Reinsurance (Components of Written Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Direct premiums written, property and casualty	$ 8,467	$ 7,976	$ 7,673
Assumed premiums written, property and casualty	169	102	77
Ceded premiums written, property and casualty	2,225	1,857	1,853
Premiums written, net, property and casualty	6,411	6,221	5,897
Direct premiums written, accident and health	507	529	527
Assumed premiums written, accident and health	47	50	48
Ceded premiums written, accident and health	1	2	2
Premiums written, net, accident and health	553	577	573
Direct premiums written, life	51	55	60
Assumed premiums written, life
Ceded premiums written, life	51	55	59
Premiums written, net, life			1
Direct premiums written	9,025	8,560	8,260
Assumed premiums written	216	152	125
Ceded premiums written	2,277	1,914	1,914
Premiums written, net	$ 6,964	$ 6,798	$ 6,471
Percentage of assumed premiums written to net premiums written [Member]
Percentage of assumed premium written to premium written net	3.10%	2.20%	1.90%
Percentage of assumed premiums written to net premiums written [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium written to premium written net	2.60%	1.60%	1.30%
Percentage of assumed premiums written to net premiums written [Member] | Accident and Health Insurance [Member]
Percentage of assumed premium written to premium written net	8.50%	8.70%	8.40%
Percentage of assumed premiums written to net premiums written [Member] | Life Insurance [Member]
Percentage of assumed premium written to premium written net