Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		11 Months Ended	96 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	May 03, 2023	May 04, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)	27,875
Intrinsic value of options	$ 8.0	$ 11.0
RLI Corp. Long-Term Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expenses	1.8	2.5
Income tax benefit from stock-based compensation	0.3	$ 0.4
Unrecognized stock-based compensation expense	$ 5.0		$ 5.0
Stock-based compensation expenses, recognition period	2 years 7 months 17 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)	28,475		224,013