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Property Casualty Loss and Loss Expenses - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 9,928		$ 9,928		$ 9,050
Prior accident years	(71)	$ (53)	(211)	$ (213)
Life Insurance Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	62	72	62	72
Commercial Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(50)	(34)	(117)	(125)
Commercial Insurance | Commercial Property Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(33)	(11)	(76)	(36)
Commercial Insurance | Commercial Casualty Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			27	(36)
Commercial Insurance | Commercial Auto Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(10)
Commercial Insurance | Workers' Compensation Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(16)	(20)	(56)	(46)
Personal Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	1	(8)	(27)	(54)
Personal Insurance | Personal Auto
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years				(12)
Personal Insurance | Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(37)	(44)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ 5	$ 0	$ 5	$ (14)

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