Reinsurance - Schedule of the Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Written and Earned [Abstract]
Gross Written									$ 350,267	$ 373,055	$ 386,529
Ceded Written									(8,869)	(10,307)	(11,228)
Net premiums Written									341,398	362,748	375,301
Gross Earned									355,025	379,011	387,122
Ceded Earned									(8,869)	(10,307)	(11,228)
Net premiums Earned	$ 87,377	$ 85,118	$ 82,749	$ 90,912	$ 92,097	$ 89,918	$ 90,728	$ 95,961	$ 346,156	$ 368,704	$ 375,894