Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves as of January 1				$ 11,087,199	$ 10,799,200
Less: reinsurance recoverables	[1]			1,236,200	1,169,300
Net reserves as of January 1				9,851,000	9,629,900
Other adjustments				(700)	3,200
Incurred loss and LAE, net of reinsurance, related to:
Current year				3,099,000	2,460,200
Prior years		$ (61,500)	$ (86,400)	(173,000)	(261,700)
Total incurred loss and LAE, net of reinsurance		1,491,848	718,556	2,926,039	2,198,512
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]			390,600	206,000
Prior years	[2]			1,743,200	1,702,700
Total paid loss and LAE, net of reinsurance	[2]			2,133,800	1,908,700
Foreign exchange effect				120,500	(22,600)
Net reserves as of September 30		10,763,000	9,900,300	10,763,000	9,900,300
Reinsurance recoverables as of September 30	[1]	1,693,400	1,157,800	1,693,400	1,157,800
Reserves as of September 30		$ 12,456,375	$ 11,058,100	$ 12,456,375	$ 11,058,100

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

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