Note 3 - Rate and Regulatory Matters (Details Textual)																					1 Months Ended		2 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 01, 2019	Aug. 30, 2019 USD ($)	Jun. 28, 2019 USD ($)	Jun. 24, 2019 USD ($)	Jun. 16, 2019 USD ($)	Jun. 11, 2019 USD ($)	May 30, 2019 USD ($)	Apr. 01, 2019 USD ($)	Dec. 01, 2018	Oct. 16, 2018	Sep. 26, 2018 USD ($)	Apr. 20, 2018 USD ($)	Mar. 23, 2018 USD ($)	Mar. 22, 2018 USD ($)	Nov. 16, 2016 USD ($)	Sep. 28, 2016	May 25, 2016	Feb. 12, 2015	Nov. 06, 2014	Nov. 12, 2013	Jun. 30, 2016	Dec. 22, 2015	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017	Oct. 24, 2019 USD ($)	Mar. 01, 2018 USD ($)	Dec. 20, 2017 USD ($)	Nov. 02, 2017 USD ($)	May 01, 2017	Dec. 31, 2016 USD ($)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent																										21.00%	35.00%
Regulatory Liabilities, Total																							$ 245,092,000		$ 245,092,000	$ 227,207,000
Minnesota1 [Member]
Effect of Tax Cuts and Jobs Act, Refund to Customers																							11,500,000
NORTH DAKOTA
Accrued Refund Liabilities Resulting from Tax Rate Reduction																								$ 800,000
Federal Energy Regulatory Commission [Member]
Accrued Refund Liabilities Resulting from Tax Rate Reduction																							200,000		200,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]
Current Return on Equity Used in Transmission Rates																10.32%				12.38%		10.32%
Proposed Reduced Return on Equity Used in Transmission Rates																		8.67%		9.15%	9.70%
Additional Incentive Basis Point																			0.50%
Expected Percentage of Return on Equity										10.41%						10.82%
Regulatory Liabilities, Total																							1,600,000		1,600,000								$ 2,700,000
Contract with Customer, Refund Liability, Total																							1,600,000		1,600,000
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | Maximum [Member]
Proposed Reduced Return on Equity Used in Transmission Rates										13.08%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | The 2016 General Rate Case [Member]
Percentage of Allowed Rate of Return on Rate Base																																7.5056%
Percentage of Allowed Rate of Return on Equity																																9.41%
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member]
Utility Incentive Percentage in Next Rolling Twelve Months																	13.50%
Utility Incentive Percentage in Next Rolling Year Two																	12.00%
Utility Incentive Percentage in Next Rolling Year Three																	10.00%
Assumed Savings of Utility																	1.70%
Financial Incentive, Maximum Percentage of Spending, Year One																	40.00%
Financial Incentive, Maximum Percentage of Spending, Year Two																	35.00%
Financial Incentive, Maximum Percentage of Spending, Year Three																	30.00%
Expected Rate of Financial Incentive Reduction																	50.00%
Financial Incentives Recognized During Period								$ 3,000,000.0
Financial Incentive Offered to MPUC				$ 4,000,000.0
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Subsequent Event [Member]
Financial Incentive Request Approved																												$ 3,000,000.0
Otter Tail Power Company [Member] | Minnesota1 [Member]
Environmental Cost Recovery Rider Rate									0.00%
Otter Tail Power Company [Member] | Minnesota1 [Member] | Transmission Cost Recovery Rider [Member]
Loss Contingency, Estimate of Possible Loss																							2,600,000		2,600,000
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2010 General Rate Case [Member]
Percentage of Allowed Rate of Return on Rate Base																															7.97%
Percentage of Allowed Rate of Return on Equity																															10.30%
General Rate Revenue Increase Requested																														$ 12,800,000	$ 13,100,000
Percentage of Increase in Base Rate Revenue Requested																															8.72%
Public Utilities, Interim Rate Requirement, Decrease in Amount																													$ 4,500,000
Public Utilities, Interim Rate Requirement, Amount																													$ 8,300,000
Otter Tail Power Company [Member] | North Dakota Public Service Commission [Member] | The 2017 General Rate Case [Member]
Percentage of Allowed Rate of Return on Equity													10.30%
Public Utilities, Requested Rate Increase (Decrease), Amount											$ 4,600,000		$ 7,100,000	$ 13,100,000
Public Utilities, Requested Rate Increase (Decrease), Percentage											3.10%		4.80%
General Rate Revenue Increase Requested, Decrease													$ 6,000,000.0
General Rate Revenue Increase Requested, Decrease, Amount Related to Tax Reform													4,800,000
General Rate Revenue Increase Requested, Decrease, Amount Related to Updates Other Than Tax Reform													$ 1,200,000
Percentage of Requested Allowed Rate of Return on Equity											9.77%
Equity to Total Capitalization Ratio Basis for Return on Equity											52.50%
Otter Tail Power Company [Member] | North Dakota 1 [Member] | Subsequent Event [Member]
Environmental Cost Recovery Rider Rate	0.00%
Otter Tail Power Company [Member] | North Dakota 1 [Member] | Transmission Cost Recovery Rider [Member] | The 2019 Annual Update [Member]
Annual Revenue Requesting Recovery		$ 5,700,000
Number of New Projects Included in Request for Recovery		7
Otter Tail Power Company [Member] | South Dakota Public Utilities Commission [Member] | The 2018 General Rate Case [Member]
Percentage of Allowed Rate of Return on Equity							8.75%
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 3,300,000
Increase in Annual Non-fuel Rates Requested, Step One, Percentage												10.10%
Increase in Annual Non-fuel Rates Requested, Step Two, Percentage												1.70%
Contract with Customer, Liability, Revenue Recognized							$ 2,200,000																	$ 1,000,000.0
Understated Amount of OTP's Electric Transmission Plant in Service			$ 44,000,000
Annual Revenue Requirement Shortfall Resulted from Understatement			341,000
Increase in Non-fuel Annual Revenue Resulted from Increased in General Rate Case			$ 2,600,000
Increase in Non-fuel Annual Revenue Resulted from Increased in General Rate Case, Percentage			7.70%
Non-fuel Annual Revenue Increased, Perecenage of Adjusted Requsted Annual Revenue			69.00%
Adjusted Requested Annual Revenue Increased, Amount			$ 3,700,000
Adjusted Requested Annual Revenue Increased, Percentage			11.10%
Authorized Return of Equity			8.75%
Percentage of Excess Weather-normalized Revenue, Refund to Customer			50.00%
Maximum ROE, Input for Refund to Customer			9.50%
Percentage of Any Earnings Above Maximum ROE			100.00%
Otter Tail Power Company [Member] | Big Stone South - Ellendale MVP [Member] | Federal Energy Regulatory Commission [Member]
Current Project Cost																									$ 106,000,000
Expanded Capacity of Projects																									345
Extended Distance of Transmission Line																									162
Percentage of Assets of Project																									100.00%
Otter Tail Power Company [Member] | Astoria Station Project [Member]
Expanded Capacity of Projects (MW)																									245
Current Project Capitalized Cost																									$ 36,800,000
Expected Project Cost																							158,000,000		158,000,000
Otter Tail Power Company [Member] | Merricourt Project [Member] | EDF [Member]
Expanded Capacity of Projects (MW)															150
Expected Project Cost																							$ 258,000,000		258,000,000
Asset Purchase Agreement, Purchase Price					$ 37,700,000	$ 37,700,000									$ 34,700,000
Turnkey Engineering, Procurement and Construction Services Agreement, Costs															$ 200,500,000
Current Project Cost																									$ 54,800,000