Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,030,979	$ 1,915,278
Available-for-sale, allowance for credit losses	632	0
Equity securities, cost	280,337	262,131
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	17,246	16,682
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	8,097	9,402
Property and equipment, accumulated depreciation	$ 67,555	$ 62,703
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	100,000,000
Common stock, shares issued (in shares)	67,985,378	67,799,229
Common stock, shares outstanding (in shares)	45,055,164	44,869,015
Treasury stock, shares (in shares)	22,930,214	22,930,214