Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	$ 8,975,000,000	$ 8,336,000,000	$ 7,229,000,000	$ 6,677,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(215,000,000)	(159,000,000)	(428,000,000)
Net loss and loss expense reserves for asbestos and environmental claims	$ 98,000,000	$ 92,000,000
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
Certain accident, life and health loss reserves	75,000,000	$ 64,000,000	76,000,000
Commercial Insurance
Property Casualty Insurance Underwriters
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(123,000,000)	(76,000,000)	(353,000,000)
Commercial Insurance | Workers' Compensation Line
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	1,036,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(66,000,000)	(63,000,000)	(66,000,000)
Commercial Insurance | Commercial Property Line
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	525,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(55,000,000)	(44,000,000)	(97,000,000)
Commercial Insurance | Commercial Casualty Line
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	2,927,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	15,000,000	25,000,000	(120,000,000)
Commercial Insurance | Commercial Auto Line
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	811,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable		23,000,000	(43,000,000)
Personal Insurance
Property Casualty Insurance Underwriters
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(64,000,000)	(61,000,000)	(50,000,000)
Personal Insurance | Homeowner
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	348,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(53,000,000)	(54,000,000)	(14,000,000)
Personal Insurance | Personal Auto
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	327,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(15,000,000)		(31,000,000)
Excess and Surplus Lines Insurance
Property Casualty Insurance Underwriters
Certain accident, life and health loss reserves	875,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	$ (11,000,000)	$ (9,000,000)	$ 7,000,000