Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 4,918	$ 4,647	$ 4,918	$ 4,647	$ 4,750	$ 4,660	$ 4,577	$ 4,438
Prior accident years	(49)	(70)	(111)	(92)
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	52	44	52	44
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(58)	(63)	(87)	(77)
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(23)	(41)	(35)	(56)
Commercial Lines | Commercial Casualty Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(20)	(23)	(23)	(20)
Commercial Lines | Other Commercial Lines [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(15)		(28)
Commercial Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	13	11	21	23
Commercial Lines | Commercial Property Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(14)		(22)	(15)
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	10	2	(8)	(1)
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	(1)	$ (9)	(15)	(14)
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)
Accident Year 2015 | Personal Lines | Personal Auto [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ 8