Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Gross loss and loss expense reserves, beginning of period	$ 4,280	$ 4,137
Less reinsurance receivable	375	326
Net loss and loss expense reserves, beginning of period	3,905	3,811
Current accident year	655	588
Prior accident years	(116)	(58)
Total incurred	539	530
Current accident year	132	129
Prior accident years	375	359
Total paid	507	488
Net loss and loss expense reserves, end of period	3,937	3,853
Plus reinsurance receivable	352	326
Gross loss and loss expense reserves, end of period	$ 4,289	$ 4,179