Incurred Consolidated Property Casualty Insurance Loss and Loss Expenses on Assumed and Ceded Business (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Reinsurance [Line Items]
Net incurred loss and loss expenses	$ 571	$ 656	$ 1,840	$ 2,032
Property, Liability and Casualty Insurance Segment
Reinsurance [Line Items]
Direct incurred loss and loss expenses	562	649	1,745	2,157
Assumed incurred loss and loss expenses	(3)	8	3	28
Ceded incurred loss and loss expenses	(34)	(50)	(44)	(291)
Net incurred loss and loss expenses	$ 525	$ 607	$ 1,704	$ 1,894