Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves as of January 1		$ 10,799,242		$ 11,597,200	$ 11,952,500
Less: reinsurance recoverables	[1]	1,169,300		1,289,400	1,302,100
Net reserves as of January 1		9,629,900		10,307,800	10,650,400
Other adjustments		2,400		(1,900)	56,900	[2]
Incurred loss and LAE, net of reinsurance, related to:
Current year		3,285,200		2,555,300	2,709,700
Prior years		(368,000)		(215,500)	(215,200)
Total incurred loss and LAE, net of reinsurance		2,917,166		2,339,790	2,494,565
Paid loss and LAE, net of reinsurance, related to:
Current year	[3]	734,300		417,600	520,800
Prior years	[3]	1,866,500		2,390,400	2,178,100
Total paid loss and LAE, net of reinsurance	[3]	2,600,800		2,808,000	2,698,900
Foreign exchange effect		(97,700)		(207,800)	(195,100)
Net reserves as of December 31		9,851,000	[4]	9,629,900	10,307,800
Reinsurance recoverables as of December 31	[1]	1,236,200	[4]	1,169,300	1,289,400
Reserves as of December 31		$ 11,087,199	[4]	$ 10,799,242	$ 11,597,200

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

[2] Represents reserves acquired in connection with a loss portfolio transfer transaction.

[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, 2016. 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.