Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Available-for-sale, amortized cost	$ 2,219,502	$ 2,061,467
Available-for-sale, allowance for credit losses	204	397
Equity securities, cost	308,374	293,190
Premiums and reinsurance balances receivable, allowances for uncollectible amounts	18,410	17,658
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	9,220	8,634
Property and equipment, accumulated depreciation	$ 71,928	$ 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)	68,153,557	68,072,794
Common stock, shares outstanding (in shares)	45,223,343	45,142,580
Treasury stock, shares (in shares)	22,930,214	22,930,214