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Note 9 - Commitments and Contingencies (Details Textual) - USD ($)			1 Months Ended	9 Months Ended	12 Months Ended
	Jul. 30, 2020	Oct. 01, 2019	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission[Member]
Estimated Liability of Refund Obligation			$ 2,800	$ 2,800
Otter Tail Power Company [Member] | Hoot Lake and Seven Hydroelectric Plants [Member]
Reduction in Depreciable Lives, Increase in Depreciation Expense	$ 1,400,000
Otter Tail Power Company [Member] | Transmission Obligation [Member]
Transmission Obligation, Annual Payment			$ 675,000
Otter Tail Power Company [Member] | Construction Programs [Member]
Contract Expiration Year				2022	2021
Long-term Purchase Commitment, Amount				$ 58,000,000	$ 317,000,000
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year				2040
Otter Tail Power Company [Member] | OTP Land Easements Through 2034 [Member]
Long-term Purchase Commitment, Amount				$ 9,700,000
T.O Plastics [Member] | Supply Agreement [Member]
Long-term Purchase Commitment, Period (Year)		6 years
Long-term Purchase Commitment, Minimum Annual Amount		$ 1,900,000

X

- Definition

Represents the contract expiration year.

+ References

No definition available.

+ Details

Name:	ottr_ContractExpirationYear
Namespace Prefix:	ottr_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This element represents amount of estimated liability of refund obligation.

+ References

No definition available.

+ Details

Name:	ottr_EstimatedLiabilityOfRefundObligation
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The minimum annual amount of a long-term purchase commitment.

+ References

No definition available.

+ Details

Name:	ottr_LongtermPurchaseCommitmentMinimumAnnualAmount
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of increase in depreciation expense from the reduction in depreciable life.

+ References

No definition available.

+ Details

Name:	ottr_ReductionInDepreciableLivesIncreaseInDepreciationExpense
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of annual payment related to transmission obligation.

+ References

No definition available.

+ Details

Name:	ottr_TransmissionObligationAnnualPayment
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The minimum amount the entity agreed to spend under the long-term purchase commitment.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermPurchaseCommitmentPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	dei_LegalEntityAxis=ottr_OtterTailPowerCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ottr_FederalEnergyRegulatoryCommission1111Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ottr_HootLakeAndSevenHydroelectricPlantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=ottr_TransmissionObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ottr_ConstructionProgramsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ottr_CoalPurchaseCommitments2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ottr_OtpLandEasementsThrough2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=ottr_TOPlasticsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ottr_SupplyAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: