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Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)
Income Tax Disclosure [Abstract]
Period for unrecognized tax benefits not expected change	12 months
Unrecognized tax benefits, interest on income taxes accrued	$ 0

X

- Definition

Represents the period over which there is no significant expected change in the unrecognized tax benefits.

+ References

No definition available.

+ Details

Name:	ottr_PeriodForUnrecognizedTaxBenefitsNotExpectedChange
Namespace Prefix:	ottr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of interest expense accrued for an underpayment of income taxes.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant