Property Casualty Loss and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	$ 92	$ 85	$ 102	$ 201
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	65	59	65	59
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	66		78
Commercial Lines | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	8	5	15	27
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years	26		22
Excess And Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Favorable reserve development on prior accident years			$ 2