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Note 3 - Rate and Regulatory Matters (Details Textual) - USD ($)
 1 Months Ended 2 Months Ended 3 Months Ended 12 Months Ended
Nov. 21, 2019
Jul. 16, 2019
Jul. 09, 2019
Jun. 11, 2019
May 30, 2019
Sep. 26, 2018
Apr. 20, 2018
Mar. 23, 2018
Mar. 22, 2018
Nov. 17, 2017
Nov. 16, 2016
Sep. 28, 2016
May 25, 2016
Feb. 12, 2015
Jan. 05, 2015
Nov. 06, 2014
Nov. 12, 2013
Apr. 30, 2019
Jun. 30, 2016
Dec. 22, 2015
Sep. 30, 2019
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Mar. 01, 2018
Jan. 01, 2018
Nov. 02, 2017
Oct. 31, 2017
Dec. 31, 2016
Apr. 16, 2016
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                                             21.00% 21.00% 35.00%            
Regulatory Liabilities, Total                                             $ 247,386,000 $ 227,207,000              
Minnesota1 [Member]                                                              
Effect of Tax Cuts and Jobs Act, Refund to Customers                                         $ 11,500,000                    
Federal Energy Regulatory Commission [Member]                                                              
Additional Refund from Reducing ROE                                             200,000                
Otter Tail Power Company [Member]                                                              
Environmental Cost Recovery Rider Rate                                               0.00%              
Otter Tail Power Company [Member] | Litigation Case with Minnesota Customers [Member]                                                              
Loss Contingency, Estimate of Possible Loss                                             $ 2,600,000                
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member]                                                              
Current Return on Equity Used in Transmission Rates                       10.32%         12.38%     10.32%     12.38%                
Proposed Reduced Return on Equity Used in Transmission Rates                       9.88%   8.67%     9.15%   9.70%       9.88%                
Additional Incentive Basis Point                               0.50%                              
Expected Percentage of Return on Equity                       10.82%                                      
Regulatory Liabilities, Total                                         $ 1,600,000                 $ 2,700,000  
Contract with Customer, Refund Liability, Total                                             $ 3,000,000.0 $ 1,600,000              
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | First Complaint Period [member]                                                              
Current Return on Equity Used in Transmission Rates                       10.82%                     10.38%                
Additional Incentive Basis Point                             0.50%                                
Contract with Customer, Refund Liability, Total                                             $ 1,300,000                
Otter Tail Power Company [Member] | Federal Energy Regulatory Commission [Member] | Second Complaint Period [Member]                                                              
Current Return on Equity Used in Transmission Rates 9.88%                     12.38%                                      
Additional Incentive Basis Point                             0.50%                                
Contract with Customer, Refund Liability, Total                                             1,500,000                
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%
Estimated Interim Rate Refund                   $ 9,000,000.0                                          
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Environmental Cost Recovery Rider [Member] | The 2016 General Rate Case [Member]                                                              
Revenues Collected Under Riders, Subject to Customer Refund                                                         $ 900,000    
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Transmission Cost Recovery Rider [Member] | The 2016 General Rate Case [Member]                                                              
Revenues Collected Under Riders, Subject to Customer Refund                                                         $ 1,400,000    
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | ECR and TCR Riders [Member] | The 2016 General Rate Case [Member]                                                              
Revenues Collected Under Riders, Subject to Refund, Period of Refund                                                 12 months            
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                                             2,700,000 $ 3,000,000.0 $ 2,900,000            
Financial Incentives Reserve Increase (Decrease) During Period                                             (300,000)   300,000            
Otter Tail Power Company [Member] | Minnesota Public Utilities Commission [Member] | Conservation Improvement Program [Member] | Recognized in 2017 [Member]                                                              
Financial Incentives Recognized During Period                                                 $ 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]                                                              
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%                                                  
Refunds of Excess Revenues Collected, Amounts Collected Under Higher Interim Tax Rates                                   $ 800,000                          
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     $ 4,100,000                                                        
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                
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                                             $ 58,700,000                
Otter Tail Power Company [Member] | EDF [Member] | Merricourt Project [Member]                                                              
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                                             $ 81,700,000