| Cover | Oct. 30, 2019 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 30, 2019 |
| Entity File Number | 001-33067 |
| Entity Registrant Name | SELECTIVE INSURANCE GROUP, INC. |
| Entity Central Index Key | 0000230557 |
| Entity Tax Identification Number | 22-2168890 |
| Entity Incorporation, State or Country Code | NJ |
| Entity Address, Address Line One | 40 Wantage Avenue |
| Entity Address, City or Town | Branchville |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07890 |
| City Area Code | 973 |
| Local Phone Number | 948-3000 |
| 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 $2 per share |
| Trading Symbol | SIGI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
