Earned Consolidated Property Casualty Insurance Premiums 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
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Direct earned premiums	$ 898	$ 817	$ 2,605	$ 2,404
Assumed earned premiums	1	3	7	10
Ceded earned premiums	(48)	(51)	(137)	(170)
Net earned premiums	$ 851	$ 769	$ 2,475	$ 2,244