Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Direct premiums written	$ 11,568	$ 11,141	$ 10,946
Assumed premiums written	360	377	297
Ceded premiums written	4,583	4,449	4,255
Net written premiums	7,345	7,069	6,988
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums written	11,094	10,655	10,451
Assumed premiums written	310	327	245
Ceded premiums written	4,583	4,449	4,255
Net written premiums	6,821	6,533	6,441
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums written	474	486	495
Assumed premiums written	50	50	52
Ceded premiums written	0	0	0
Net written premiums	$ 524	$ 536	$ 547
Percentage of assumed premiums written to net premiums written
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	4.90%	5.30%	4.30%
Percentage of assumed premiums written to net premiums written | Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	4.50%	5.00%	3.80%
Percentage of assumed premiums written to net premiums written | Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	9.50%	9.30%	9.50%