Liability for Loss and LAE - Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Activity in liability for loss and loss adjustment expense
Reserves as of January 1				$ 14,357,635	$ 12,970,600
Less: reinsurance recoverables	[1]			2,026,100	1,703,700
Net reserves as of January 1				12,331,500	11,266,900
Other adjustments				(1,100)	100
Incurred loss and LAE, net of reinsurance, related to:
Current year				2,082,600	2,340,800
Prior years		$ (48,400)	$ (85,500)	(101,700)	(141,500)
Total incurred loss and LAE, net of reinsurance		1,039,127	1,087,206	1,980,914	2,199,276
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]			200,000	298,000
Prior years	[2]			1,653,100	1,484,300
Total paid loss and LAE, net of reinsurance	[2]			1,853,100	1,782,300
Foreign currency exchange rate effect				(103,300)	63,100
Net reserves as of June 30		12,354,900	11,747,100	12,354,900	11,747,100
Reinsurance recoverables as of June 30	[1]	2,038,800	1,757,400	2,038,800	1,757,400
Reserves as of June 30		$ 14,393,680	$ 13,504,500	$ 14,393,680	$ 13,504,500

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

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