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Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 29 Months Ended | 96 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | May 03, 2023 | May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Options granted (in shares) |  |  | 280,680 |  |  |  |
RLI Corp. Long-Term Incentive Plans |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Stock-based compensation expenses | $ 2 | $ 2 | $ 6 | $ 6 |  |  |
Unrecognized stock-based compensation expense | 7 |  | $ 7 |  | $ 7 |  |
Stock-based compensation expenses, recognition period |  |  | 2 years 6 months 10 days |  |  |  |
RLI Corp. Long-Term Incentive Plans | Maximum |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Income tax benefit from stock-based compensation | $ 1 | $ 1 | $ 1 | $ 1 |  |  |
RLI Corp. Long-Term Incentive Plans | Employee Stock Option |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period |  |  | 5 years |  |  |  |
Term of options |  |  | 8 years |  |  |  |
RLI Corp. Long-Term Incentive Plans | Employee Stock Option | Minimum |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Age and period of service of the participant to be eligible for retirement |  |  | 75 years |  |  |  |
RLI Corp. Long-Term Incentive Plan (2015 LTIP) |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Options granted (in shares) |  |  |  |  |  | 6,582,776 |
RLI Corp. Long-Term Incentive Plan (2015 LTIP) | Maximum |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Shares authorized for grant |  |  |  |  |  | 8,000,000 |
RLI Corp. Long-Term Incentive Plan (2023 LTIP) |  |  |  |  |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] |  |  |  |  |  |  |
Shares authorized for grant |  |  |  |  |  |  | 8,009,782
Options granted (in shares) |  |  | 317,581 |  | 1,130,763 |  |

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