Statutory Information and Dividend Restrictions - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($) company	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Statutory Accounting Practices [Line Items]
Number of insurance companies | company		3
Fair value of investment in affiliate included in statutory surplus		$ 352,000	$ 294,000	$ 327,000
Cost basis of investment in affiliate included in statutory surplus		65,000
Equity		1,521,967	1,413,514	1,177,341	$ 1,229,361
Liquid assets		$ 39,000
Unrestricted portion of net assets as a percent of normal annual expenditures.		66.70%
RLI Ins.
Statutory Accounting Practices [Line Items]
Authorized control level RBC		$ 296,000	273,000	250,000
Consolidated surplus, statutory basis		$ 1,800,000	1,500,000	1,400,000
Dividend restriction as percentage of policyholder surplus		10.00%
Payments of Ordinary Dividends		$ 152,000	145,000	13,000
Extraordinary dividend paid after seeking and receiving approval from the Illinois regulatory authorities		$ 0	$ 0	$ 0
Ordinary dividend capacity available for distribution	$ 241,000