Note 3 - Rate and Regulatory Matters - Summary of Status of Updates for Previous Two Years for Various Rate Riders (Details) - Otter Tail Power Company [Member]	3 Months Ended
Mar. 31, 2020 USD ($) kWh	Mar. 31, 2019 USD ($)
Annual Revenue	$ 10,166,000	$ 6,880,000
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 1,086,000	887,000
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2019 Incentive and Cost Recovery [Member]
Request date	May 01, 2020
Effective Date Requested or Approved	Oct. 01, 2020
Annual Revenue	$ 8,247,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00485
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2018 Incentive and Cost Recovery [Member]
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 11,926,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00710
Approval date	Dec. 27, 2019
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2017 Incentive and Cost Recovery [Member]
Effective Date Requested or Approved	Nov. 01, 2018
Annual Revenue	$ 10,283,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00600
Approval date	Oct. 04, 2018
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 715,000	641,000
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Annual Update - Scenario A [Member]
Request date	Nov. 30, 2018
Effective Date Requested or Approved	Jun. 01, 2019
Annual Revenue	$ 6,475,000
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Annual Update - Scenario B [Member]
Annual Revenue	$ 2,708,000
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2017 Rate Reset [Member]
Effective Date Requested or Approved	Nov. 01, 2017
Annual Revenue	$ (3,311,000)
Approval date	Oct. 30, 2017
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ 0	(1,000)
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Annual Update [Member]
Effective Date Requested or Approved	Dec. 01, 2018
Annual Revenue	$ 0
Approval date	Nov. 29, 2018
Minnesota1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	$ 3,254,000	1,316,000
Minnesota1 [Member] \| Renewable Resource Adjustment [Member] \| The 2018 Annual Update [Member]
Effective Date Requested or Approved	Nov. 01, 2018
Annual Revenue	$ 5,886
Rate rider rate (Kilowatt-Hour) \| kWh	0.00219
Approval date	Aug. 29, 2018
Minnesota1 [Member] \| Renewable Resource Adjustment [Member] \| The 2019 Annual Update - Revised [Member]
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 12,506,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00467
Approval date	Dec. 19, 2019
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 1,482,000	1,772,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 5,739,000
Approval date	Dec. 18, 2019
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 7,469,000
Approval date	Feb. 27, 2018
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Supplemental Update [Member]
Effective Date Requested or Approved	Feb. 01, 2019
Annual Revenue	$ 4,801,000
Approval date	Dec. 06, 2018
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ 0	575,000
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Annual Update [Member]
Effective Date Requested or Approved	Feb. 01, 2019
Annual Revenue	$ (378,000)
Approval date	Dec. 19, 2018
Rate of base	(0.31%)
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Effective Date Requested or Approved	Nov. 01, 2019
Annual Revenue	$ 0
Approval date	Oct. 22, 2019
Rate of base	0.00%
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 7,718,000
Approval date	Feb. 27, 2018
Rate of base	5.593%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	$ 1,129,000	729,000
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2020 Annual Update [Member]
Effective Date Requested or Approved	Apr. 01, 2020
Annual Revenue	$ 5,762,000
Approval date	Mar. 18, 2020
Rate of base	5.637%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2019 Annual Update [Member]
Effective Date Requested or Approved	Jun. 01, 2019
Annual Revenue	$ (235,000)
Approval date	May 01, 2019
Rate of base	(0.224%)
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 9,650,000
Approval date	Feb. 27, 2018
Rate of base	7.493%
North Dakota 1 [Member] \| Generation Cost Recovery [Member]
Annual Revenue	$ 948,000	248,000
North Dakota 1 [Member] \| Generation Cost Recovery [Member] \| The 2020 Annual Update [Member]
Request date	Mar. 02, 2020
Effective Date Requested or Approved	Jul. 01, 2020
Annual Revenue	$ 6,184,000
Rate of base	6.041%
North Dakota 1 [Member] \| Generation Cost Recovery [Member] \| The 2019 Initial Request [Member]
Effective Date Requested or Approved	Jul. 01, 2019
Annual Revenue	$ 2,720,000
Approval date	May 15, 2019
Rate of base	2.547%
South Dakota 1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 344,000	244,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 562,000	473,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2020 Annual Update [Member]
Effective Date Requested or Approved	Mar. 01, 2020
Annual Revenue	$ 2,327,000
Approval date	Feb. 19, 2020
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Effective Date Requested or Approved	Mar. 01, 2019
Annual Revenue	$ 1,638,000
Approval date	Feb. 20, 2019
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Rate Reset [Member]
Effective Date Requested or Approved	Oct. 01, 2019
Annual Revenue	$ 2,046,000
Approval date	Sep. 17, 2019
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Interim Rate Reset [Member]
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ 1,171,000
Approval date	Oct. 18, 2018
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ 0	(4,000)
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Interim Rate Reset [Member]
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ (189,000)
Rate rider rate (Kilowatt-Hour) \| kWh	(0.00075)
Approval date	Oct. 18, 2018
South Dakota 1 [Member] \| Phase-In Rate Plan [Member]
Annual Revenue	$ 646,000	$ 0
South Dakota 1 [Member] \| Phase-In Rate Plan [Member] \| The 2019 Initial Request [Member]
Effective Date Requested or Approved	Sep. 01, 2019
Annual Revenue	$ 864,000
Approval date	Aug. 21, 2019
Rate of base	3.345%