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] $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) kWh	Jun. 30, 2019 USD ($)
Annual Revenue	$ 7,016	$ 4,537	$ 17,182	$ 11,417
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	1,521	1,841	$ 2,607	2,728
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
Rate rider rate (Kilowatt-Hour) \| kWh			0.00485
Request date			May 01, 2020
Effective Date Requested or Approved			Oct. 01, 2020
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
Rate rider rate (Kilowatt-Hour) \| kWh			0.00710
Approval date			Dec. 27, 2019
Effective Date Requested or Approved			Jan. 01, 2020
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
Rate rider rate (Kilowatt-Hour) \| kWh			0.00600
Approval date			Oct. 04, 2018
Effective Date Requested or Approved			Nov. 01, 2018
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	(314)	(56)	$ 401	585
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Annual Update [Member]
Request date			May 07, 2020
Effective Date Requested or Approved			Jan. 01, 2021
Annual Revenue			$ 10,264
Request date			May 07, 2020
Effective Date Requested or Approved			Jan. 01, 2021
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2017 Rate Reset [Member]
Effective Date Requested or Approved			Nov. 01, 2017
Annual Revenue			$ (3,311)
Approval date			Oct. 30, 2017
Effective Date Requested or Approved			Nov. 01, 2017
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	0	0	$ 0	(1)
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
Effective Date Requested or Approved			Dec. 01, 2018
Rate of base			0.00%
Minnesota1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	3,088	1,317	$ 6,342	2,633
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
Effective Date Requested or Approved			Nov. 01, 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
Rate rider rate (Kilowatt-Hour) \| kWh			0.00467
Approval date			Dec. 19, 2019
Effective Date Requested or Approved			Jan. 01, 2020
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	840	874	$ 2,322	2,646
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
Approval date			Dec. 18, 2019
Effective Date Requested or Approved			Jan. 01, 2020
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
Approval date			Feb. 27, 2018
Effective Date Requested or Approved			Mar. 01, 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
Approval date			Dec. 06, 2018
Effective Date Requested or Approved			Feb. 01, 2019
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	0	(12)	$ 0	563
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)
Approval date			Dec. 19, 2018
Effective Date Requested or Approved			Feb. 01, 2019
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
Effective Date Requested or Approved			Nov. 01, 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
Approval date			Feb. 27, 2018
Effective Date Requested or Approved			Mar. 01, 2018
Rate of base			5.593%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	1,070	(93)	$ 2,199	636
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
Approval date			Mar. 18, 2020
Effective Date Requested or Approved			Apr. 01, 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)
Approval date			May 01, 2019
Effective Date Requested or Approved			Jun. 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
Approval date			Feb. 27, 2018
Effective Date Requested or Approved			Mar. 01, 2018
Rate of base			7.493%
North Dakota 1 [Member] \| Generation Cost Recovery [Member]
Annual Revenue	900	222	$ 1,848	470
North Dakota 1 [Member] \| Generation Cost Recovery [Member] \| The 2020 Annual Update [Member]
Request date			Jun. 10, 2020
Effective Date Requested or Approved			Jul. 01, 2020
Annual Revenue			$ 6,184
Request date			Jun. 10, 2020
Effective Date Requested or Approved			Jul. 01, 2020
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
Approval date			May 15, 2019
Effective Date Requested or Approved			Jul. 01, 2019
Rate of base			2.547%
South Dakota 1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	210	96	$ 554	340
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	371	371	$ 933	844
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
Approval date			Feb. 19, 2020
Effective Date Requested or Approved			Mar. 01, 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
Approval date			Feb. 20, 2019
Effective Date Requested or Approved			Mar. 01, 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
Approval date			Sep. 17, 2019
Effective Date Requested or Approved			Oct. 01, 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
Approval date			Oct. 18, 2018
Effective Date Requested or Approved			Oct. 18, 2018
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	0	(23)	$ 0	(27)
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)
Rate rider rate (Kilowatt-Hour) \| kWh			(0.00075)
Approval date			Oct. 18, 2018
Effective Date Requested or Approved			Oct. 18, 2018
South Dakota 1 [Member] \| Phase-in Rate Plan Recovery [Member] \| The 2020 Annual Update [Member]
Request date			Jun. 01, 2020
Effective Date Requested or Approved			Sep. 01, 2020
Annual Revenue			$ 1,931
Request date			Jun. 01, 2020
Effective Date Requested or Approved			Sep. 01, 2020
Rate of base			7.753%
South Dakota 1 [Member] \| Phase-In Rate Plan [Member]
Annual Revenue	$ (670)	$ 0	$ (24)	$ 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
Approval date			Aug. 21, 2019
Effective Date Requested or Approved			Sep. 01, 2019
Rate of base			3.345%