Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 4,169	$ 4,280
Less reinsurance receivable	356	375
Net loss and loss expense reserves, beginning of period	3,813	3,905
Current accident year	534	655
Prior accident years	(10)	(116)
Total incurred	524	539
Current accident year	121	132
Prior accident years	392	375
Total paid	513	507
Net loss and loss expense reserves, end of period	3,824	3,937
Plus reinsurance receivable	349	352
Gross loss and loss expense reserves, end of period	$ 4,173	$ 4,289