Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	$ 5,219,000,000	$ 5,035,000,000	$ 4,660,000,000	$ 4,438,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	119,000,000	168,000,000	184,000,000
Net loss and loss expense reserves for asbestos and environmental claims	$ 84,000,000	$ 85,000,000
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	54,000,000	$ 50,000,000	58,000,000
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	73,000,000	129,000,000	154,000,000
Commercial Insurance | Workers' Compensation Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	992,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	54,000,000	69,000,000	93,000,000
Commercial Insurance | Commercial Property Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	277,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	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 adjustment 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	1,987,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(11,000,000)	20,000,000	63,000,000
Commercial Insurance | Commercial Auto Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	595,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(33,000,000)	(31,000,000)	(31,000,000)
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	14,000,000	4,000,000	(5,000,000)
Personal Insurance | Personal Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	307,000,000
Prior year claims and claims adjustment 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	343,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	29,000,000	34,000,000	35,000,000
Reinsurance assumed and other non segment [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	$ 3,000,000	$ 1,000,000	$ 0