Summary of Components of Net Written Premium (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Direct									$ 1,613,825	$ 1,489,422	$ 1,403,865
Assumed									299,136	223,843	165,046
Ceded written premiums									(441,876)	(441,935)	(382,687)
Net written premiums	$ 346,119	$ 352,412	$ 379,292	$ 393,262	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 1,471,085	$ 1,271,330	$ 1,186,224