Property Casualty Loss and Loss Expenses - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 9,005	$ 8,045	$ 9,005	$ 8,045	$ 8,807	$ 8,336	$ 7,603	$ 7,229
Prior accident years	(53)	(43)	(213)	(143)
Life and Health Loss Reserves
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	72	68	72	68
Commercial Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(34)	(4)	(125)	(51)
Commercial Insurance | Commercial Property Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(11)	(24)	(36)	(36)
Commercial Insurance | Workers' Compensation Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(20)	(16)	(46)	(43)
Commercial Insurance | Commercial Auto Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		16		15
Commercial Insurance | Commercial Casualty Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		23	(36)	25
Personal Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(8)	(8)	(54)	(56)
Personal Insurance | Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(44)	(51)
Personal Insurance | Personal Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(12)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ 0	$ (7)	$ (14)	$ (13)