Statutory Information and Dividend Restrictions - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) company	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Statutory Accounting Practices [Line Items]
Number of insurance companies | company	3
Fair value of investment in affiliate included in statutory surplus	$ 294,000	$ 327,000	$ 271,000
Cost basis of investment in affiliate included in statutory surplus	65,000
Equity	1,413,514	1,177,341	1,229,361	$ 1,135,978
Liquid assets	145,000
RLI Ins.
Statutory Accounting Practices [Line Items]
Authorized control level RBC	273,000	250,000	225,000
Consolidated surplus, statutory basis	$ 1,500,000	1,400,000	1,200,000
Dividend restriction as percentage of policyholder surplus	10.00%
Payments of Ordinary Dividends	$ 145,000	13,000	70,000
Extraordinary dividend paid after seeking and receiving approval from the Illinois regulatory authorities	0	$ 0	$ 110,000
Non restricted net assets	$ 7,000