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, 2012	Mar. 31, 2011
Reinsurance [Line Items]
Net incurred loss and loss expenses	$ 582	$ 575
Property, Liability and Casualty Insurance Segment
Reinsurance [Line Items]
Direct incurred loss and loss expenses	536	527
Assumed incurred loss and loss expenses	5	15
Ceded incurred loss and loss expenses	(2)	(12)
Net incurred loss and loss expenses	$ 539	$ 530