Summary of Reserves for Losses and LAE Activity (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,705,380	$ 1,510,451
Provision for Losses and LAE for Claims Occurring in the Current Year	184,179	162,515
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	1,966	7,085
Incurred Losses and LAE	186,145	169,600
Losses and LAE Paid for Claims Occurring During:
Current Year	(9,206)	(16,287)
Prior Years	(155,860)	(126,722)
Losses and LAE Payments	(165,066)	(143,009)
Foreign Currency Adjustment	4,980	(226)
Net Reserves for Losses and LAE at End of Period	1,731,439	1,536,816
Reinsurance Recoverables on Unpaid Losses and LAE	790,388	794,576	$ 809,765
Gross Reserves for Losses and LAE at End of Period	$ 2,521,827	$ 2,331,392	$ 2,515,145