Summary of Significant Accounting Policies (Details-1) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
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	$ 700,000	$ 500,000
Purchased Gas Adjustment Clause
Purchased gas adjustment, over (under) collected	1,200,000	1,400,000
Regional Transmission Organizations
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting	$ 90,300,000	87,900,000		$ 77,200,000
Allowance for Funds Used During Construction
Authorized AFUDC rate	100.00%
Capitalized Software Costs
Capitalized software	$ 15,100,000	11,100,000
Capitalized software amortization	4,700,000	3,300,000		3,000,000
Capitalized Software Hosting Agreement Costs
Capitalized Computer Software Hosting Agreement Net	9,200,000	500,000
Capitalized Computer Software Hosting Agreement Amortization	100,000	0		0
Capitalized Computer Software Hosting Agreement Accumulated Amortization	100,000	0
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	$ 14,400,000	$ 14,100,000		$ 14,500,000
Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase agreement, maximum exposure	10,000,000
2019 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	396,000
2020 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	407,000
2021 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	392,000
2022 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	354,000
2023 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	510,000
Thereafter [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	$ 122,000
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%
Rate at which AFUDC was capitalized	7.87%	7.87%		7.93%
Columbia Environmental Project, SCR [Member] | PSCW [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate	100.00%	100.00%		100.00%
Saratoga Wind [Member] | PSCW [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate	100.00%
Electric [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	2.90%	3.00%	[1]	2.50%	[2]
Gas [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	2.10%	2.10%		2.10%	[2]
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 September 2016, the PSCW approved new depreciation rates for Columbia effective January 1, 2017.

[2] In December 2015, the PSCW approved new depreciation rates, which were implemented and became effective as of January 1, 2016.