| Document and Entity Information | Aug. 06, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | MGIC INVESTMENT CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0000876437 |
| Document Type | 8-K |
| Document Period End Date | Aug. 6, 2020 |
| Entity Incorporation State Country Code | WI |
| Entity File Number | 1-10816 |
| Entity Tax Identification Number | 39-1486475 |
| Entity Address, Address Line One | 250 East Kilbourn Avenue |
| Entity Address, City or Town | Milwaukee |
| Entity Address, State or Province | WI |
| Entity Address, Postal Zip Code | 53202 |
| City Area Code | (414) |
| Local Phone Number | 347-6480 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | MTG |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
