Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,374,256	$ 2,346,267
Available-for-sale, allowance for credit losses	594	441
Equity securities, cost	347,354	324,501
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	18,381	18,067
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	10,983	11,188
Property and equipment, accumulated depreciation	$ 77,131	$ 75,236
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)	68,246,156	68,219,551
Common stock, shares outstanding (in shares)	45,315,942	45,289,337
Treasury stock, shares (in shares)	22,930,214	22,930,214