Property Casualty Loss and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	$ 38	$ 86	$ 140	$ 287
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	69	64	69	64
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	13		91
Commercial Lines | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	9	12	24	39
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	21		43
Excess And Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	$ 4		$ 6