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Note 9 - Commitments and Contingencies (Details Textual) - USD ($)			12 Months Ended
	Oct. 01, 2019	Aug. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Land Easement Non-lease Payments Expense			$ 617,000	$ 605,000	$ 593,000
Otter Tail Power Company [Member]
Estimated Maximum Potential Asset Impairment Charge		$ 1,100,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Estimated Liability of Refund Obligation			$ 3,000,000.0
Otter Tail Power Company [Member] | Construction Programs [Member]
Contract Expiration Year			2021
Long-term Purchase Commitment, Amount			$ 317,000,000
Other Commitment, Total			$ 283,000	$ 252,000	$ 280,000
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			$ 10,200,000
T.O Plastics [Member] | Supply Agreement [Member]
Long-term Purchase Commitment, Amount	$ 1,900,000
Long-term Purchase Commitment, Period	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

Amount of non-lease land easement payments charged to rent expense in the Electric segment.

+ References

No definition available.

+ Details

Name:	ottr_LandEasementNonleasePaymentsExpense
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

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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: