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Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended		41 Months Ended	96 Months Ended
	Sep. 30, 2026	Jun. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	May 03, 2023	May 04, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)				386,975
Special cash dividend declared		$ 2
RLI Corp. Long-Term Incentive Plans
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation expenses	$ 2		$ 2	$ 5	$ 6
Unrecognized stock-based compensation expense	7			$ 7		$ 7
Stock-based compensation expenses, recognition period				2 years 4 months 2 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]
Options granted (in shares)				439,784		1,620,422
RLI Corp. Long-Term Incentive Plan (2023 LTIP) \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares authorized for grant								8,009,782