Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)
Investments Schedule [Abstract]
Individual fixed maturity held in unrealized loss position | Security	448
Individual fixed maturity held in unrealized loss position longer than 12 months | Security	44
Impairment losses recognized | $	$ 0	$ 0
Net realized losses on investments | $	$ (31,000)	$ (181,000)