Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves as of January 1		$ 11,087,199		$ 10,799,200	$ 11,597,200
Less: reinsurance recoverables	[1]	1,236,200		1,169,300	1,289,400
Net reserves as of January 1		9,851,000		9,629,900	10,307,800
Other adjustments		(293,700)	[2]	2,400	(1,900)
Incurred loss and LAE, net of reinsurance, related to:
Current year		3,918,800		3,285,200	2,555,300
Prior years		(298,600)		(368,000)	(215,500)
Total incurred loss and LAE, net of reinsurance		3,620,197		2,917,166	2,339,790
Paid loss and LAE, net of reinsurance, related to:
Current year	[3]	853,200		734,300	417,600
Prior years	[3]	2,225,200		1,866,500	2,390,400
Total paid loss and LAE, net of reinsurance	[3]	3,078,400		2,600,800	2,808,000
Foreign exchange effect		122,100		(97,700)	(207,800)
Net reserves as of December 31		10,221,200	[4]	9,851,000	9,629,900
Reinsurance recoverables as of December 31	[1]	1,650,100	[4]	1,236,200	1,169,300
Reserves as of December 31		$ 11,871,250	[4]	$ 11,087,199	$ 10,799,200

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

[2] Primarily represents the impact on net reserves arising from the sale of PacificComp on December 31, 2017.

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

[4] Includes unallocated LAE, which aggregate to 1.3 percent of gross loss and LAE reserves as of December 31, 2017. Net loss and LAE Reserves by component are shown in the preceding tables, and consolidated gross loss and LAE reserves is presented in the Consolidated Balance Sheets.