Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves as of January 1		$ 11,871,250		$ 11,087,200		$ 10,799,200
Less: reinsurance recoverables	[1]	1,650,100		1,236,200		1,169,300
Net reserves as of January 1		10,221,200		9,851,000		9,629,900
Other adjustments		900		(293,700)	[2]	2,400
Incurred loss and LAE, net of reinsurance, related to:
Current year		3,849,400		3,918,800		3,285,200
Prior years		(329,000)		(298,600)		(368,000)
Total incurred loss and LAE, net of reinsurance		3,520,431		3,620,197		2,917,166
Paid loss and LAE, net of reinsurance, related to:
Current year	[3]	838,900		853,200		734,300
Prior years	[3]	2,419,000		2,225,200		1,866,500
Total paid loss and LAE, net of reinsurance	[3]	3,257,900		3,078,400		2,600,800
Foreign currency exchange rate effect		(91,700)		122,100		(97,700)
Net reserves as of December 31		10,392,900	[4]	10,221,200		9,851,000
Reinsurance recoverables as of December 31	[1]	1,857,400	[4]	1,650,100		1,236,200
Reserves as of December 31		$ 12,250,294		$ 11,871,250		$ 11,087,200

[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.

[3] Includes paid loss and LAE, net of reinsurance, related to commutations.

[4] Includes unallocated LAE, which aggregate to 1.1 percent of gross loss and LAE reserves as of December 31, 2018. 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.