| Document and Entity Information | May 01, 2019 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 4, 2019 |
| Entity File Number | 000-50070 |
| Entity Registrant Name | SAFETY INSURANCE GROUP, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identificatoin Number | 13-4181699 |
| Entity Address, Address Line One | 20 Custom House Street |
| Entity Address, State or Province | MA |
| Entity Address, City or Town | Boston |
| Entity Address, Postal Zip Code | 02110 |
| City Area Code | (617) |
| Local Phone Number | 951-0600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | SAFT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001172052 |
| Amendment Flag | false |
