Earned Consolidated Property Casualty Insurance Premiums on Assumed and Ceded Business (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Direct earned premiums	$ 3,236	$ 3,080	$ 3,068
Assumed earned premiums	12	10	12
Ceded earned premiums	(219)	(166)	(169)
Net earned premiums	$ 3,029	$ 2,924	$ 2,911