Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-Term Debt	$ 6,000	$ 18,599
Long-Term Debt	692,351	592,523
Less: Current Maturities net of Unamortized Debt Issuance Costs	183	172
Unamortized Long-Term Debt Issuance Costs	(2,587)	(2,349)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	689,581	590,002
Total Short-Term and Long-Term Debt (with current maturities)	695,764	608,773
Unamortized Long-Term Debt Issuance Costs	2,587	2,349
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
Senior Unsecured Notes 6.15%, Series B, 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 A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
Senior Unsecured Notes 3.07%, Series A, Due October 10, 2029 [Member]
Long-Term Debt	10,000
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-Term Debt	26,000
Senior Unsecured Notes 4.07%, Series 2018A, Due February 7, 2048 [Member]
Long-Term Debt	100,000	100,000
Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	351	523
Senior Unsecured Notes 3.82%, Sericies C, Due October 10, 2049 [Member]
Long-Term Debt	64,000
Otter Tail Power Company [Member]
Short-Term Debt		9,384
Long-Term Debt	612,000	512,000
Unamortized Long-Term Debt Issuance Costs	(2,231)	(1,942)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	609,769	510,058
Total Short-Term and Long-Term Debt (with current maturities)	609,769	519,442
Unamortized Long-Term Debt Issuance Costs	2,231	1,942
Otter Tail Power Company [Member] \| Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]
Long-Term Debt	140,000	140,000
Otter Tail Power Company [Member] \| Senior Unsecured Notes 6.15%, Series B, 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 A, Due February 27, 2029 [Member]
Long-Term Debt	60,000	60,000
Otter Tail Power Company [Member] \| Senior Unsecured Notes 3.07%, Series A, Due October 10, 2029 [Member]
Long-Term Debt	10,000
Otter Tail Power Company [Member] \| Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]
Long-Term Debt	50,000	50,000
Otter Tail Power Company [Member] \| Senior Unsecured Notes 5.47%, Series B, Due February 27, 2044 [Member]
Long-Term Debt	90,000	90,000
Otter Tail Power Company [Member] \| Senior Unsecured Notes 3.52%, Series 2019B, Due October 10, 2039 [Member]
Long-Term Debt	26,000	0
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%, Sericies C, Due October 10, 2049 [Member]
Long-Term Debt	64,000	0
Parent Company [Member]
Short-Term Debt	6,000	9,215
Long-Term Debt	80,351	80,523
Less: Current Maturities net of Unamortized Debt Issuance Costs	183	172
Unamortized Long-Term Debt Issuance Costs	(356)	(407)
Total Long-Term Debt net of Unamortized Debt Issuance Costs	79,812	79,944
Total Short-Term and Long-Term Debt (with current maturities)	85,995	89,331
Unamortized Long-Term Debt Issuance Costs	356	407
Parent Company [Member] \| The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]
Long-Term Debt	80,000	80,000
Parent Company [Member] \| Partnership in Assisting Community Expansion (PACE) Note, 2.54%, due March 18, 2021 [Member]
Long-Term Debt	$ 351	$ 523

Amount, after accumulated amortization, of debt discount.

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