Summary of Activity in Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,705,380	$ 1,510,451	$ 1,393,126
Acquired Net Reserves	16,159
Provision for Losses and LAE for Claims Occurring in the Current Year	817,537	771,955	693,976
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	76,242	34,310	(28,528)
Incurred Losses and LAE	893,779	806,265	665,448
Losses and LAE Paid for Claims Occurring During:
Current Year	(124,754)	(144,095)	(119,485)
Prior Years	(554,733)	(478,979)	(412,544)
Losses and LAE Payments	(679,487)	(623,074)	(532,029)
Foreign Currency Adjustment	(12,047)	11,738	(16,094)
Net Reserves for Losses and LAE at End of Period	1,923,784	1,705,380	1,510,451
Reinsurance Recoverables on Unpaid Losses and LAE	820,189	809,765	779,276
Gross Reserves for Losses and LAE at End of Period	$ 2,743,973	$ 2,515,145	$ 2,289,727