Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 6,692	$ 5,997	$ 6,692	$ 5,997	$ 6,409	$ 6,088	$ 5,954	$ 5,646
Prior accident years	(11)	(52)	(91)	(203)
Life and Health Loss Reserves
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	55	59	55	59
Commercial Lines
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(8)	(33)	(59)	(153)
Commercial Lines | Commercial Casualty Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(10)	(8)	36	(65)
Commercial Lines | Workers' Compensation
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(6)	(20)	(32)	(62)
Commercial Lines | Commercial Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	10		14	(7)
Commercial Lines | Commercial Property Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		(7)		(15)
Personal Lines
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		(6)	(28)	(17)
Personal Lines | Maximum
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	1
Personal Lines | Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(10)	15
Personal Lines | Personal Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		(3)	(19)	(23)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ (1)	$ (4)	$ 8	$ (11)