Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	$ 5,646,000,000	$ 5,219,000,000	$ 5,035,000,000	$ 4,660,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(167,000,000)	(119,000,000)	(168,000,000)
Net loss and loss expense reserves for asbestos and environmental claims	$ 89,000,000	$ 84,000,000
Percentage of reserves for asbestos and environmental claims	2.00%	2.00%
Maximum
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reinsurance retention	$ 500,000
Life and Health Loss Reserves
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	61,000,000	$ 54,000,000	50,000,000
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(157,000,000)	(73,000,000)	(129,000,000)
Commercial Insurance | Workers' Compensation Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	991,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(58,000,000)	(54,000,000)	(69,000,000)
Commercial Insurance | Commercial Property Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	314,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(47,000,000)	(33,000,000)	(29,000,000)
Commercial Insurance | Other Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustments expense, (favorable) and unfavorable		(30,000,000)	(42,000,000)
Commercial Insurance | Commercial Casualty Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	2,158,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(47,000,000)	11,000,000	(20,000,000)
Commercial Insurance | Commercial Auto Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	637,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable		33,000,000	31,000,000
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustments expense, (favorable) and unfavorable	13,000,000	(14,000,000)	(4,000,000)
Personal Insurance | Personal Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	324,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable			18,000,000
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	284,000,000
Prior year claims and claims adjustments expense, (favorable) and unfavorable	(24,000,000)	(29,000,000)	(34,000,000)
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustments expense, (favorable) and unfavorable	$ 1,000,000	$ (3,000,000)	$ (1,000,000)