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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) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Short-Term Debt |  | $ 48,600 | $ 6,000
Long-Term Debt |  | 767,215 | 692,351
Less: Current Maturities net of Unamortized Debt Issuance Costs |  | 215 | 183
Unamortized Long-Term Debt Issuance Costs |  | 2,726 | 2,587
Total Long-Term Debt net of Unamortized Debt Issuance Costs |  | 764,274 | 689,581
Total Short-Term and Long-Term Debt (with current maturities) |  | 813,089 | 695,764
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member] |  |  |
Long-Term Debt |  | 80,000 | 80,000
Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 [Member] |  |  |
Long-Term Debt |  | 140,000 | 140,000
Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 [Member] |  |  |
Long-Term Debt |  | 30,000 | 30,000
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] |  |  |
Long-Term Debt |  | 42,000 | 42,000
Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 [Member] |  |  |
Long-Term Debt |  | 60,000 | 60,000
Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 [Member] |  |  |
Long-Term Debt | [1] | 10,000 | 10,000
Senior Unsecured Notes 3.22%, Series 2020A, due February 25, 2030 [Member] |  |  |
Long-Term Debt |  | 10,000 |
Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] |  |  |
Long-Term Debt | [1] | 40,000 |
Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 [Member] |  |  |
Long-Term Debt |  | 50,000 | 50,000
Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] |  |  |
Long-Term Debt |  | 26,000 | 26,000
Senior Unsecured Notes 3.62%. Series 2020C, due February 25, 2040 [Member] |  |  |
Long-Term Debt |  | 10,000 |
Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 [Member] |  |  |
Long-Term Debt |  | 90,000 | 90,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] |  |  |
Long-Term Debt |  | 100,000 | 100,000
Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 [Member] |  |  |
Long-Term Debt |  | 64,000 | 64,000
Senior Unsecured Notes 3.92%, Series 2020D, due February 25, 2050 [Member] |  |  |
Long-Term Debt |  | 15,000 |
PACE Note, 2.54%, due March 18, 2021 [Member] |  |  |
Long-Term Debt |  | 215 | 351
Parent Company [Member] |  |  |
Short-Term Debt |  | 48,600 | 6,000
Long-Term Debt |  | 80,215 | 80,351
Less: Current Maturities net of Unamortized Debt Issuance Costs |  | 215 | 183
Unamortized Long-Term Debt Issuance Costs |  | 318 | 356
Total Long-Term Debt net of Unamortized Debt Issuance Costs |  | 79,682 | 79,812
Total Short-Term and Long-Term Debt (with current maturities) |  | 128,497 | 85,995
Parent Company [Member] | The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member] |  |  |
Long-Term Debt |  | 80,000 | 80,000
Parent Company [Member] | PACE Note, 2.54%, due March 18, 2021 [Member] |  |  |
Long-Term Debt |  | 215 | 351
Otter Tail Power Company [Member] |  |  |
Long-Term Debt |  | 687,000 | 612,000
Unamortized Long-Term Debt Issuance Costs |  | 2,408 | 2,231
Total Long-Term Debt net of Unamortized Debt Issuance Costs |  | 684,592 | 609,769
Total Short-Term and Long-Term Debt (with current maturities) |  | 684,592 | 609,769
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.63%, Series 2011A, due December 1, 2021 [Member] |  |  |
Long-Term Debt |  | 140,000 | 140,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.15%, Series 2007B, due August 20, 2022 [Member] |  |  |
Long-Term Debt |  | 30,000 | 30,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] |  |  |
Long-Term Debt |  | 42,000 | 42,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.68%, Series 2013A, due February 27, 2029 [Member] |  |  |
Long-Term Debt |  | 60,000 | 60,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.07%, Series 2019A, due October 10, 2029 [Member] |  |  |
Long-Term Debt | [1] | 10,000 | 10,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.22%, Series 2020A, due February 25, 2030 [Member] |  |  |
Long-Term Debt |  | 10,000 |
Otter Tail Power Company [Member] | Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] |  |  |
Long-Term Debt | [1] | 40,000 |
Otter Tail Power Company [Member] | Senior Unsecured Notes 6.47%, Series 2007D, due August 20, 2037 [Member] |  |  |
Long-Term Debt |  | 50,000 | 50,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member] |  |  |
Long-Term Debt |  | 26,000 | 26,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.62%. Series 2020C, due February 25, 2040 [Member] |  |  |
Long-Term Debt |  | 10,000 |
Otter Tail Power Company [Member] | Senior Unsecured Notes 5.47%, Series 2013B, due February 27, 2044 [Member] |  |  |
Long-Term Debt |  | 90,000 | 90,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member] |  |  |
Long-Term Debt |  | 100,000 | 100,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.82%, Series 2019C, due October 10, 2049 [Member] |  |  |
Long-Term Debt |  | 64,000 | $ 64,000
Otter Tail Power Company [Member] | Senior Unsecured Notes 3.92%, Series 2020D, due February 25, 2050 [Member] |  |  |
Long-Term Debt |  | $ 15,000 |
[1] | Holder is COBANK, a cooperative lender. Interest payments are subject to cash credits which may result in a lower effective interest rate.

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