Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	9 Months Ended		96 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	May 03, 2023	May 04, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)				172,250
Intrinsic value of options				$ 17.0	$ 6.0
RLI Corp. Long-Term Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2.0	$ 2.4		6.6	6.2
Income tax benefit from stock-based compensation	0.3	$ 0.4		1.1	$ 1.0
Unrecognized stock-based compensation expense	$ 7.0		$ 7.0	$ 7.0
Stock-based compensation expenses, recognition period				2 years 11 months 19 days
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)			165,063