Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 5,213	$ 4,918	$ 5,213	$ 4,918	$ 5,128	$ 5,035	$ 4,750	$ 4,660
Prior accident years	(38)	(49)	(76)	(111)
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	68	52	68	52
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(26)	(58)	(37)	(87)
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(12)	(23)	(31)	(35)
Commercial Lines | Commercial Casualty Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(7)	(20)	8	(23)
Commercial Lines | Other Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(11)	(15)	(19)	(28)
Commercial Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	9	13	20	21
Commercial Lines | Commercial Property Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(5)	(14)	(15)	(22)
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(3)	(10)	(13)	(8)
Personal Lines | Personal Auto [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years		14		6
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(9)	(1)	(22)	(15)
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years			(4)	(1)
Consolidated Property and Casualty Insurance Entity | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ (3)		$ (14)	$ (7)
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Lines | Personal Auto [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years		$ 8