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Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property Casualty Insurance Underwriters
Gross losses	$ 9,937,000,000
Prior accident years	(236,000,000)	$ (215,000,000)	$ (159,000,000)
Net loss and loss expense reserves for asbestos and environmental claims	$ 119,000,000	$ 98,000,000
Percentage of reserves for asbestos and environmental claims	2.00%	2.00%
Maximum
Property Casualty Insurance Underwriters
Reinsurance retention	$ 500,000
Life and Health Loss Reserves
Property Casualty Insurance Underwriters
Gross losses	66,000,000	$ 75,000,000	64,000,000
Commercial Insurance
Property Casualty Insurance Underwriters
Prior accident years	(138,000,000)	(123,000,000)	(76,000,000)
Commercial Insurance | Workers' Compensation Line
Property Casualty Insurance Underwriters
Gross losses	1,020,000,000
Prior accident years	(83,000,000)	(66,000,000)	(63,000,000)
Commercial Insurance | Commercial Property Line
Property Casualty Insurance Underwriters
Gross losses	493,000,000
Prior accident years	(74,000,000)	(55,000,000)	(44,000,000)
Commercial Insurance | Commercial Casualty Line
Property Casualty Insurance Underwriters
Gross losses	3,248,000,000
Prior accident years	26,000,000	15,000,000	25,000,000
Commercial Insurance | Commercial Auto Line
Property Casualty Insurance Underwriters
Gross losses	874,000,000
Prior accident years			23,000,000
Personal Insurance
Property Casualty Insurance Underwriters
Prior accident years	(26,000,000)	(64,000,000)	(61,000,000)
Personal Insurance | Homeowner
Property Casualty Insurance Underwriters
Gross losses	423,000,000
Prior accident years	(54,000,000)	(53,000,000)	(54,000,000)
Personal Insurance | Personal Auto
Property Casualty Insurance Underwriters
Gross losses	410,000,000
Prior accident years	20,000,000	(15,000,000)
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Gross losses	1,043,000,000
Prior accident years	$ 8,000,000	$ (11,000,000)	$ (9,000,000)

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