Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - Property, Liability and Casualty Insurance Product Line [Member] - 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 Premiums Written	$ 4,854	$ 4,646	$ 4,461
Assumed Premiums Written	125	103	77
Ceded Premiums Written	(139)	(169)	(177)
Premiums Written, Net, Property and Casualty	$ 4,840	$ 4,580	$ 4,361