Summary of Components of Net Earned 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									$ 1,553,852	$ 1,425,582	$ 1,330,265
Assumed									256,509	187,609	171,978
Ceded									(447,138)	(426,771)	(401,898)
Net Earned Premiums	$ 362,543	$ 347,038	$ 331,015	$ 322,627	$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 1,363,223	$ 1,186,420	$ 1,100,345