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Note 5 - Common Shares and Earnings Per Share (Details Textual) - Distribution Agreement With KeyBanc [Member] | Nov. 08, 2019 USD ($)
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Sale of Stock, Maximum Aggregate Sales Price | $ 75,000,000
Sale of Stock, Commission Percent | 2.00%

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- Definition The maximum commission percent for sale of stock. + References No definition available. + Details
Name: | ottr_SaleOfStockCommissionPercent
Namespace Prefix: | ottr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The maximum aggregate sales price for a sale of stock. + References No definition available. + Details
Name: | ottr_SaleOfStockMaximumAggregateSalesPrice
Namespace Prefix: | ottr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=ottr_DistributionAgreementWithKeybancMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |