Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	$ 5,035,000,000	$ 4,660,000,000	$ 4,438,000,000	$ 4,241,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(168,000,000)	(184,000,000)	(98,000,000)
Recovery of liability for unpaid claims and claims adjustment expense incurred prior years		(86,000,000)
Increase in (favorable) reserve developement on prior accident			(49,000,000)
Net loss and loss expense reserves for asbestos and environmental claims	85,000,000	84,000,000
Environmental claims related to mold	$ 45,000,000	$ 45,000,000
Percentage of reserves for asbestos and environmental claims	1.80%	1.90%
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	50,000,000	$ 58,000,000	47,000,000
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(129,000,000)	(154,000,000)	(57,000,000)
Commercial Insurance | Workers' Compensation Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	1,001,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(69,000,000)	(93,000,000)	(51,000,000)
Commercial Insurance | Commercial Property Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	306,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(29,000,000)		(34,000,000)
Commercial Insurance | Other Commercial Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(42,000,000)
Commercial Insurance | Homeowner Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable			(14,000,000)
Commercial Insurance | Commercial Casualty Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	1,957,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(20,000,000)	(63,000,000)	5,000,000
Commercial Insurance | Commercial Auto Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	521,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	31,000,000	31,000,000	39,000,000
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(4,000,000)	5,000,000	(12,000,000)
Personal Insurance | Personal Auto
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	280,000,000
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	229,000,000
Prior year claims and claims adjustment expense, (favorable) and unfavorable	(34,000,000)	(35,000,000)	(29,000,000)
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	$ (1,000,000)	$ 0	$ 0