Reinsurance - Schedule of the Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Premiums Written and Earned [Abstract]
Gross Written									$ 351,696	$ 350,267	$ 373,055
Ceded Written									(9,344)	(8,869)	(10,307)
Net premiums Written									342,352	341,398	362,748
Gross Earned									359,670	355,025	379,011
Ceded Earned									(9,344)	(8,869)	(10,307)
Net premiums Earned	$ 88,837	$ 85,184	$ 88,995	$ 87,310	$ 87,377	$ 85,118	$ 82,749	$ 90,912	$ 350,326	$ 346,156	$ 368,704