Employee Benefits - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	8 Months Ended	12 Months Ended			96 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	May 03, 2023	May 04, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)		201,125
Intrinsic value of options		$ 19	$ 22	$ 12
RLI Corp. Long-Term Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expenses		9	9	6
Income tax benefit from stock-based compensation		1	$ 1	$ 1
Unrecognized stock-based compensation expense	$ 6	$ 6
Stock-based compensation expenses, recognition period		2 years 8 months 8 days
RLI Corp. Long-Term Incentive Plan (2023 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)	195,538
RLI Corp. Long-Term Incentive Plan (2023 LTIP) | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant						4,004,891
RLI Corp. Long-Term Incentive Plan (2023 LTIP) | 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 Plan (2023 LTIP) | 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)					3,291,388
RLI Corp. Long-Term Incentive Plan (2015 LTIP) | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant					4,000,000