Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,753,631	$ 2,346,267
Available-for-sale, allowance for credit losses	327	441
Equity securities, cost	327,964	324,501
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	18,478	18,067
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	11,400	11,188
Property and equipment, accumulated depreciation	$ 67,911	$ 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)	68,308,385	68,219,551
Common stock, shares outstanding (in shares)	45,378,171	45,289,337
Treasury stock, shares (in shares)	22,930,214	22,930,214