Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		17 Months Ended	96 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	May 03, 2023	May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)			165,802
RLI Corp. Long-Term Incentive Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2.0	$ 2.0	$ 6.0	$ 7.0
Income tax benefit from stock-based compensation			1.0	$ 1.0
Unrecognized stock-based compensation expense	8.0		$ 8.0		$ 8.0
Stock-based compensation expenses, recognition period			2 years 7 months 20 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	$ 1.0	$ 1.0
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]
Shares authorized for grant						4,000,000
Options granted (in shares)						3,291,388
RLI Corp. Long-Term Incentive Plan (2023 LTIP)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares authorized for grant							4,004,891
Options granted (in shares)			186,483		382,021