Reinsurance - 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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Net incurred loss and loss expenses	$ 642	$ 571	$ 1,841	$ 1,840
Property, Liability And Casualty Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	597	562	1,725	1,745
Assumed incurred loss and loss expenses	1	(3)	8	3
Ceded incurred loss and loss expenses	(5)	(34)	(33)	(44)
Net incurred loss and loss expenses	$ 593	$ 525	$ 1,700	$ 1,704