Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,046,785	$ 2,945,273
Available-for-sale, allowance for credit losses	704	339
Equity securities, cost	347,772	328,019
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	19,415	18,696
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,131	11,250
Property and equipment, accumulated depreciation	$ 74,416	$ 68,633
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)	68,555,985	68,399,966
Common stock, shares outstanding (in shares)	45,625,771	45,469,752
Treasury stock, shares (in shares)	22,930,214	22,930,214