Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Direct premiums written	$ 10,946	$ 10,345	$ 9,787
Assumed premiums written	297	319	325
Ceded premiums written	4,255	3,702	3,024
Net written premiums	6,988	6,962	7,088
Property and Casualty Insurance [Member]
Direct premiums written	10,451	9,852	9,283
Assumed premiums written	245	270	276
Ceded premiums written	4,255	3,702	3,024
Net written premiums	6,441	6,420	6,535
Long term care Insurance Product Line [Member]
Direct premiums written	495	493	504
Assumed premiums written	52	49	49
Ceded premiums written	0	0	0
Net written premiums	$ 547	$ 542	$ 553
Percentage of assumed premiums written to net premiums written [Member]
Percentage of assumed premium written to premium written net	4.30%	4.60%	4.60%
Percentage of assumed premiums written to net premiums written [Member] | Property and Casualty Insurance [Member]
Percentage of assumed premium written to premium written net	3.80%	4.20%	4.20%
Percentage of assumed premiums written to net premiums written [Member] | Long term care Insurance Product Line [Member]
Percentage of assumed premium written to premium written net	9.50%	9.00%	8.90%