Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 3,082,273	$ 3,054,391
Available-for-sale, allowance for credit losses	237	306
Equity securities, cost	362,527	354,022
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	21,699	21,438
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,085	10,608
Property and equipment, accumulated depreciation	$ 75,558	$ 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)	68,640,095	68,570,261
Common stock, shares outstanding (in shares)	45,709,881	45,640,047
Treasury stock, shares (in shares)	22,930,214	22,930,214