Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,346,267	$ 2,061,467
Available-for-sale, allowance for credit losses	441	397
Equity securities, cost	324,501	293,190
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	18,067	17,658
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,188	8,634
Property and equipment, accumulated depreciation	$ 75,236	$ 68,682
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)	68,219,551	68,072,794
Common stock, shares outstanding (in shares)	45,289,337	45,142,580
Treasury stock, shares (in shares)	22,930,214	22,930,214