Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Activity in liability for loss and loss adjustment expense
Reserves as of January 1				$ 12,250,294	$ 11,871,300
Less: reinsurance recoverables	[1]			1,857,400	1,650,100
Net reserves as of January 1				10,392,900	10,221,200
Other adjustments				(3,200)	1,200
Incurred loss and LAE, net of reinsurance, related to:
Current year				2,649,900	2,579,300
Prior years		$ (58,900)	$ (75,000)	(152,900)	(212,800)
Total incurred loss and LAE, net of reinsurance		907,736	957,703	2,497,037	2,366,491
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]			484,400	444,900
Prior years	[2]			2,238,900	1,928,900
Total paid loss and LAE, net of reinsurance	[2]			2,723,300	2,373,800
Foreign currency exchange rate effect				(39,100)	(77,000)
Net reserves as of September 30		10,124,300	10,138,100	10,124,300	10,138,100
Reinsurance recoverables as of September 30	[1]	1,509,700	1,716,800	1,509,700	1,716,800
Reserves as of September 30		$ 11,634,021	$ 11,854,900	$ 11,634,021	$ 11,854,900

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

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