Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 4,750	$ 4,577	$ 4,660	$ 4,438
Prior accident years	(62)	(22)
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	54	46
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(29)	(14)
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(13)	(15)
Commercial Lines | Other Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(13)
Commercial Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	8	11
Commercial Lines | Commercial Property Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(8)	(11)
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(18)	(3)
Personal Lines | Home Owner Segment
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(8)	(7)
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(14)	(5)
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(1)
Consolidated Property and Casualty Insurance Entity | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ (7)	$ (11)