Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Direct premiums earned	$ 10,351	$ 9,960	$ 9,574
Assumed premiums earned	324	325	306
Ceded premiums earned	3,754	3,073	2,609
Net earned premiums	6,921	7,212	7,271
Property and Casualty Insurance [Member]
Direct premiums earned	9,853	9,452	9,063
Assumed premiums earned	274	277	258
Ceded premiums earned	3,754	3,073	2,609
Net earned premiums	6,373	6,656	6,712
Accident and Health Insurance [Member]
Direct premiums earned	498	508	511
Assumed premiums earned	50	48	48
Ceded premiums earned	0	0	0
Net earned premiums	$ 548	$ 556	$ 559
Percentage of assumed premiums earned to net premiums earned [Member]
Percentage of assumed premium earned to premium earned net	4.70%	4.50%	4.20%
Percentage of assumed premiums earned to net premiums earned [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium earned to premium earned net	4.30%	4.20%	3.80%
Percentage of assumed premiums earned to net premiums earned [Member] | Accident and Health Insurance [Member]
Percentage of assumed premium earned to premium earned net	9.10%	8.60%	8.60%