Property Casualty Loss and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 4,393	$ 4,238	$ 4,393	$ 4,238	$ 4,408	$ 4,241	$ 4,219	$ 4,169
Prior accident years	25	38	120	140
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	42	69	42	69
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	24		84
Commercial Lines | Commercial Property
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	8		33
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	15		45
Commercial Lines | Commercial Casualty
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	8		25
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(5)		13
Personal Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(12)		(24)
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	6		23
Consolidated Property and Casualty Insurance Entity | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ 5	$ 9	$ 19	$ 24