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Summary of Significant Accounting Policies (Details-1) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Trade Receivables, Allowance for Doubtful Accounts, and Concentration Risk      
Late payment charge on upaid receivables 1.00%    
Inventories      
Renewable energy credit allowance included in inventory $ 600,000 $ 700,000  
Purchased Gas Adjustment Clause      
Purchased gas adjustment, over (under) collected 1,900,000 1,200,000  
Regional Transmission Organizations      
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting 75,600,000 90,300,000 $ 87,900,000
Allowance for Funds Used During Construction      
AFUDC - equity funds 2,300,000 3,300,000 1,200,000
AFUDC - borrowed funds 800,000 1,100,000 400,000
Capitalized Software Costs      
Capitalized software 24,600,000 15,100,000  
Capitalized software amortization 5,100,000 4,700,000 3,300,000
Capitalized Software Hosting Agreement Costs      
Capitalized Computer Software Hosting Agreement Net 14,700,000 9,200,000  
Capitalized Computer Software Hosting Agreement Amortization 1,400,000 100,000 0
Capitalized Computer Software Hosting Agreement Accumulated Amortization 1,400,000 100,000  
Impairment of Long-Lived Assets      
Impairment of long-lived assets $ 0 0 0
Excise taxes      
License fee tax rate, electric, retail sales 3.19%    
License fee tax rate, electric, resale by purchaser 1.59%    
License fee tax rate, natural gas 0.97%    
License fee tax expense $ 13,900,000 $ 14,400,000 $ 14,100,000
Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase agreement, maximum exposure 10,000,000.0    
2020 [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due 280,000    
2021 [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due 267,000    
2022 [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due 250,000    
2023 [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due 181,000    
2024 [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due 70,000    
Thereafter [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
Repurchase-to-Maturity Transactions, loans due $ 119,000    
Minimum [Member]      
Capitalized Software Hosting Agreement Costs      
Capitalized software hosting agreement, useful life 5 years    
Minimum [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
MGE guarantee to the financial institution, term 1 year    
Minimum [Member] | Capitalized Software [Member]      
Property, Plant, and Equiptment      
Property, plant, and equipment, useful life 5 years    
Maximum [Member]      
Capitalized Software Hosting Agreement Costs      
Capitalized software hosting agreement, useful life 10 years    
Maximum [Member] | Chattel Paper Agreements [Member]      
Transfers and Servicing      
MGE guarantee to the financial institution, term 10 years    
Maximum [Member] | Capitalized Software [Member]      
Property, Plant, and Equiptment      
Property, plant, and equipment, useful life 15 years    
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%
Rate at which AFUDC was capitalized 7.00% 7.87% 7.87%
Saratoga Wind [Member]      
Allowance for Funds Used During Construction      
AFUDC - equity funds $ 4,100,000    
Saratoga Wind [Member] | PSCW [Member]      
Allowance for Funds Used During Construction      
Authorized AFUDC rate - Signficiant projects (100%) 100.00%    
Electric [Member]      
Property, Plant, and Equiptment      
Composite straight-line depreciation rates 3.60% [1] 2.90% 3.00%
Gas [Member]      
Property, Plant, and Equiptment      
Composite straight-line depreciation rates 2.10% 2.10% 2.10%
Non Regulated Energy [Member]      
Property, Plant, and Equiptment      
Composite straight-line depreciation rates 2.30% 2.30% 2.30%
MGE Power Elm Road [Member]      
Principles of Consolidation      
Ownership percentage by parent 100.00%    
MGE Power West Campus [Member]      
Principles of Consolidation      
Ownership percentage by parent 100.00%    
[1] In the December 2018 rate settlement, the PSCW approved new depreciation rates for Columbia effective January 1, 2019.