Summary of Reserves for Losses and LAE Activity (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,923,784	$ 1,705,380
Provision for Losses and LAE for Claims Occurring in the Current Year	218,029	184,179
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	52,323	1,966
Incurred Losses and LAE	270,352	186,145
Losses and LAE Paid for Claims Occurring During:
Current Year	(19,251)	(9,206)
Prior Years	(203,287)	(155,860)
Losses and LAE Payments	(222,538)	(165,066)
Foreign Currency Adjustment	8,084	4,980
Net Reserves for Losses and LAE at End of Period	1,979,682	1,731,439
Reinsurance Recoverables on Unpaid Losses and LAE	830,732	790,388	$ 820,189
Gross Reserves for Losses and LAE at End of Period	$ 2,810,414	$ 2,521,827	$ 2,743,973