Summary of Components of Net Earned Premium (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance [Abstract]
Direct									$ 1,330,265	$ 1,223,840	$ 1,183,120
Assumed									171,978	175,153	193,431
Ceded									(401,898)	(414,906)	(440,656)
Net earned premiums	$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 251,545	$ 254,082	$ 242,328	$ 236,132	$ 1,100,345	$ 984,087	$ 935,895