Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,946,920	$ 2,945,273
Available-for-sale, allowance for credit losses	434	339
Equity securities, cost	340,184	328,019
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	18,768	18,696
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	10,251	11,250
Property and equipment, accumulated depreciation	$ 72,404	$ 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)	68,526,410	68,399,966
Common stock, shares outstanding (in shares)	45,596,196	45,469,752
Treasury stock, shares (in shares)	22,930,214	22,930,214