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Note 12 - Fair Value of Financial Instruments (Details Textual) - London Interbank Offered Rate (LIBOR) [Member] | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Otter Tail Corporation Credit Agreement [Member] |  |
Debt Instrument, Basis Spread on Variable Rate | 1.50% | 1.50%
OTP Credit Agreement [Member] |  |
Debt Instrument, Basis Spread on Variable Rate | 1.25% | 1.25%

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DebtInstrumentAxis=ottr_OtterTailCorporationCreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=ottr_OTPCreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |