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Note 14 - Income Tax Expense - Effective Income Tax Rate (Details) (Parentheticals)	3 Months Ended		6 Months Ended
	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