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Note 9 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Oct. 01, 2019	Aug. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
Otter Tail Power Company [Member]
Estimated Maximum Potential Asset Impairment Charge		$ 1.1
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation			$ 3.0
Otter Tail Power Company [Member] | Construction Programs [Member]
Contract Expiration Year			2021	2021
Long-term Purchase Commitment, Amount			$ 250.0	$ 317.0
Otter Tail Power Company [Member] | Coal Purchase Commitments 2 [Member]
Contract Expiration Year			2040
Otter Tail Power Company [Member] | Coal Purchase Commitments 3 [Member]
Contract Expiration Year			2020
Otter Tail Power Company [Member] | OTP Land Easements [Member]
Long-term Purchase Commitment, Amount			$ 10.1
T.O Plastics [Member] | Supply Agreement [Member]
Long-term Purchase Commitment, Amount	$ 1.9
Long-term Purchase Commitment, Period (Year)	6 years

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

Estimated maximum potential asset impairment charge related to state commission mandated reduction in remaining life of asset.

+ References

No definition available.

+ Details

Name:	ottr_EstimatedMaximumPotentialAssetImpairmentCharge
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_FederalEnergyRegulatoryCommissionMember
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_CoalPurchaseCommitments3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ottr_OTPLandEasementsMember
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: