Summary of Components of Net Written Premium (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
Insurance [Abstract]
Direct									$ 1,489,422	$ 1,403,865	$ 1,277,728
Assumed									223,843	165,046	175,774
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 282,868	$ 277,001	$ 306,535	$ 319,820	$ 1,271,330	$ 1,186,224	$ 1,043,860