Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves, beginning of period		$ 11,597,216	$ 11,952,500		$ 12,239,800
Less: reinsurance recoverables, beginning of period	[1]	1,289,400	1,302,100		1,305,900
Net reserves, beginning of period		10,307,800	10,650,400		10,933,900
Other adjustments		(1,900)	56,900	[2]
Incurred loss, net of reinsurance, related to:
Current year		2,555,300	2,709,700		2,682,300
Prior years		(215,500)	(215,200)		(203,000)
Total incurred loss and LAE, net of reinsurance		2,339,790	2,494,565		2,479,353
Paid loss, net of reinsurance, related to:
Current year	[3]	417,600	520,800		518,500
Prior years	[3]	2,390,400	2,178,100		2,236,800
Total paid loss and LAE, net of reinsurance		2,808,000	2,698,900		2,755,300
Foreign exchange effect		(207,800)	(195,100)		(7,500)
Net reserves, ending of period		9,629,900	10,307,800		10,650,400
Plus: reinsurance recoverables, ending of period	[1]	1,169,300	1,289,400		1,302,100
Reserves, ending of period		$ 10,799,242	$ 11,597,216		$ 11,952,500

[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.