Employee Benefits - Stock Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended			60 Months Ended	92 Months Ended
	Nov. 10, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	May 06, 2015	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)		344,100
Special cash dividend declared	$ 7.00
Intrinsic value of options		$ 21.8	$ 12.2	$ 15.5
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant					4,000,000
Options granted (in shares)					2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant		4,000,000				4,000,000
Options granted (in shares)		363,368				3,263,788
Stock-based compensation expenses		$ 8.8	6.4	5.4
Income tax benefit from stock-based compensation		1.4	$ 1.0	$ 0.9
Unrecognized stock-based compensation expense		$ 6.6				$ 6.6
Stock-based compensation expenses, recognition period		2 years 3 months 25 days
RLI Corp. Long-Term Incentive Plan (LTIP 2010 and LTIP 2015) | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period		5 years
Term of options		8 years
Age and period of service of the participant to be eligible for retirement		75 years