Unpaid Losses and Loss Expenses - Reconciliation of Beginning and Ending Liability for Unpaid Losses and Loss Expenses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gross liability, beginning of year	$ 23,068	$ 22,718	$ 22,839
Reinsurance recoverable, beginning of year	1,739	1,817	2,053
Net liability, beginning of year	21,329	20,901	20,786
Net incurred losses and loss expenses related to [Abstract]
Current year	8,121	8,174	7,245
Prior years	(614)	(767)	(746)
Net incurred losses and loss expenses, total	7,507	7,407	6,499
Net payments for losses and loss expenses related to [Abstract]
Current year	2,323	2,746	2,280
Prior years	4,493	4,300	4,074
Net payments for losses and loss expenses, total	6,816	7,046	6,354
Foreign currency translation effect	2	67	(30)
Net liability, end of year	22,022	21,329	20,901
Reinsurance recoverable, end of year	1,941	1,739	1,817
Gross liability, end of year	$ 23,963	$ 23,068	$ 22,718