equity Used in Transmission Rates											10.82%										10.38%
Additional Incentive Basis Point													0.50%
Contract with Customer, Refund Liability, Total																			1,300,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | Second Complaint Period [Member]
Current Return on Equity Used in Transmission Rates		9.88%									12.38%
Additional Incentive Basis Point													0.50%
Contract with Customer, Refund Liability, Total																			1,500,000
Otter Tail Power Company [Member] | Astoria Station Project [Member]
Current Project Capitalized Cost																			$ 78,000,000.0
Expanded Capacity of Projects (MW)																			245
Otter Tail Power Company [Member] | 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																			$ 95,800,000

X

- Definition

Represents additional incentive basis point.

+ References

No definition available.

+ Details

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- Definition

Amount of refund as a result of reducing the reasonable ROE established in the FERC's refund order.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

The amount of annual revenue requirement shortfall resulted from the understatement.

+ References

No definition available.

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- Definition

Represents the amount of purchase price under the asset purchase agreement.

+ References

No definition available.

+ Details

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- Definition

The percentage of authorized return of equity.

+ References

No definition available.

+ Details

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X

- Definition

The capitalized cost of current projects.

+ References

No definition available.

+ Details

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- Definition

Represents the current return on equity used in MISO's transmission rates.

+ References

No definition available.

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- Definition

Represents the ratio of equity to total capitalization on which the percentage of allowed rate of return on equity is based for ratemaking purposes.

+ 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

The expanded capacity of projects expressed as MW.

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No definition available.

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- Definition

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+ References

No definition available.

+ Details

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X

- Definition

Represents the amount of increase to the yearly general rate revenue requested.

+ References

No definition available.

+ Details

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- Definition

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+ References

No definition available.

+ Details

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X

- 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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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

Represents the percentage requested in the first step of a request for an annual increase in non-fuel rates.

+ References

No definition available.

+ Details

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X

- Definition

Represents the amount requested in the second step of a request for an annual increase in non-fuel rates.

+ References

No definition available.

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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

Represents the proposed reduced return on equity used in MISO's transmission rates.

+ 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.

+ References

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.

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- Definition

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.

+ References

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-Publisher FASB
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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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Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

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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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