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Note 7 - Retained Earnings and Dividend Restriction (Details Textual) - USD ($)	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Capitalization, Long-term Debt and Equity, Total	$ 1,616,344,000	$ 1,471,063,000
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Minimum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	47.50%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Maximum [Member]
Public Utilities, Requested Equity Capital Structure, Percentage	58.10%
Otter Tail Power Company [Member] | Maximum Allowed Capitalization Under Minnesota Capital Structure Filing [Member]
Equity to Total Capitalization Ratio	52.80%
Net Assets Restricted from Distribution	$ 619,000,000
Capitalization, Long-term Debt and Equity, Total	$ 1,704,607,000

X

- Definition

Represents the ratio of Equity to total capitalization, which includes as equity minority interest and preferred stock as well as all of the components of common equity as presented on the balance sheet. Total capitalization is calculated as equity plus short-term debt plus long-term debt as presented on the balance sheet.

+ References

No definition available.

+ Details

Name:	ottr_EquityToTotalCapitalizationRatio
Namespace Prefix:	ottr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents net assets restricted from distribution.

+ References

No definition available.

+ Details

Name:	ottr_NetAssetsRestrictedFromDistribution
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument.

+ References

No definition available.

+ Details

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

X

- Definition

Percentage of public utility's requested equity to capital, including debt and equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedEquityCapitalStructurePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	dei_LegalEntityAxis=ottr_OtterTailPowerCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ottr_MinnesotaPublicUtilitiesCommissionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ottr_MaximumAllowedCapitalizationUnderMinnesotaCapitalStructureFilingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: