Deferred Policy Acquisition Costs (Deferred Policy Acquisition Costs And Asset Reconciliation Including The Amortized Deferred Policy Acquisition Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs [Roll Forward]
Deferred policy acquisition costs asset at January 1	$ 637	$ 616	$ 578
Capitalized deferred policy acquisition costs	948	889	846
Amortized deferred policy acquisition costs	(913)	(863)	(829)
Amortized shadow deferred policy acquisition costs	(2)	(5)	21
Deferred policy acquisition costs asset at December 31	670	637	616
Property Casualty Insurance Segment [Member]
Deferred Costs [Roll Forward]
Deferred policy acquisition costs asset at January 1	408	388	379
Capitalized deferred policy acquisition costs	897	840	801
Amortized deferred policy acquisition costs	(867)	(820)	(792)
Deferred policy acquisition costs asset at December 31	438	408	388
Life Insurance Product Line [Member]
Deferred Costs [Roll Forward]
Deferred policy acquisition costs asset at January 1	229	228	199
Capitalized deferred policy acquisition costs	51	49	45
Amortized deferred policy acquisition costs	(46)	(43)	(37)
Amortized shadow deferred policy acquisition costs	(2)	(5)	21
Deferred policy acquisition costs asset at December 31	$ 232	$ 229	$ 228