Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effects of Reinsurance [Line Items]
Direct premiums earned	$ 10,936	$ 10,886	$ 10,351
Assumed premiums earned	367	308	324
Ceded premiums earned	4,315	4,270	3,754
Net earned premiums	6,988	6,924	6,921
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums earned	10,447	10,400	9,853
Assumed premiums earned	317	258	274
Ceded premiums earned	4,315	4,270	3,754
Net earned premiums	6,449	6,388	6,373
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums earned	489	486	498
Assumed premiums earned	50	50	50
Ceded premiums earned	0	0	0
Net earned premiums	$ 539	$ 536	$ 548
Percentage of assumed premiums earned to net premiums earned
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	5.30%	4.40%	4.70%
Percentage of assumed premiums earned to net premiums earned | Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	4.90%	4.00%	4.30%
Percentage of assumed premiums earned to net premiums earned | Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	9.30%	9.30%	9.10%