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
	Mar. 31, 2013	Mar. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Net incurred loss and loss expenses	$ 568	$ 582
Property, Liability And Casualty Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	535	536
Assumed incurred loss and loss expenses	2	5
Ceded incurred loss and loss expenses	(13)	(2)
Net incurred loss and loss expenses	$ 524	$ 539