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Property, Plant, and Equipment (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Property, Plant and Equipment [Line Items]      
In-service utility plant, net $ 1,630,286 $ 1,530,199  
Construction work in progress 139,099 112,484  
Total property, plant, and equipment 1,769,385 1,642,683  
Allowance for Funds Used During Construction      
AFUDC equity recognized 5,900 2,300 $ 3,300
Two Creeks Units [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross (excluding AFUDC) 62,900    
Allowance for Funds Used During Construction      
AFUDC equity recognized 2,400 1,000  
Badger Hollow I Units [Member]      
Property, Plant and Equipment [Line Items]      
Construction work in progress 54,700 18,700  
Allowance for Funds Used During Construction      
AFUDC equity recognized 2,000 $ 200  
Badger Hollow II Units [Member]      
Property, Plant and Equipment [Line Items]      
Construction work in progress 5,200    
Allowance for Funds Used During Construction      
AFUDC equity recognized $ 200    
PSCW [Member]      
Allowance for Funds Used During Construction      
Authorized AFUDC rate 50.00% 50.00% 50.00%
Authorized AFUDC rate - Signficiant projects (100%) 100.00% 100.00% 100.00%
PSCW [Member] | Two Creeks Units [Member]      
Allowance for Funds Used During Construction      
Authorized AFUDC rate - Signficiant projects (100%) 100.00%    
PSCW [Member] | Badger Hollow I Units [Member]      
Allowance for Funds Used During Construction      
Authorized AFUDC rate - Signficiant projects (100%) 100.00%    
PSCW [Member] | Badger Hollow II Units [Member]      
Allowance for Funds Used During Construction      
Authorized AFUDC rate - Signficiant projects (100%) 100.00%    
First Mortgage Bonds [Member] | 7.70%, 2028 Series [Member]      
Plant related debt items      
Long-term debt, gross [1] $ 1,200 $ 1,200  
Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 2,105,108 1,952,807  
Less: Accumulated depreciation and amortization 721,382 674,251  
In-service utility plant, net 1,383,726 1,278,556  
Construction work in progress [2] 137,725 111,707  
Electric [Member] | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 1,608,658 1,480,684  
Gas [Member] | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 496,450 472,123  
Non Regulated Energy [Member] | Non-regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 320,691 323,266  
Less: Accumulated depreciation and amortization 74,131 71,623  
In-service utility plant, net 246,560 251,643  
Construction work in progress 1,374 777  
MGE [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, net 1,630,314 1,530,227  
Construction work in progress 139,099 112,484  
Total property, plant, and equipment 1,769,413 1,642,711  
MGE [Member] | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 2,105,136 1,952,835  
Less: Accumulated depreciation and amortization 721,382 674,251  
In-service utility plant, net 1,383,754 1,278,584  
Construction work in progress [2] 137,725 111,707  
MGE [Member] | Electric [Member] | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 1,608,675 1,480,701  
MGE [Member] | Gas [Member] | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 496,461 472,134  
MGE [Member] | Non Regulated Energy [Member] | Non-regulated [Member]      
Property, Plant and Equipment [Line Items]      
In-service utility plant, gross 320,691 323,266  
Less: Accumulated depreciation and amortization 74,131 71,623  
In-service utility plant, net 246,560 251,643  
Construction work in progress $ 1,374 $ 777  
[1] MGE's utility plant is subject to the lien of its Indenture of Mortgage and Deed of Trust, under which its first mortgage bonds are issued. The Mortgage Indenture provides that dividends or any other distribution or purchase of MGE shares may not be made if the aggregate amount thereof since December 31, 1945, would exceed the earned surplus (retained earnings) accumulated subsequent to December 31, 1945. As of December 31, 2020, approximately $529.9 million was available for the payment of dividends under this covenant.
[2] In 2019, the PSCW authorized construction of Badger Hollow I. Construction of the project is expected to be completed in April 2021. As of December 31, 2020, and 2019, MGE had $54.7 million and $18.7 million (excluding AFUDC), respectively, in construction work in progress. After tax, MGE recognized $2.0 million and $0.2 million, respectively, of AFUDC equity related to Badger Hollow I for the years ended December 31, 2020 and 2019. Phase II of the project was authorized by the PSCW in 2020. Construction of Badger Hollow II is expected to be completed in December 2022. As of December 31, 2020, MGE had $5.2 million (excluding AFUDC) in construction work in progress. After tax, MGE recognized $0.2 million of AFUDC equity related to Badger Hollow II for the year ended December 31, 2020. MGE received specific approval to recover 100% AFUDC on these projects.