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Note 10 - Short-term and Long-term Borrowings (Details Textual) - USD ($)
12 Months Ended
Oct. 10, 2019
Dec. 31, 2019
Dec. 31, 2018
Oct. 02, 2019
Sep. 12, 2019
Feb. 07, 2018
Sep. 23, 2016
Aug. 14, 2013
Oct. 29, 2012
Dec. 01, 2011
Short-term Debt, Maximum Amount Outstanding During Period   $ 109,200,000                
Short-term Debt, Average Outstanding Amount   $ 36,200,000                
Short-term Debt, Weighted Average Interest Rate, at Point in Time   3.20%                
Line of Credit Facility, Maximum Borrowing Capacity   $ 340,000,000                
Otter Tail Corporation Credit Agreement [Member]                    
Short-term Debt, Maximum Amount Outstanding During Period   38,900,000                
Short-term Debt, Average Outstanding Amount   $ 21,900,000                
Short-term Debt, Weighted Average Interest Rate, at Point in Time   3.80% 3.80%              
Line of Credit Facility, Maximum Borrowing Capacity   $ 170,000,000                
Otter Tail Corporation Credit Agreement [Member] | Maximum [Member]                    
Debt to Total Capitalization Ratio   60.00%                
Otter Tail Corporation Credit Agreement [Member] | Minimum [Member]                    
Interest and Dividend Coverage Ratio   1.50                
Otter Tail Corporation Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument, Basis Spread on Variable Rate   1.50% 1.50%              
Otter Tail Corporation Credit Agreement [Member] | Unsecured Revolving Credit Facility [Member]                    
Line of Credit Facility, Maximum Borrowing Capacity                 $ 130,000,000  
Reduced Line of Credit Facility, Borrowing Capacity by Outstanding Letters of Credit   $ 40,000,000                
Otter Tail Corporation Credit Agreement [Member] | Revolving Credit Facility [Member]                    
Line of Credit Facility, Maximum Borrowing Capacity                 170,000,000  
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions                 290,000,000  
OTP Credit Agreement [Member]                    
Short-term Debt, Average Outstanding Amount   $ 14,300,000                
Short-term Debt, Weighted Average Interest Rate, at Point in Time   3.60% 3.00%              
Line of Credit Facility, Maximum Borrowing Capacity   $ 170,000,000   $ 73,200,000            
Line of Credit Facility, Maximum Amount Outstanding During Period, by Outstanding Letters of Credit   $ 50,000,000                
OTP Credit Agreement [Member] | Maximum [Member]                    
Debt to Total Capitalization Ratio   60.00%                
OTP Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]                    
Debt Instrument, Basis Spread on Variable Rate   1.25% 1.25%              
OTP Credit Agreement [Member] | Revolving Credit Facility [Member]                    
Line of Credit Facility, Maximum Borrowing Capacity                 170,000,000  
Line of Credit Facility, Maximum Borrowing Capacity, Subject to Conditions                 $ 250,000,000  
OTP Credit Agreement [Member] | Revolving Credit Facility [Member] | Otter Tail Power Company [Member]                    
Repayments of Debt $ 69,900,000                  
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member]                    
Proceeds from Issuance of Debt $ 100,000,000                  
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount 10.00%                  
Debt Instrument, Redemption Price, Percentage 100.00%                  
The 2019 Note Purchase Agreement [Member] | Otter Tail Power Company [Member] | Senior Unsecured Note [Member[                    
Debt Instrument, Face Amount         $ 175,000,000          
Series 2019 A Senior Unsecured Note Due October 10, 2029 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 10,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.07%          
Series 2019 B Senior Unsecured Note Due October 10, 2039 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 26,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.52%          
Series 2019 C Senior Unsecured Note Due October 10, 2049 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 64,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.82%          
Series 2020 A Senior Unsecured Notes Due February 25, 2030 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 10,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.22%          
Series 2020 B Senior Unsecured Notes Due August 20, 2030 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 40,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.22%          
Series 2020 C Senior Unsecured Notes Due February 25, 2040 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 10,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.62%          
Series 2020 D Senior Unsecured Notes Due February 25, 2050 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount         $ 15,000,000          
Debt Instrument, Interest Rate, Stated Percentage         3.92%          
Series 2018 A senior Unsecured Notes Due February 7, 2048 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount           $ 100,000,000        
Debt Instrument, Interest Rate, Stated Percentage           4.07%        
The 2018 Note Purchase Agreement [Member]                    
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount           10.00%        
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid           100.00%        
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control           100.00%        
The 3.55% Guaranteed Senior Notes, Due December 15, 2026 [Member]                    
Debt Instrument, Face Amount             $ 80,000,000      
Debt Instrument, Interest Rate, Stated Percentage   3.55% 3.55%       3.55%      
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount             10.00%      
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid             100.00%      
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control             100.00%      
The 9.000% Notes, due December 15, 2016 [Member]                    
Debt Instrument, Interest Rate, Stated Percentage     9.00%              
Repayments of Debt     $ 52,330,000              
Series A Senior Unsecured Notes due on February 27, 2029 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount               $ 60,000,000    
Debt Instrument, Interest Rate, Stated Percentage               4.68%    
Series B Senior Unsecured Notes due on February 27, 2044 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount               $ 90,000,000    
Debt Instrument, Interest Rate, Stated Percentage               5.47%    
Note Purchase Agreement 2013 [Member]                    
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount               10.00%    
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid               100.00%    
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control               100.00%    
Priority Debt to Total Capitalization   20.00%                
Interest-bearing Debt, Maximum Percentage of Total Capitalization   60.00%                
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member]                    
Debt Instrument, Interest Rate, Stated Percentage   4.63% 4.63%              
Senior Unsecured Notes 4.63%, Due December 1, 2021 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount                   $ 140,000,000
Debt Instrument, Interest Rate, Stated Percentage                   4.63%
The 2007 Notes [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount   $ 122,000,000                
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member]                    
Debt Instrument, Interest Rate, Stated Percentage   6.15% 6.15%              
Senior Unsecured Notes 6.15%, Series B, Due August 20, 2022 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount   $ 30,000,000                
Debt Instrument, Interest Rate, Stated Percentage   6.15%                
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member]                    
Debt Instrument, Interest Rate, Stated Percentage   6.37% 6.37%              
Senior Unsecured Notes 6.37%, Series C, Due August 20, 2027 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount   $ 42,000,000                
Debt Instrument, Interest Rate, Stated Percentage   6.37%                
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member]                    
Debt Instrument, Interest Rate, Stated Percentage   6.47% 6.47%              
Senior Unsecured Notes 6.47%, Series D, Due August 20, 2037 [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Face Amount   $ 50,000,000                
Debt Instrument, Interest Rate, Stated Percentage   6.47%                
The 2011 and 2007 Note Purchase Agreements [Member]                    
Priority Debt to Total Capitalization   20.00%                
The 2011 and 2007 Note Purchase Agreements [Member] | Maximum [Member]                    
Debt to Total Capitalization Ratio   60.00%                
The 2011 and 2007 Note Purchase Agreements [Member] | Minimum [Member]                    
Interest and Dividend Coverage Ratio   1.50                
The 2011 and 2007 Note Purchase Agreements [Member] | Otter Tail Power Company [Member]                    
Debt Instrument, Prepayment, Minimum Percentage of Aggregate Principal Amount   10.00%                
Debt Instrument, Prepayment, Percentage of the Principal Amount Prepaid   100.00%                
Debt Instrument, Percentage of Principal Amount to be Offered for Prepayment in the Event of a Change of Control   100.00%                
The 2016 Note Purchase Agreement [Member]                    
Priority Debt to Total Capitalization   10.00%