Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	$ 4,647	$ 4,408	$ 4,647	$ 4,408	$ 4,577	$ 4,438	$ 4,323	$ 4,241
Prior accident years	(70)	(66)	(92)	(95)
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reserve for loss and loss expenses	44	36	44	36
Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(63)	(57)	(77)	(60)
Commercial Lines | Workers' Compensation
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(41)		(56)	(30)
Commercial Lines | Commercial Casualty Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	(23)	(24)	(20)
Commercial Lines | Commercial Property Segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years		(22)	(15)	(24)
Commercial Lines | Commercial Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	11		23
Personal Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	2	(1)	(1)	(18)
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years	$ (9)	(8)	(14)	(17)
Consolidated Property and Casualty Insurance Entity | Catastrophe Development
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior accident years		$ (5)	$ (11)	$ (14)