Deferred Policy Acquisition Costs - Schedule of Activity in the Deferred Policy Acquisition Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$ 19,300	$ 20,412	$ 19,649
Policy acquisition costs deferred	43,960	43,209	45,604
Amortization expense during the year	(43,009)	(44,321)	(44,841)
Balance, end of year	$ 20,251	$ 19,300	$ 20,412