Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Direct premiums written	$ 10,345	$ 9,787	$ 9,608
Assumed premiums written	319	325	296
Ceded premiums written	3,702	3,024	2,556
Net written premiums	6,962	7,088	7,348
Property and Casualty Insurance [Member]
Direct premiums written	9,852	9,283	9,103
Assumed premiums written	270	276	249
Ceded premiums written	3,702	3,024	2,556
Net written premiums	6,420	6,535	6,796
Accident and Health Insurance [Member]
Direct premiums written	493	504	505
Assumed premiums written	49	49	47
Ceded premiums written	0	0	0
Net written premiums	$ 542	$ 553	$ 552
Percentage of assumed premiums written to net premiums written [Member]
Percentage of assumed premium written to premium written net	4.60%	4.60%	4.00%
Percentage of assumed premiums written to net premiums written [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium written to premium written net	4.20%	4.20%	3.70%
Percentage of assumed premiums written to net premiums written [Member] | Accident and Health Insurance [Member]
Percentage of assumed premium written to premium written net	9.00%	8.90%	8.50%