Summary of Significant Accounting Policies (Details-1) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Trade Receivables, Allowance for Doubtful Accounts, and Concentration Risk
Late payment charge on upaid receivables	1.00%
Late payment charge on unpaid receivables - suspended	1.00%
Accounts receivable, allowance for credit loss, writeoff	$ 1,400,000
Accounts receivable, allowance for credit loss, period increase	5,200,000
Accounts receivable, allowance for credit loss	7,100,000
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	157,252,000	$ 145,746,000
Inventories
Renewable energy credit allowance included in inventory	800,000	600,000
Purchased Gas Adjustment Clause
Purchased gas adjustment, over (under) collected	1,800,000	1,900,000
Regional Transmission Organizations
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting	61,800,000	75,600,000		$ 90,300,000
Allowance for Funds Used During Construction
AFUDC - equity funds	5,900,000	2,300,000		3,300,000
AFUDC - borrowed funds	2,100,000	800,000		1,100,000
Capitalized Software Costs
Capitalized software	20,400,000	24,600,000
Capitalized software - accumulated amortization	36,900,000	31,800,000
Capitalized software amortization	5,100,000	5,100,000		4,700,000
Capitalized Software Hosting Agreement Costs
Capitalized Computer Software Hosting Agreement Net	14,800,000	13,600,000
Capitalized Computer Software Hosting Agreement Amortization	1,800,000	1,400,000		100,000
Capitalized Computer Software Hosting Agreement Accumulated Amortization	3,200,000	1,400,000
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	$ 14,100,000	$ 13,900,000		$ 14,400,000
Deferred COVID Allowance For Credit Loss [Member]
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	3,800,000
Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase agreement, maximum exposure	1,500,000
Repurchase-to-Maturity Transactions, loans due	$ 700,000
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	7.03%	7.00%		7.87%
Electric [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	3.50%	3.60%	[1]	2.90%
Gas [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	2.20%	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.