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v3.19.3.a.u2

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: |