Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)
Investments Schedule [Abstract]
Individual fixed maturity held in unrealized loss position | Security	419		419
Individual fixed maturity held in unrealized loss position longer than 12 months | Security	46		46
Impairment losses recognized | $			$ 0	$ 0
Net realized losses on investments | $	$ (1,111,000)	$ (388,000)	$ (1,142,000)	$ (569,000)