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Note 1 - Summary of Significant Accounting Policies (Details Textual) - Variable Interest Entity, Not Primary Beneficiary [Member] $ in Millions	6 Months Ended
	Jun. 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%

X

- 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

X

- 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

X

- Details

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