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Note 10 - Short-term and Long-term Borrowings (Details Textual) - Otter Tail Power Company [Member] - USD ($) |  | 9 Months Ended |
--- | --- | --- | ---
Aug. 20, 2020 | Sep. 30, 2020 | Sep. 12, 2019
The 2019 Note Purchase Agreement [Member] |  |  |
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount |  | 10.00% |
Debt Instrument, Redemption Price, Percentage |  | 100.00% |
Interest-bearing Debt, Maximum Percentage of Total Capitalization |  | 60.00% |
Priority Debt to Total Capitalization |  | 20.00% |
The 2019 Note Purchase Agreement [Member] | Senior Unsecured Note [Member[ |  |  |
Debt Instrument, Face Amount |  |  | $ 175,000,000
Series 2019 A Senior Unsecured Note Due October 10, 2029 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.07%
Series 2019 B Senior Unsecured Note Due October 10, 2039 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 26,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.52%
Series 2019 C Senior Unsecured Note Due October 10, 2049 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 64,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.82%
Series 2020 A Senior Unsecured Notes Due February 25, 2030 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.22%
Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 40,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.22%
Proceeds from Issuance of Debt | $ 75,000,000 |  |
Series 2020 C Senior Unsecured Notes Due February 25, 2040 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.62%
Series 2020 D Senior Unsecured Notes Due February 25, 2050 [Member] |  |  |
Debt Instrument, Face Amount |  |  | $ 15,000,000
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.92%

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