Note 3 - Rate and Regulatory Matters - Summary of Status of Updates for Previous Two Years for Various Rate Riders (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) kWh	Jun. 30, 2016 USD ($)
Annual Revenue		$ 212,086	$ 203,482	$ 426,203	$ 409,724
Otter Tail Power Company [Member] | Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue	[1]	2,102	2,209	4,068	4,715
Otter Tail Power Company [Member] | Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2016 Incentive and Cost Recovery [Member]
Annual Revenue				$ 9,868
Rate (Kilowatt-Hour) | kWh				0.00754
R - Request Date				Mar. 31, 2017
Effective Date Requested or Approved				Oct. 01, 2017
Otter Tail Power Company [Member] | Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2015 Incentive and Cost Recovery [Member]
Annual Revenue				$ 8,590
Rate (Kilowatt-Hour) | kWh				0.00275
A - Approval Date				Jul. 19, 2016
Effective Date Requested or Approved				Oct. 01, 2016
Otter Tail Power Company [Member] | Minnesota [Member] | Conservation Improvement Program Costs and Incentives [Member] | The 2014 Incentive and Cost Recovery [Member]
Annual Revenue				$ 8,689
Rate (Kilowatt-Hour) | kWh				0.00287
A - Approval Date				Jul. 10, 2015
Effective Date Requested or Approved				Oct. 01, 2015
Otter Tail Power Company [Member] | Minnesota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		1,273	1,133	$ 3,443	3,409
Otter Tail Power Company [Member] | Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue	[2]			$ 4,736
Rate	[2]			Various
A - Approval Date	[2]			Jul. 05, 2016
Effective Date Requested or Approved	[2]			Sep. 01, 2016
Otter Tail Power Company [Member] | Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 7,203
Rate				Various
A - Approval Date				Mar. 09, 2016
Effective Date Requested or Approved				Apr. 01, 2016
Otter Tail Power Company [Member] | Minnesota [Member] | Transmission Cost Recovery Rider [Member] | The 2014 Annual Update [Member]
Annual Revenue				$ 8,388
Rate				Various
A - Approval Date				Feb. 18, 2015
Effective Date Requested or Approved				Mar. 01, 2015
Otter Tail Power Company [Member] | Minnesota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		2,812	3,153	$ 5,636	6,235
Otter Tail Power Company [Member] | Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue	[2]			$ 11,884
Rate of revenue	[2]			6.927%
A - Approval Date	[2]			Jul. 05, 2016
Effective Date Requested or Approved	[2]			Sep. 01, 2016
Otter Tail Power Company [Member] | Minnesota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 12,104
Rate of revenue				7.006%
A - Approval Date				Mar. 09, 2016
Effective Date Requested or Approved				Oct. 01, 2015
Otter Tail Power Company [Member] | North Dakota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		1,384	1,969	$ 3,895	4,205
Otter Tail Power Company [Member] | North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue				$ 6,916
Rate				Various
A - Approval Date				Dec. 14, 2016
Effective Date Requested or Approved				Jan. 01, 2017
Otter Tail Power Company [Member] | North Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 9,985
Rate				Various
A - Approval Date				Dec. 16, 2015
Effective Date Requested or Approved				Jan. 01, 2016
Otter Tail Power Company [Member] | North Dakota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		2,388	2,771	$ 4,876	5,582
Otter Tail Power Company [Member] | North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue				$ 10,359
Rate of base				7.904%
A - Approval Date				Jun. 22, 2016
Effective Date Requested or Approved				Jul. 01, 2016
Otter Tail Power Company [Member] | North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 12,249
Rate of base				9.193%
A - Approval Date				Jun. 17, 2015
Effective Date Requested or Approved				Jul. 01, 2015
Otter Tail Power Company [Member] | North Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2017 Annual Update [Member]
Annual Revenue				$ 9,917
Rate of base				7.633%
R - Request Date				Jul. 12, 2017
Effective Date Requested or Approved				Aug. 01, 2017
Otter Tail Power Company [Member] | North Dakota [Member] | Renewable Resource Adjustment [Member]
Annual Revenue		1,839	1,922	$ 3,609	3,981
Otter Tail Power Company [Member] | North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2016 Annual Update [Member]
Annual Revenue				$ 9,156
Rate of revenue				7.005%
A - Approval Date				Mar. 15, 2017
Effective Date Requested or Approved				Apr. 01, 2017
Otter Tail Power Company [Member] | North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 9,262
Rate of revenue				7.573%
A - Approval Date				Jun. 22, 2016
Effective Date Requested or Approved				Jul. 01, 2016
Otter Tail Power Company [Member] | North Dakota [Member] | Renewable Resource Adjustment [Member] | The 2014 Annual Update [Member]
Annual Revenue				$ 5,441
Rate of revenue				4.069%
A - Approval Date				Mar. 25, 2015
Effective Date Requested or Approved				Apr. 01, 2015
Otter Tail Power Company [Member] | South Dakota [Member] | Conservation Improvement Program Costs and Incentives [Member]
Annual Revenue		176	124	$ 416	283
Otter Tail Power Company [Member] | South Dakota [Member] | Transmission Cost Recovery Rider [Member]
Annual Revenue		287	411	728	1,062
Otter Tail Power Company [Member] | South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue				$ 2,053
Rate				Various
A - Approval Date				Feb. 17, 2017
Effective Date Requested or Approved				Mar. 01, 2017
Otter Tail Power Company [Member] | South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 1,895
Rate				Various
A - Approval Date				Feb. 12, 2016
Effective Date Requested or Approved				Mar. 01, 2016
Otter Tail Power Company [Member] | South Dakota [Member] | Transmission Cost Recovery Rider [Member] | The 2014 Annual Update [Member]
Annual Revenue				$ 1,538
Rate				Various
A - Approval Date				Feb. 13, 2015
Effective Date Requested or Approved				Mar. 01, 2015
Otter Tail Power Company [Member] | South Dakota [Member] | Environmental Cost Recovery Rider [Member]
Annual Revenue		$ 545	$ 627	$ 1,142	$ 1,260
Otter Tail Power Company [Member] | South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2016 Annual Update [Member]
Annual Revenue				$ 2,238
Rate (Kilowatt-Hour) | kWh				0.00536
A - Approval Date				Oct. 26, 2016
Effective Date Requested or Approved				Nov. 01, 2016
Otter Tail Power Company [Member] | South Dakota [Member] | Environmental Cost Recovery Rider [Member] | The 2015 Annual Update [Member]
Annual Revenue				$ 2,728
Rate (Kilowatt-Hour) | kWh				0.00643
A - Approval Date				Oct. 15, 2015
Effective Date Requested or Approved				Nov. 01, 2015

[1] Includes MNCIP costs recovered in base rates.

[2] Approved on a provisional basis and subject to change based on comments from the MNDOC.