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Joint Plant Ownership (Details)
$ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
MW
Mar. 31, 2020
USD ($)
Badger Hollow I Units [Member]    
Jointly Owned Utility Plant Interests [Line Items]    
Projected Solar Farm Project Costs [1] $ 65.0  
Public Utilities Property Plant And Equipment Construction Work In Progress [1],[2] $ 56.4  
Badger Hollow I Units [Member] | Public Service Commission of Wisconsin [Member]    
Allowance for Funds Used During Construction    
Authorized AFUDC rate - Signficiant projects (100%) 100.00%  
Badger Hollow I Units [Member] | MGE [Member]    
Jointly Owned Utility Plant Interests [Line Items]    
Jointly owned utility plant, ownership interest [1] 33.00%  
Jointly owned utility plant, plant capacity (in MW) | MW [1] 50  
Badger Hollow II Units [Member]    
Jointly Owned Utility Plant Interests [Line Items]    
Projected Solar Farm Project Costs [1] $ 65.0  
Public Utilities Property Plant And Equipment Construction Work In Progress [1],[2] $ 5.3  
Badger Hollow II Units [Member] | Public Service Commission of Wisconsin [Member]    
Allowance for Funds Used During Construction    
Authorized AFUDC rate - Signficiant projects (100%) 100.00%  
Badger Hollow II Units [Member] | MGE [Member]    
Jointly Owned Utility Plant Interests [Line Items]    
Jointly owned utility plant, ownership interest [1] 33.00%  
Jointly owned utility plant, plant capacity (in MW) | MW [1] 50  
Badger Hollow I and II [Member]    
Allowance for Funds Used During Construction    
AFUDC $ 1.1 $ 0.4
[1] The Badger Hollow I and Badger Hollow II solar farms are located in southwestern Wisconsin in Iowa County, near the villages of Montfort and Cobb.
[2] Excluding AFUDC.