Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves as of January 1		$ 11,871,250	$ 11,087,200
Less: reinsurance recoverables	[1]	1,650,100	1,236,200
Net reserves as of January 1		10,221,200	9,851,000
Incurred loss and LAE, net of reinsurance, related to:
Current year		756,300	747,000
Prior years		(85,700)	(47,700)
Total incurred loss and LAE, net of reinsurance		670,578	699,305
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]	100,500	114,000
Prior years	[2]	690,800	647,200
Total paid loss and LAE, net of reinsurance	[2]	791,300	761,200
Foreign exchange effect		49,900	(12,200)
Net reserves as of March 31		10,150,400	9,776,900
Reinsurance recoverables as of March 31	[1]	1,563,900	1,213,000
Reserves as of March 31		$ 11,714,269	$ 10,989,900

[1] Reinsurance recoverables in this table include only ceded loss and LAE reserves.

[2] Includes paid losses, net of reinsurance, related to commutations.