Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 6,409	$ 5,954	$ 6,409	$ 5,954	$ 6,153	$ 6,088	$ 5,886	$ 5,646
Prior accident years	(47)	(84)	(80)	(151)
Life and Health Loss Reserves
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	54	55	54	55
Commercial Casualty Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(26)
Commercial Lines
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(45)	(58)	(51)	(120)
Commercial Lines | Commercial Casualty Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(21)	(25)		(56)
Commercial Lines | Workers' Compensation
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(19)	(27)	(26)	(42)
Commercial Lines | Commercial Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			3	(9)
Commercial Lines | Commercial Property Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		(6)		(8)
Personal Lines
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		(14)	(28)	(11)
Personal Lines | Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	7		(11)	15
Personal Lines | Personal Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(7)	(15)	(20)	(20)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ 8	$ (5)	$ 9	$ (7)