Reinsurance (Components of Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Direct premiums earned, property and casualty	$ 8,354	$ 7,858	$ 7,716
Assumed premiums earned, property and casualty	197	95	66
Ceded premiums earned, property and casualty	2,229	1,919	1,849
Premiums earned, net, property and casualty	6,322	6,034	5,933
Direct premiums earned, accident and health	514	521	534
Assumed premiums earned, accident and health	47	50	49
Ceded premiums earned, accident and health	1	2	2
Premiums earned, net, accident and health	560	569	581
Direct premiums earned, life	51	55	60
Assumed premiums earned, life
Ceded premiums earned, life	51	55	59
Premiums earned, net, life			1
Direct premiums earned	8,919	8,434	8,310
Assumed premiums earned	244	145	115
Ceded premiums earned	2,281	1,976	1,910
Net earned premiums	$ 6,882	$ 6,603	$ 6,515
Percentage of assumed premiums earned to net premiums earned [Member]
Percentage of assumed premium earned to premium earned net	3.50%	2.20%	1.80%
Percentage of assumed premiums earned to net premiums earned [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium earned to premium earned net	3.10%	1.60%	1.10%
Percentage of assumed premiums earned to net premiums earned [Member] | Accident and Health Insurance [Member]
Percentage of assumed premium earned to premium earned net	8.40%	8.80%	8.40%
Percentage of assumed premiums earned to net premiums earned [Member] | Life Insurance [Member]
Percentage of assumed premium earned to premium earned net