| Cover | Aug. 04, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 23, 2021 |
| Entity Registrant Name | MGIC Investment Corporation |
| Entity Incorporation, State or Country Code | WI |
| Entity File Number | 1-10816 |
| Entity Tax Identification Number | 39-1486475 |
| Entity Address, Address Line One | 250 E. 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 |
| Title of each class | Common stock |
| Trading Symbol | MTG |
| Security Exchange Name | NYSE |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000876437 |
| Amendment Flag | false |
