Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Document And Entity Information [Line Items]
Entity Registrant Name	MGE Energy, Inc.
Entity Central Index Key	0001161728
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Amendment Flag	false
Trading Symbol	MGEE
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock Shares Outstanding		34,668,370
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Security Exchange Name	NASDAQ
Security 12(b) Title	Common Stock, $1 Par Value Per Share
Entity Tax Identification Number	39-2040501
Entity File Number	000-49965
Entity Incorporation State Country Code	WI
Entity Address Address Line 1	133 South Blair Street
EntityAddressCityOrTown	Madison
Entity Address State Or Province	WI
Entity Address Postal Zip Code	53788
City Area Code	608
Local Phone Number	252-7000
Document Transition Report	false
Entity Interactive Data Current	Yes
Document Quarterly Report	true
MGE [Member]
Document And Entity Information [Line Items]
Entity Registrant Name	Madison Gas and Electric Company
Entity Central Index Key	0000061339
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock Shares Outstanding		17,347,894
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Tax Identification Number	39-0444025
Entity File Number	000-1125
Entity Incorporation State Country Code	WI
Entity Address Address Line 1	133 South Blair Street
EntityAddressCityOrTown	Madison
Entity Address State Or Province	WI
Entity Address Postal Zip Code	53788
City Area Code	608
Local Phone Number	252-7000
Document Transition Report	false
Entity Interactive Data Current	Yes
Document Quarterly Report	true