Represents the contract expiration year.

No definition available.

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

- 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

- 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

- 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

- 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

- 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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

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