Deferred Policy Acquisition Costs - Summary of Activity with Respect to Deferred Policy Acquisition Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Beginning balance	$ 16,639	$ 18,602
Policy acquisition costs deferred	35,736	35,905
Amortization	(35,663)	(37,868)	$ (36,224)
Ending balance	$ 16,712	$ 16,639	$ 18,602