Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Fixed maturity securities, fair value	$ 599,885	$ 639,309
Fixed maturity securities, cost	473,315	461,236
Equity securities, cost	$ 14,532	$ 8,503
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	50,000,000	50,000,000
Common stock, shares issued	20,528,230	20,504,165
Common stock, shares outstanding	19,269,980	19,245,915
Treasury stock, shares	1,258,250	1,258,250