Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Activity in liability for loss and loss adjustment expense
Reserves as of January 1		$ 11,928,359	$ 12,250,300
Less: reinsurance recoverables	[1]	1,583,900	1,857,400
Net reserves as of January 1		10,344,500	10,392,900
Other adjustments			(200)
Incurred loss and LAE, net of reinsurance, related to:
Current year		1,086,400	829,900
Prior years		(59,000)	(37,400)
Total incurred loss and LAE, net of reinsurance		1,027,444	792,498
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]	87,300	75,300
Prior years	[2]	770,200	914,800
Total paid loss and LAE, net of reinsurance	[2]	857,500	990,100
Foreign currency exchange rate effect		(7,500)	17,200
Net reserves as of March 31		10,506,900	10,212,300
Reinsurance recoverables as of March 31	[1]	1,559,500	1,642,800
Reserves as of March 31		$ 12,066,438	$ 11,855,100

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

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