Property Casualty Loss And Loss Expenses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	$ 4,660,000,000	$ 4,438,000,000	$ 4,241,000,000	$ 4,169,000,000
Prior year claims and claims adjustment expense	(184,000,000)	(98,000,000)	(147,000,000)
Recovery Of Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Prior Years	(86,000,000)
Increase in favorable reserve developement on prior accident		49,000,000	249,000,000
Net loss and loss expense reserves for asbestos and environmental claims	84,000,000	81,000,000
Environmental claims related to mold	$ 45,000,000	$ 51,000,000
Percentage of reserves for asbestos and environmental claims	1.90%	2.00%
Maximum
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Reinsurance retention	$ 500,000
Certain Accident, Life and Health Lines
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Certain accident, life and health loss reserves	58,000,000	$ 47,000,000	70,000,000
Commercial Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	(154,000,000)	(57,000,000)	(95,000,000)
Commercial Insurance | Workers' Compensation Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	(93,000,000)	(51,000,000)	(14,000,000)
Commercial Insurance | Commercial Property Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense		(34,000,000)	(22,000,000)
Commercial Insurance | Homeowner Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense		(14,000,000)	(19,000,000)
Commercial Insurance | Commercial Casualty Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	(63,000,000)	5,000,000	(70,000,000)
Commercial Insurance | Commercial Auto Line
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	31,000,000	39,000,000
Personal Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	5,000,000	(12,000,000)	(39,000,000)
Excess and Surplus Lines Insurance
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Prior year claims and claims adjustment expense	$ (35,000,000)	$ (29,000,000)	$ (13,000,000)