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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		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) kWh	Sep. 30, 2019 USD ($)
Annual Revenue	$ 11,258	$ 4,399	$ 28,440	$ 15,816
Annual Revenue	11,258	4,399	28,440	15,816
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	2,096	1,518	4,703	4,246
Annual Revenue	2,096	1,518	$ 4,703	4,246
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2019 Incentive and Cost Recovery [Member]
Request date			Aug. 18, 2020
Effective Date Requested or Approved			Oct. 01, 2020
Annual Revenue			$ 8,247
Rate rider rate (Kilowatt-Hour) | kWh			0.00485
Request date			Aug. 18, 2020
Effective Date Requested or Approved			Oct. 01, 2020
Annual Revenue			$ 8,247
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
Annual Revenue			$ 11,926
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
Annual Revenue			$ 10,283
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	2,001	(284)	2,402	301
Annual Revenue	2,001	(284)	$ 2,402	301
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
Request date			Oct. 22, 2020
Effective Date Requested or Approved			Jan. 01, 2021
Annual Revenue			$ 10,260
Request date			Oct. 22, 2020
Effective Date Requested or Approved			Jan. 01, 2021
Annual Revenue			$ 10,260
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
Annual Revenue			$ (3,311)
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	0	0	0	(1)
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%
Annual Revenue			$ 0
Minnesota1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue	3,318	1,316	9,660	3,949
Annual Revenue	3,318	1,316	$ 9,660	3,949
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
Annual Revenue			$ 5,886
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
Annual Revenue			$ 12,506
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	809	908	3,131	3,554
Annual Revenue	809	908	$ 3,131	3,554
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2020 Annual Update [Member]
Effective Date Requested or Approved			Jan. 01, 2021
Annual Revenue			$ 5,570
Approval date			Aug. 31, 2020
Effective Date Requested or Approved			Jan. 01, 2021
Annual Revenue			$ 5,570
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
Annual Revenue			$ 5,739
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
Annual Revenue			$ 7,469
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
Annual Revenue			$ 4,801
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	0	(7)	0	556
Annual Revenue	0	(7)	$ 0	556
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%)
Annual Revenue			$ (378)
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%
Annual Revenue			$ 0
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%
Annual Revenue			$ 7,718
North Dakota 1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue	1,256	(20)	3,455	616
Annual Revenue	1,256	(20)	$ 3,455	616
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%
Annual Revenue			$ 5,762
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%)
Annual Revenue			$ (235)
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%
Annual Revenue			$ 9,650
North Dakota 1 [Member] | Generation Cost Recovery [Member]
Annual Revenue	1,187	137	3,035	607
Annual Revenue	1,187	137	$ 3,035	607
North Dakota 1 [Member] | Generation Cost Recovery [Member] | The 2020 Annual Update [Member]
Effective Date Requested or Approved			Jul. 01, 2020
Annual Revenue			$ 6,184
Approval date			Jun. 10, 2020
Effective Date Requested or Approved			Jul. 01, 2020
Rate of base			6.041%
Annual Revenue			$ 6,184
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%
Annual Revenue			$ 2,720
South Dakota 1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	194	100	748	440
Annual Revenue	194	100	748	440
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue	450	743	1,383	1,587
Annual Revenue	450	743	$ 1,383	1,587
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
Annual Revenue			$ 2,327
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Annual Update [Member]
Request date			Feb. 20, 2019
Effective Date Requested or Approved			Mar. 01, 2019
Annual Revenue			$ 1,638
Request date			Feb. 20, 2019
Effective Date Requested or Approved			Mar. 01, 2019
Annual Revenue			$ 1,638
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
Annual Revenue			$ 2,046
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
Annual Revenue			$ 1,171
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue	0	(2)	0	(29)
Annual Revenue	0	(2)	$ 0	(29)
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
Annual Revenue			$ (189)
South Dakota 1 [Member] | Phase-in Rate Plan Recovery [Member] | The 2020 Annual Update [Member]
Effective Date Requested or Approved			Sep. 01, 2020
Annual Revenue			$ 1,625
Approval date			Aug. 06, 2020
Effective Date Requested or Approved			Sep. 01, 2020
Rate of base			6.521%
Annual Revenue			$ 1,625
South Dakota 1 [Member] | Phase-In Rate Plan [Member]
Annual Revenue	(53)	(10)	(77)	(10)
Annual Revenue	$ (53)	$ (10)	$ (77)	$ (10)
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%
Annual Revenue			$ 864

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