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Stockholders' Equity (Details) - USD ($)
$ in Thousands
 9 Months Ended
May 03, 2021
Jul. 15, 2020
Sep. 30, 2021
Dec. 31, 2020
Class of Stock [Line Items]        
Total capitalization     $ 1,547,101 $ 1,495,398
Otter Tail Power Company        
Class of Stock [Line Items]        
Equity to total capitalization ratio     52.60%  
Net assets restricted from distribution     $ 674,900  
Minimum | Otter Tail Power Company | Minnesota Public Utilities Commission        
Class of Stock [Line Items]        
Public utilities, requested equity capital structure, percentage   47.50%    
Maximum | Otter Tail Power Company        
Class of Stock [Line Items]        
Total capitalization     $ 1,700,000  
Maximum | Otter Tail Power Company | Minnesota Public Utilities Commission        
Class of Stock [Line Items]        
Public utilities, requested equity capital structure, percentage   58.10%    
Second Shelf Registration        
Class of Stock [Line Items]        
Shelf registration (in shares) 1,500,000