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Note 12 - Fair Value of Financial Instruments (Details Textual) | 6 Months Ended | 12 Months Ended
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Jun. 30, 2020 | Dec. 31, 2019
Otter Tail Corporation Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |
Debt Instrument, Basis Spread on Variable Rate | 1.50% | 1.50%

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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
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Namespace Prefix: | us-gaap_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |