| Document and Entity Information | Sep. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 29, 2021 |
| Entity Registrant Name | STATE AUTO FINANCIAL CORPORATION |
| Entity Incorporation, State or Country Code | OH |
| Entity File Number | 000-19289 |
| Entity Tax Identification Number | 31-1324304 |
| Entity Address, Address Line One | 518 East Broad Street |
| Entity Address, City or Town | Columbus |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 43215-3976 |
| City Area Code | 614 |
| Local Phone Number | 464-5000 |
| 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 | 0000874977 |
| Title of 12(b) Security | Common shares, without par value |
| Trading Symbol | STFC |
| Security Exchange Name | NASDAQ |
