Summary of Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Restricted Cash [Abstract]
Restricted cash	$ 2,100,000		$ 3,700,000
Receivable - Margin Account [Abstract]
Receivable, margin account balance, net of collateral posted against derivative positions	2,500,000		1,300,000
Derivative liability, collateral offset	$ 100,000		0
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	$ 500,000		300,000
Purchased Gas Adjustment Clause
Purchased gas adjustment, over (under) collected	1,400,000		900,000
Regional Transmission Organizations
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting	87,900,000		77,200,000		$ 68,600,000
Capitalized Software Costs
Capitalized software	11,100,000		12,300,000
Capitalized software amortization	3,300,000		3,000,000		2,200,000
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		$ 14,500,000		$ 14,700,000
Revision To Cash Flow Statement [Abstract]
Revision to classification of prior year cash flow item	2,100,000
Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase agreement, maximum exposure	10,000,000
2018 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	538,000
2019 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	542,000
2020 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	537,000
2021 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	431,000
2022 [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	385,000
Thereafter [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
Repurchase-to-Maturity Transactions, loans due	$ 697,000
Minimum [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
MGE guarantee to the financial institution, term	1 year
Minimum [Member] | Capitalized Software [Member]
Capitalized Software Costs
Property, plant, and equipment, useful life	5 years
Maximum [Member] | Chattel Paper Agreements [Member]
Transfers and Servicing
MGE guarantee to the financial institution, term	10 years
Maximum [Member] | Capitalized Software [Member]
Capitalized Software Costs
Property, plant, and equipment, useful life	10 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%		7.93%		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	3.00%	[1]	2.50%	[2]	2.60%
Gas [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	2.10%		2.10%	[2]	1.70%
Non Regulated Energy [Member]
Property, Plant, and Equiptment
Composite straight-line depreciation rates	2.30%		2.30%		2.40%
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.