Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 01, 2019	Jun. 30, 2018
Document and Entity Information [Line Items]
Entity Registrant Name	MGE Energy Inc
Entity Central Index Key	0001161728
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Trading Symbol	MGEE
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Well known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,180,572,457
Entity Common Stock Shares Outstanding		34,668,370
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
MGE [Member]
Document and Entity Information [Line Items]
Entity Registrant Name	Madison Gas and Electric Company
Entity Central Index Key	0000061339
Entity Well known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock Shares Outstanding		17,347,894