Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Direct premiums earned	$ 10,886	$ 10,351	$ 9,960
Assumed premiums earned	308	324	325
Ceded premiums earned	4,270	3,754	3,073
Net earned premiums	6,924	6,921	7,212
Property and Casualty Insurance [Member]
Direct premiums earned	10,400	9,853	9,452
Assumed premiums earned	258	274	277
Ceded premiums earned	4,270	3,754	3,073
Net earned premiums	6,388	6,373	6,656
Long term care Insurance Product Line [Member]
Direct premiums earned	486	498	508
Assumed premiums earned	50	50	48
Ceded premiums earned	0	0	0
Net earned premiums	$ 536	$ 548	$ 556
Percentage of assumed premiums earned to net premiums earned [Member]
Percentage of assumed premium earned to premium earned net	4.40%	4.70%	4.50%
Percentage of assumed premiums earned to net premiums earned [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium earned to premium earned net	4.00%	4.30%	4.20%
Percentage of assumed premiums earned to net premiums earned [Member] | Long term care Insurance Product Line [Member]
Percentage of assumed premium earned to premium earned net	9.30%	9.10%	8.60%