Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		29 Months Ended	96 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	May 03, 2023	May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)			280,680
RLI Corp. Long-Term Incentive Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2	$ 2	$ 6	$ 6
Unrecognized stock-based compensation expense	7		$ 7		$ 7
Stock-based compensation expenses, recognition period			2 years 6 months 10 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	$ 1	$ 1	$ 1
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)			317,581		1,130,763