Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Direct premiums written	$ 11,894	$ 11,568	$ 11,141
Assumed premiums written	331	360	377
Ceded premiums written	4,569	4,583	4,449
Net written premiums	7,656	7,345	7,069
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums written	11,421	11,094	10,655
Assumed premiums written	281	310	327
Ceded premiums written	4,569	4,583	4,449
Net written premiums	7,133	6,821	6,533
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums written	473	474	486
Assumed premiums written	50	50	50
Ceded premiums written	0	0	0
Net written premiums	$ 523	$ 524	$ 536
Percentage of assumed premiums written to net premiums written
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	4.30%	4.90%	5.30%
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	3.90%	4.50%	5.00%
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	9.60%	9.50%	9.30%