Employee Benefits - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended			20 Months Ended	96 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024	May 03, 2023	May 04, 2023
Share Based Compensation
Options granted (in shares)	379,344
RLI Corp. Long-Term Incentive Plans
Share Based Compensation
Stock-based compensation expenses	$ 8	$ 9	$ 9
Income tax benefit from stock-based compensation	1	$ 1	$ 1
Unrecognized stock-based compensation expense	$ 7			$ 7
Stock-based compensation expenses, recognition period	2 years 7 months 6 days
RLI Corp. Long-Term Incentive Plan (2023 LTIP)
Share Based Compensation
Options granted (in shares)				813,182
RLI Corp. Long-Term Incentive Plan (2023 LTIP) | Maximum
Share Based Compensation
Shares authorized for grant						8,009,782
RLI Corp. Long-Term Incentive Plan (2023 LTIP) | Employee Stock Option
Share Based Compensation
Award vesting period	5 years
Term of options	8 years
RLI Corp. Long-Term Incentive Plan (2023 LTIP) | Employee Stock Option | Minimum
Share Based Compensation
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
Options granted (in shares)					6,582,776
RLI Corp. Long-Term Incentive Plan (2015 LTIP) | Maximum
Share Based Compensation
Shares authorized for grant					8,000,000