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Note 15 - Asset Retirement Obligations (AROs) (Details Textual) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Asset Retirement Obligation, Liabilities Incurred | $ 0 | $ 0
Otter Tail Power Company [Member] |  |
Asset Retirement Obligation, Liabilities Incurred | $ 0 |
North Dakota 1 [Member] |  |
Number of Wind Turbines | 92 |

X
- Definition Represents the number of wind turbines as of a specified date. + References No definition available. + Details
Name: | ottr_NumberOfWindTurbines
Namespace Prefix: | ottr_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
Name: | us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=ottr_OtterTailPowerCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_StatementGeographicalAxis=ottr_NorthDakota1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |