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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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