Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,302,684	$ 3,054,391
Available-for-sale, allowance for credit losses	234	306
Equity securities, cost	405,663	354,022
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	22,049	21,438
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	10,055	10,608
Property and equipment, accumulated depreciation	$ 74,821	$ 74,279
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	68,740,134	68,570,261
Common stock, shares outstanding (in shares)	45,809,920	45,640,047
Treasury stock, shares (in shares)	22,930,214	22,930,214