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, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) kWh	Jun. 30, 2018 USD ($)
Annual Revenue		$ 5,314	$ 8,512	$ 13,459	$ 18,628
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	[1]	2,618	2,368	$ 4,770	4,884
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2018 Incentive and Cost Recovery [Member]
R - Request Date				Apr. 01, 2019
Effective Date Requested or Approved				Oct. 01, 2019
Annual Revenue				$ 11,926
Rate rider rate (Kilowatt-Hour) | kWh				0.00710
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
A - Approval Date				Oct. 04, 2018
Minnesota1 [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2016 Incentive and Cost Recovery [Member]
Effective Date Requested or Approved				Oct. 01, 2017
Annual Revenue				$ 9,868
Rate rider rate (Kilowatt-Hour) | kWh				0.00536
A - Approval Date				Sep. 15, 2017
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		(56)	(458)	$ 585	(487)
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario A [Member]
R - Request Date				Nov. 30, 2018
Effective Date Requested or Approved				Jun. 01, 2019
Annual Revenue				$ 6,475
Rate				Various
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2018 Annual Update - Scenario B [Member]
Annual Revenue				$ 2,708
Rate				Various
Minnesota1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Effective Date Requested or Approved				Nov. 01, 2017
Annual Revenue				$ (3,311)
A - Approval Date				Oct. 30, 2017
Rate				Various
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		0	(18)	$ (1)	(49)
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Effective Date Requested or Approved				Nov. 01, 2017
Annual Revenue				$ (1,943)
A - Approval Date				Oct. 30, 2017
Rate of base				(0.935%)
Minnesota1 [Member] | Environmental Cost Recovery Rider [Member] | The 2018 Annual Update [Member]
Effective Date Requested or Approved				Dec. 01, 2018
Annual Revenue				$ 0
A - Approval Date				Nov. 29, 2018
Rate of base				0.00%
Minnesota1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue		1,317	659	$ 2,633	1,184
Minnesota1 [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
Effective Date Requested or Approved				Nov. 01, 2017
Annual Revenue				$ 1,279
Rate rider rate (Kilowatt-Hour) | kWh				0.00049
A - Approval Date				Oct. 30, 2017
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
A - Approval Date				Aug. 29, 2018
Minnesota1 [Member] | Renewable Resource Adjustment [Member] | The 2019 Annual Update [Member]
R - Request Date				Jun. 21, 2019
Effective Date Requested or Approved				Nov. 01, 2019
Annual Revenue				$ 12,571
Rate rider rate (Kilowatt-Hour) | kWh				0.00469
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		874	1,165	$ 2,646	3,227
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
A - Approval Date				Feb. 27, 2018
Rate				Various
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
A - Approval Date				Dec. 06, 2018
Rate				Various
North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Effective Date Requested or Approved				Jan. 01, 2018
Annual Revenue				$ 7,959
A - Approval Date				Nov. 29, 2017
Rate				Various
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		(12)	1,830	$ 563	3,651
North Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Rate Reset [Member]
Effective Date Requested or Approved				Jan. 01, 2018
Annual Revenue				$ 8,537
A - Approval Date				Dec. 20, 2017
Rate of base				6.629%
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)
A - Approval Date				Dec. 19, 2018
Rate of base				(0.31%)
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
A - Approval Date				Feb. 27, 2018
Rate of base				5.593%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member]
Annual Revenue		(93)	2,079	$ 636	4,046
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2017 Rate Reset [Member]
Effective Date Requested or Approved				Jan. 01, 2018
Annual Revenue				$ 9,989
A - Approval Date				Dec. 20, 2017
Rate of base				7.756%
North Dakota 1 [Member] | Renewable Resource Adjustment [Member] | The 2019 Annual Update [Member]
Effective Date Requested or Approved				Jun. 01, 2019
Annual Revenue				$ (235)
A - 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
A - Approval Date				Feb. 27, 2018
Rate of base				7.493%
North Dakota 1 [Member] | Generation Cost Recovery [Member]
Annual Revenue		222	0	$ 470	0
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
A - Approval Date				May 15, 2019
Rate of base				2.547%
South Dakota 1 [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue		96	122	$ 340	351
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		371	250	$ 844	786
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
A - Approval Date				Feb. 20, 2019
Rate				Various
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Effective Date Requested or Approved				Mar. 01, 2018
Annual Revenue				$ 1,779
A - Approval Date				Feb. 28, 2018
Rate				Various
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Rate Reset [Member]
R - Request Date				Jul. 31, 2019
Effective Date Requested or Approved				Oct. 01, 2019
Annual Revenue				$ 2,050
Rate				Various
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
A - Approval Date				Oct. 18, 2018
Rate				Various
South Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Effective Date Requested or Approved				Mar. 01, 2017
Annual Revenue				$ 2,053
A - Approval Date				Feb. 17, 2017
Rate				Various
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		$ (23)	$ 515	$ (27)	$ 1,035
South Dakota 1 [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Effective Date Requested or Approved				Nov. 01, 2017
Annual Revenue				$ 2,082
Rate rider rate (Kilowatt-Hour) | kWh				0.00483
A - Approval Date				Oct. 13, 2017
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)
A - Approval Date				Oct. 18, 2018
South Dakota 1 [Member] | Phase-In Rate Plan [Member] | The 2019 Initial Request [Member]
R - Request Date				May 31, 2019
Effective Date Requested or Approved				Sep. 01, 2019
Annual Revenue				$ 1,027
Rate of base				3.942%

[1] Includes MNCIP costs recovered in base rates.