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]

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]	12 Months Ended
	Dec. 31, 2019 USD ($) kWh Boe	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Annual Revenue	$ 26,092,000	$ 35,962,000	$ 47,850,000
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 8,271,000	8,127,000	6,008,000
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2018 Incentive and Cost Recovery [Member]
Approval date	Dec. 27, 2019
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 11,926,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00710
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2017 Incentive and Cost Recovery [Member]
Approval date	Oct. 04, 2018
Effective Date Requested or Approved	Nov. 01, 2018
Annual Revenue	$ 10,283,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00600
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2016 Incentive and Cost Recovery [Member]
Approval date	Sep. 15, 2017
Effective Date Requested or Approved	Oct. 01, 2017
Annual Revenue	$ 9,868,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00536
Minnesota1 [Member] \| Conservation Improvement Program Costs and Incentives [Member] \| The 2015 Incentive and Cost Recovery [Member]
Approval date	Jul. 19, 2016
Effective Date Requested or Approved	Oct. 01, 2016
Annual Revenue	$ 8,590,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00275
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 2,497,000	(2,039,000)	2,973,000
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Annual Update - Scenario A [Member]
Effective Date Requested or Approved	Jun. 01, 2019
Annual Revenue	$ 6,475,000
Request date	Nov. 30, 2018
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]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 01, 2017
Annual Revenue	$ (3,311,000)
Minnesota1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Jul. 05, 2016
Effective Date Requested or Approved	Sep. 01, 2016
Annual Revenue	$ 4,736,000
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ (1,000)	(24,000)	8,148,000
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2017 Rate Reset [Member]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 01, 2017
Annual Revenue	$ (1,943,000)
Rate of base	(0.935%)
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Jul. 05, 2016
Effective Date Requested or Approved	Sep. 01, 2016
Annual Revenue	$ 11,884,000
Rate of base	6.927%
Minnesota1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Annual Update [Member]
Approval date	Nov. 29, 2018
Effective Date Requested or Approved	Dec. 01, 2018
Annual Revenue	$ 0
Rate of base	0.00%
Minnesota1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	$ 5,513,000	3,067,000	(196,000)
Minnesota1 [Member] \| Renewable Resource Adjustment [Member] \| The 2017 Rate Reset [Member]
Approval date	Oct. 30, 2017
Effective Date Requested or Approved	Nov. 01, 2017
Annual Revenue	$ 1,279,000
Rate rider rate (Kilowatt-Hour) \| kWh	0.00049
Minnesota1 [Member] \| Renewable Resource Adjustment [Member] \| The 2018 Annual Update [Member]
Approval date	Aug. 29, 2018
Effective Date Requested or Approved	Nov. 01, 2018
Annual Revenue	$ 5,886,000
Rate rider rate (Kilowatt-Hour) \| Boe	0.00219
Minnesota1 [Member] \| Renewable Resource Adjustment [Member] \| The 2019 Annual Update - Revised [Member]
Approval date	Dec. 19, 2019
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 12,506,000
Rate rider rate (Kilowatt-Hour) \| Boe	0.00467
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 5,292,000	7,016,000	8,729,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Dec. 14, 2016
Effective Date Requested or Approved	Jan. 01, 2017
Annual Revenue	$ 6,916,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Approval date	Dec. 18, 2019
Effective Date Requested or Approved	Jan. 01, 2020
Annual Revenue	$ 5,739,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 7,469,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Supplemental Update [Member]
Approval date	Dec. 06, 2018
Effective Date Requested or Approved	Feb. 01, 2019
Annual Revenue	$ 4,801,000
North Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2017 Annual Update [Member]
Approval date	Nov. 29, 2017
Effective Date Requested or Approved	Jan. 01, 2018
Annual Revenue	$ 7,959,000
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ 550,000	7,318,000	9,782,000
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2017 Rate Reset [Member]
Approval date	Dec. 20, 2017
Effective Date Requested or Approved	Jan. 01, 2018
Annual Revenue	$ 8,537,000
Rate of base	6.629%
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Jun. 22, 2016
Effective Date Requested or Approved	Jul. 01, 2016
Annual Revenue	$ 10,359,000
Rate of base	7.904%
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Annual Update [Member]
Approval date	Dec. 19, 2018
Effective Date Requested or Approved	Feb. 01, 2019
Annual Revenue	$ (378,000)
Rate of base	(0.31%)
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Approval date	Oct. 22, 2019
Effective Date Requested or Approved	Nov. 01, 2019
Annual Revenue	$ 0
Rate of base	0.00%
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 7,718,000
Rate of base	5.593%
North Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2017 Annual Update [Member]
Approval date	Jul. 12, 2017
Effective Date Requested or Approved	Aug. 01, 2017
Annual Revenue	$ 9,917,000
Rate of base	7.633%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member]
Annual Revenue	$ 230,000	8,529,000	7,620,000
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2017 Rate Reset [Member]
Approval date	Dec. 20, 2017
Effective Date Requested or Approved	Jan. 01, 2018
Annual Revenue	$ 9,989,000
Rate of base	7.756%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2016 Annual Update [Member]
Approval date	Mar. 15, 2017
Effective Date Requested or Approved	Apr. 01, 2017
Annual Revenue	$ 9,156,000
Rate of base	7.005%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2020 Annual Update [Member]
Effective Date Requested or Approved	Apr. 01, 2020
Annual Revenue	$ 3,828,000
Request date	Dec. 31, 2019
Rate of base	3.744%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2019 Annual Update [Member]
Approval date	May 01, 2019
Effective Date Requested or Approved	Jun. 01, 2019
Annual Revenue	$ (235,000)
Rate of base	(0.224%)
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2018 Rate Reset for Effect of TCJA [Member]
Approval date	Feb. 27, 2018
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 9,650,000
Rate of base	7.493%
North Dakota 1 [Member] \| Renewable Resource Adjustment [Member] \| The 2015 Annual Update [Member]
Approval date	Jun. 22, 2016
Effective Date Requested or Approved	Jul. 01, 2016
Annual Revenue	$ 9,262,000
Rate of base	7.573%
North Dakota 1 [Member] \| Generation Cost Recovery [Member]
Annual Revenue	$ 878,000	0	0
North Dakota 1 [Member] \| Generation Cost Recovery [Member] \| The 2019 Initial Request [Member]
Approval date	May 15, 2019
Effective Date Requested or Approved	Jul. 01, 2019
Annual Revenue	$ 2,720,000
Rate of base	2.547%
South Dakota 1 [Member] \| Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	$ 851,000	628,000	598,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member]
Annual Revenue	$ 2,165,000	1,664,000	1,843,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Feb. 17, 2017
Effective Date Requested or Approved	Mar. 01, 2017
Annual Revenue	$ 2,053,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2020 Annual Update [Member]
Effective Date Requested or Approved	Mar. 01, 2020
Annual Revenue	$ 2,407,000
Request date	Oct. 31, 2019
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Annual Update [Member]
Approval date	Feb. 20, 2019
Effective Date Requested or Approved	Mar. 01, 2019
Annual Revenue	$ 1,638,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2015 Annual Update [Member]
Approval date	Feb. 12, 2016
Effective Date Requested or Approved	Mar. 01, 2016
Annual Revenue	$ 1,895,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2017 Annual Update [Member]
Approval date	Feb. 28, 2018
Effective Date Requested or Approved	Mar. 01, 2018
Annual Revenue	$ 1,779,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2019 Rate Reset [Member]
Approval date	Sep. 17, 2019
Effective Date Requested or Approved	Oct. 01, 2019
Annual Revenue	$ 2,046,000
South Dakota 1 [Member] \| Transmission Cost Recovery Rider [Member] \| The 2018 Interim Rate Reset [Member]
Approval date	Oct. 18, 2018
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ 1,171,000
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member]
Annual Revenue	$ (29,000)	1,676,000	2,345,000
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2016 Annual Update [Member]
Approval date	Oct. 26, 2016
Effective Date Requested or Approved	Nov. 01, 2016
Annual Revenue	$ 2,238,000
Rate rider rate (Kilowatt-Hour) \| Boe	0.00536
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2017 Annual Update [Member]
Approval date	Oct. 13, 2017
Effective Date Requested or Approved	Nov. 01, 2017
Annual Revenue	$ 2,082,000
Rate rider rate (Kilowatt-Hour) \| Boe	0.00483
South Dakota 1 [Member] \| Environmental Cost Recovery Rider [Member] \| The 2018 Interim Rate Reset [Member]
Approval date	Oct. 18, 2018
Effective Date Requested or Approved	Oct. 18, 2018
Annual Revenue	$ (189,000)
Rate rider rate (Kilowatt-Hour) \| Boe	(0.00075)
South Dakota 1 [Member] \| Phase-In Rate Plan [Member]
Annual Revenue	$ (125,000)	$ 0	$ 0
South Dakota 1 [Member] \| Phase-In Rate Plan [Member] \| The 2019 Initial Request [Member]
Approval date	Aug. 21, 2019
Effective Date Requested or Approved	Sep. 01, 2019
Annual Revenue	$ 864,000
Rate of base	3.345%