Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,173	$ 4,289	$ 4,169	$ 4,280
Less reinsurance receivable	349	352	356	375
Net loss and loss expense reserves, beginning of period	3,824	3,937	3,813	3,905
Current accident year	675	725	1,209	1,380
Prior accident years	(92)	(85)	(102)	(201)
Total incurred	583	640	1,107	1,179
Current accident year	249	282	370	414
Prior accident years	272	290	664	665
Total paid	521	572	1,034	1,079
Net loss and loss expense reserves, end of period	3,886	4,005	3,886	4,005
Plus reinsurance receivable	333	332	333	332
Gross loss and loss expense reserves, end of period	$ 4,219	$ 4,337	$ 4,219	$ 4,337