Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,054,391	$ 2,945,273
Available-for-sale, allowance for credit losses	306	339
Equity securities, cost	354,022	328,019
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	21,438	18,696
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	10,608	11,250
Property and equipment, accumulated depreciation	$ 74,279	$ 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)	68,570,261	68,399,966
Common stock, shares outstanding (in shares)	45,640,047	45,469,752
Treasury stock, shares (in shares)	22,930,214	22,930,214