Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)
Investments Schedule [Abstract]
Individual fixed maturity held in unrealized loss position | Security	507		507
Individual fixed maturity held in unrealized loss position longer than 12 months | Security	101		101
Net unrealized gains on equity securities	$ 573,000		$ 259,000
Impairment losses recognized			0	$ 0
Net realized losses on investments	$ (329,000)	$ (192,000)	(1,471,000)	$ (761,000)
Sale of equity and fixed maturity securities classified as available-for-sale			1,100,000
Net realized losses on fixed maturity securities			$ 400,000