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Note 14 - Income Tax Expense - Effective Income Tax Rate (Details) (Parentheticals) | 3 Months Ended | 6 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Composite Federal and State Statutory Rate | 26.00% | 26.00% | 26.00% | 26.00%

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- Definition Represents the effective income tax rate reconciliation, at federal and state statutory income tax rate. + References No definition available. + Details
Name: | ottr_EffectiveIncomeTaxRateReconciliationAtFederalAndStateStatutoryIncomeTaxRate
Namespace Prefix: | ottr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration