Reinsurance (Components of Written Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Direct Premiums Written, Property and Casualty	$ 7,976	$ 7,673	$ 7,981
Assumed Premiums Written, Property and Casualty	102	77	66
Ceded Premiums Written, Property and Casualty	1,857	1,853	1,916
Premiums Written, Net, Property and Casualty	6,221	5,897	6,131
Direct Premiums Written, Accident and Health	529	527	539
Assumed Premiums Written, Accident and Health	50	48	50
Ceded Premiums Written, Accident and Health	2	2	6
Premiums Written, Net, Accident and Health	577	573	583
Direct Premiums Written, Life	55	60	83
Assumed Premiums Written, Life
Ceded Premiums Written, Life	55	59	83
Premiums Written, Net, Life		1
Direct Premiums Written	8,560	8,260	8,603
Assumed Premiums Written	152	125	116
Ceded Premiums Written	1,914	1,914	2,005
Premiums Written, Net	$ 6,798	$ 6,471	$ 6,714
Percentage of assumed premiums written to net premiums written [Member]
Percentage of Assumed Premium Written to Premium Written Net	2.20%	1.90%	1.70%
Percentage of assumed premiums written to net premiums written [Member] | Property and Casualty Insurance [Member]
Percentage of Assumed Premium Written to Premium Written Net	1.60%	1.30%	1.10%
Percentage of assumed premiums written to net premiums written [Member] | Accident and Health Insurance [Member]
Percentage of Assumed Premium Written to Premium Written Net	8.70%	8.40%	8.60%
Percentage of assumed premiums written to net premiums written [Member] | Life Insurance [Member]
Percentage of Assumed Premium Written to Premium Written Net