Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Fixed maturity securities, held-to-maturity, allowance for credit losses	$ 274,000	$ 0
Fixed maturity securities, fair value	621,654,000	621,343,000
Fixed maturity securities, available-for-sale, allowance for credit losses	0	0
Fixed maturity securities, amortized cost	387,665,000	425,698,000
Amounts recoverable from reinsurers, allowance for credit losses	452,000	0
Premiums receivable, allowance for credit losses	4,791,000	5,112,000	$ 5,390,000
Equity securities, cost	$ 35,787,000	$ 24,457,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	50,000,000	50,000,000
Common stock, shares issued	20,589,309	20,560,833
Common stock, shares outstanding	19,331,059	19,302,583
Treasury stock, shares	1,258,250	1,258,250