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Note 1 - Summary of Significant Accounting Policies (Details Textual) - Variable Interest Entity, Not Primary Beneficiary [Member] $ in Millions | 9 Months Ended
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Sep. 30, 2020 USD ($)
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount | $ 50
Variable Interest Entity Reporting Entity Involvement, Maximum Loss Exposure, Percentage | 35.00%

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- Definition Percentage of the reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References No definition available. + Details
Name: | ottr_VariableInterestEntityReportingEntityInvolvementMaximumLossExposurePercentage
Namespace Prefix: | ottr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121559654&loc=d3e5747-111685 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=121559654&loc=d3e5728-111685 + Details
Name: | us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_InvestmentsByConsolidatedAndNonconsolidatedEntitiesAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |