Document and Entity Information
| Document and Entity Information | Aug. 20, 2021 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | MGE Energy, Inc. |
| Entity Central Index Key | 0001161728 |
| Document Type | 8-K |
| Document Period End Date | Aug. 20, 2021 |
| Amendment Flag | false |
| Trading Symbol | MGEE |
| Entity Emerging Growth Company | false |
| Security Exchange Name | NASDAQ |
| Security 12b Title | Common Stock, $1 Par Value Per Share |
| Entity Tax Identification Number | 39-2040501 |
| Entity File Number | 000-49965 |
| Entity Address Address Line1 | 133 South Blair Street |
| Entity Address City Or Town | Madison |
| Entity Address State Or Province | WI |
| Entity Address Postal Zip Code | 53788 |
| City Area Code | 608 |
| Local Phone Number | 252-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Issuer Tender Offer | false |
| Pre-commencement Tender Offer | false |
| Entity Incorporation State Country Code | WI |