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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Net incurred loss and loss expenses	$ 631	$ 687	$ 1,199	$ 1,269
Property, Liability And Casualty Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	593	647	1,128	1,183
Assumed incurred loss and loss expenses	5	1	7	6
Ceded incurred loss and loss expenses	(15)	(8)	(28)	(10)
Net incurred loss and loss expenses	$ 583	$ 640	$ 1,107	$ 1,179