Note 10 - Short-term and Long-term Borrowings - Breakdown of Assignment of Consolidated Short-term and Long-term Debt Outstanding (Details) (Parentheticals)	3 Months Ended	12 Months Ended
	Mar. 31, 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