Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,217,866	$ 3,054,391
Available-for-sale, allowance for credit losses	228	306
Equity securities, cost	381,138	354,022
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	21,791	21,438
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,025	10,608
Property and equipment, accumulated depreciation	$ 74,768	$ 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)	68,676,268	68,570,261
Common stock, shares outstanding (in shares)	45,746,054	45,640,047
Treasury stock, shares (in shares)	22,930,214	22,930,214