| Document and Entity Information | Jan. 22, 2020 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 22, 2020 |
| Entity File Number | 001-09463 |
| Entity Registrant Name | RLI Corp |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 37-0889946 |
| Entity Address, Address Line One | 9025 North Lindbergh Drive |
| Entity Address, State or Province | IL |
| Entity Address, City or Town | Peoria |
| Entity Address, Postal Zip Code | 61615 |
| City Area Code | 309 |
| Local Phone Number | 692-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock $0.01 par value |
| Trading Symbol | RLI |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000084246 |
| Amendment Flag | false |
