Statutory Information and Dividend Restrictions - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) company	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Statutory Accounting Practices [Line Items]
Number of insurance companies | company	3
Fair value of investment in affiliate included in statutory surplus	$ 271,200	$ 238,000	$ 190,900
Cost basis of investment in affiliate included in statutory surplus	64,600
Equity	1,229,361	1,135,978	995,388	$ 806,842
Remaining unrestricted equity, liquid assets	87,900
RLI Ins.
Statutory Accounting Practices [Line Items]
Authorized control level RBC	225,100	203,900	191,000
Consolidated surplus, statutory basis	$ 1,200,000	1,100,000	1,000,000
Dividend restriction as percentage of policyholder surplus	10.00%
Payments of Ordinary Dividends	$ 70,000	110,000	59,000
Extraordinary dividend paid after seeking and receiving approval from the Illinois regulatory authorities	110,000	$ 0	$ 0
Non restricted net assets	$ 26,100