Property Casualty Loss And Loss Expenses - Additional Information (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]
Favorable reserve development on prior accident years	$ 285	$ 304	$ 188
Net loss and loss expense reserves for asbestos and environmental claims	136	134
Percentage of reserves for asbestos and environmental claims	3.50%	3.50%
Maximum
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reinsurance retention	0.5
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	59	63	46
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	234	269	147
Commercial Lines | Commercial Casualty Segment
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	132	186	154
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	97	39
Adverse reserve development on prior accident years			$ 48