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Note 11 - Pension Plan and Other Postretirement Benefits - Measurement Dates (Details) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Pension Plan [Member] |  |
Net Periodic Pension Cost | Jan. 01, 2019 | Jan. 01, 2018
End of Year Benefit Obligations | January 1, 2019 projected to December 31, 2019 | January 1, 2018 projected to December 31, 2018
Market Value of Assets | Dec. 31, 2019 | Dec. 31, 2018
Other Postretirement Benefits Plan [Member] |  |
Net Periodic Postretirement Benefit Cost | Jan. 01, 2019 | Jan. 01, 2018
End of Year Benefit Obligations | January 1, 2019 projected to December 31, 2019 | January 1, 2018 projected to December 31, 2018

X
- Definition Represents the measurement date for defined benefit plan benefit obligations. + References No definition available. + Details
Name: | ottr_DefinedBenefitPlanBenefitObligationsMeasurementDate
Namespace Prefix: | ottr_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the measurement date for the fair value of plan assets under a defined benefit plan. + References No definition available. + Details
Name: | ottr_DefinedBenefitPlanFairValueOfPlanAssetsMeasurementDate
Namespace Prefix: | ottr_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the measurement date of net benefit costs under a defined benefit plan. + References No definition available. + Details
Name: | ottr_DefinedBenefitPlanNetBenefitCostsMeasurementDate
Namespace Prefix: | ottr_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the measurement date information of net benefit costs under a defined benefit plan. + References No definition available. + Details
Name: | ottr_DefinedBenefitPlanNetBenefitCostsMeasurementDateInformation
Namespace Prefix: | ottr_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the measurement date of net post retirement benefit costs under a defined benefit plan. + References No definition available. + Details
Name: | ottr_DefinedBenefitPlanmNetPostretirementBenefitCostsMeasurementDate
Namespace Prefix: | ottr_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |