Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 5,423	$ 5,213	$ 5,423	$ 5,213	$ 5,293	$ 5,219	$ 5,128	$ 5,035
Prior accident years	(31)	(38)	(79)	(76)
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	45	68	45	68
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(42)	(26)	(77)	(37)
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(18)	(12)	(31)	(31)
Commercial Lines | Commercial Casualty Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(14)	(7)	(10)	8
Commercial Lines | Other Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(8)	(11)	(12)	(19)
Commercial Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	5	9	4	20
Commercial Lines | Commercial Property Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(7)	(5)	(28)	(15)
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	17	(3)	16	(13)
Personal Lines | Homeowner [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	14		17
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(4)	(9)	(14)	(22)
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ (2)		(4)	(4)
Consolidated Property and Casualty Insurance Entity | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years		$ (3)	$ (7)	$ (14)