Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		23 Months Ended	96 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	May 03, 2023	May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)	47,780
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
Unrecognized stock-based compensation expense	$ 6.0		$ 6.0
Stock-based compensation expenses, recognition period	2 years 6 months 18 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]
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)	49,530		862,712