Summary of Components of Net Written 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									$ 1,403,865	$ 1,277,728	$ 1,232,012
Assumed									165,046	175,774	200,341
Ceded written premiums									(382,687)	(409,642)	(432,215)
Net written premiums	$ 282,868	$ 277,001	$ 306,535	$ 319,820	$ 244,719	$ 251,939	$ 258,244	$ 288,958	$ 1,186,224	$ 1,043,860	$ 1,000,138