Summary of Reserves for Losses and LAE Activity (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,510,451	$ 1,393,126
Provision for Losses and LAE for Claims Occurring in the Current Year	578,272	504,599
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	44,609	(11,644)
Incurred Losses and LAE	622,881	492,955
Losses and LAE Paid for Claims Occurring During:
Current Year	(75,707)	(72,897)
Prior Years	(358,684)	(323,761)
Losses and LAE Payments	(434,391)	(396,658)
Foreign Currency Adjustment	11,688	(9,213)
Net Reserves for Losses and LAE at End of Period	1,710,629	1,480,210
Reinsurance Recoverables on Unpaid Losses and LAE	847,505	787,917	$ 779,276
Gross Reserves for Losses and LAE at End of Period	$ 2,558,134	$ 2,268,127	$ 2,289,727