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Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		26 Months Ended	96 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	May 03, 2023	May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)			233,805
RLI Corp. Long-Term Incentive Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2	$ 2	$ 4	$ 4
Unrecognized stock-based compensation expense	9		$ 9		$ 9
Stock-based compensation expenses, recognition period			2 years 6 months 25 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)			270,006		1,083,188