Reinsurance (Earned Consolidated Property Casualty Insurance Premiums On Assumed And Ceded Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct earned premiums	$ 5,052	$ 4,857	$ 4,667
Assumed earned premiums	132	77	29
Ceded earned premiums	(230)	(224)	(216)
Property, Liability and Casualty Insurance Product Line [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct earned premiums	4,752	4,567	4,396
Assumed earned premiums	132	77	29
Ceded earned premiums	(162)	(162)	(154)
Net earned premiums	$ 4,722	$ 4,482	$ 4,271