Reinsurance - Effects of Reinsurance Premium Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Direct Written	$ 254,530	$ 262,110	$ 247,498
Assumed Written	28,072	37,381	35,500
Ceded Written	(425)	(450)	(372)
Written Total	282,177	299,041	282,626
Direct Earned	248,803	265,256	233,652
Assumed Earned	29,843	38,522	33,707
Ceded Earned	(425)	(450)	(372)
Earned Total	$ 278,221	$ 303,328	$ 266,987