Summary of Activity in Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,510,451	$ 1,393,126	$ 1,308,136
Provision for Losses and LAE for Claims Occurring in the Current Year	771,955	693,976	637,267
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	34,310	(28,528)	(64,669)
Incurred Losses and LAE	806,265	665,448	572,598
Losses and LAE Paid for Claims Occurring During:
Current Year	(144,095)	(119,485)	(159,802)
Prior Years	(478,979)	(412,544)	(320,863)
Losses and LAE Payments	(623,074)	(532,029)	(480,665)
Foreign Currency Adjustment	11,738	(16,094)	(6,943)
Net Reserves for Losses and LAE at End of Period	1,705,380	1,510,451	1,393,126
Reinsurance Recoverables on Unpaid Losses and LAE	809,765	779,276	809,518
Gross Reserves for Losses and LAE at End of Period	$ 2,515,145	$ 2,289,727	$ 2,202,644