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Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) (Parentheticals) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2020 | Dec. 31, 2019
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.55% | 3.55%
Long-Term Debt, Due Date | Dec. 15, 2026 | Dec. 15, 2026
Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.63% | 4.63%
Long-Term Debt, Due Date | Dec. 01, 2021 | Dec. 01, 2021
Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.15% | 6.15%
Long-Term Debt, Due Date | Aug. 20, 2022 | Aug. 20, 2022
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.37% | 6.37%
Long-Term Debt, Due Date | Aug. 20, 2027 | Aug. 20, 2027
Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.68% | 4.68%
Long-Term Debt, Due Date | Feb. 27, 2029 | Feb. 27, 2029
Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.07% | 3.07%
Long-Term Debt, Due Date | Oct. 10, 2029 | Oct. 10, 2029
Senior Unsecured Notes 3.22%, Series 2020A, due February 25, 2030 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.22% |
Long-Term Debt, Due Date | Feb. 25, 2030 |
Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.47% | 6.47%
Long-Term Debt, Due Date | Aug. 20, 2037 | Aug. 20, 2037
Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.52% | 3.52%
Long-Term Debt, Due Date | Oct. 10, 2039 | Oct. 10, 2039
Senior Unsecured Notes 3.62%. Series 2020C, due February 25, 2040 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.62% |
Long-Term Debt, Due Date | Feb. 25, 2040 |
Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.47% | 5.47%
Long-Term Debt, Due Date | Feb. 27, 2044 | Feb. 27, 2044
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.07% | 4.07%
Long-Term Debt, Due Date | Feb. 07, 2048 | Feb. 07, 2048
Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.82% | 3.82%
Long-Term Debt, Due Date | Oct. 10, 2049 | Oct. 10, 2049
Senior Unsecured Notes 3.92%, Series 2020D, due February 25, 2050 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.92% |
Long-Term Debt, Due Date | Feb. 25, 2050 |
PACE Note, 2.54%, due March 18, 2021 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 2.54% | 2.54%
Long-Term Debt, Due Date | Mar. 18, 2021 | Mar. 18, 2021
Parent Company [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.55% | 3.55%
Long-Term Debt, Due Date | Dec. 15, 2026 | Dec. 15, 2026
Parent Company [Member] | PACE Note, 2.54%, due March 18, 2021 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 2.54% | 2.54%
Long-Term Debt, Due Date | Mar. 18, 2021 | Mar. 18, 2021
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.63% | 4.63%
Long-Term Debt, Due Date | Dec. 01, 2021 | Dec. 01, 2021
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.15% | 6.15%
Long-Term Debt, Due Date | Aug. 20, 2022 | Aug. 20, 2022
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.37% | 6.37%
Long-Term Debt, Due Date | Aug. 20, 2027 | Aug. 20, 2027
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.68% | 4.68%
Long-Term Debt, Due Date | Feb. 27, 2029 | Feb. 27, 2029
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.07% | 3.07%
Long-Term Debt, Due Date | Oct. 10, 2029 | Oct. 10, 2029
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.22%, Series 2020A, due February 25, 2030 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.22% |
Long-Term Debt, Due Date | Feb. 25, 2030 |
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.47% | 6.47%
Long-Term Debt, Due Date | Aug. 20, 2037 | Aug. 20, 2037
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.52% | 3.52%
Long-Term Debt, Due Date | Oct. 10, 2039 | Oct. 10, 2039
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.62%. Series 2020C, due February 25, 2040 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.62% |
Long-Term Debt, Due Date | Feb. 25, 2040 |
Otter Tail Power Company [Member] | Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.47% | 5.47%
Long-Term Debt, Due Date | Feb. 27, 2044 | Feb. 27, 2044
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.07% | 4.07%
Long-Term Debt, Due Date | Feb. 07, 2048 | Feb. 07, 2048
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.82% | 3.82%
Long-Term Debt, Due Date | Oct. 10, 2049 | Oct. 10, 2049
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.92%, Series 2020D, due February 25, 2050 [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.92% |
Long-Term Debt, Due Date | Feb. 25, 2050 |

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