Earned Consolidated Property Casualty Insurance Premiums on Assumed and Ceded Business (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Direct earned premiums	$ 868	$ 807	$ 1,707	$ 1,587
Assumed earned premiums	3	2	6	7
Ceded earned premiums	(45)	(79)	(89)	(119)
Net earned premiums	$ 826	$ 730	$ 1,624	$ 1,475