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Note 9 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions |  |  | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jul. 30, 2020 | Oct. 01, 2019 | Jun. 30, 2020 | Dec. 31, 2019
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission[Member] |  |  |  |
Estimated Liability of Refund Obligation |  |  | $ 2.9 |
Otter Tail Power Company [Member] | Hoot Lake and Seven Hydroelectric Plants [Member] | Subsequent Event [Member] |  |  |  |
Reduction in Depreciable Lives, Increase in Depreciation Expense | $ 1.4 |  |  |
Otter Tail Power Company [Member] | Construction Programs [Member] |  |  |  |
Contract Expiration Year |  |  | 2022 | 2021
Long-term Purchase Commitment, Amount |  |  | $ 185.0 | $ 317.0
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 |  |  | $ 9.9 |
T.O Plastics [Member] | Supply Agreement [Member] |  |  |  |
Long-term Purchase Commitment, Period (Year) |  | 6 years |  |
Long-term Purchase Commitment, Minimum Annual Amount |  | $ 1.9 |  |

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 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_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
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: |