Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Direct premiums earned	$ 11,491	$ 11,337	$ 10,936
Assumed premiums earned	338	355	367
Ceded premiums earned	4,401	4,380	4,315
Net earned premiums	7,428	7,312	6,988
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums earned	11,021	10,857	10,447
Assumed premiums earned	288	305	317
Ceded premiums earned	4,401	4,380	4,315
Net earned premiums	6,908	6,782	6,449
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums earned	470	480	489
Assumed premiums earned	50	50	50
Ceded premiums earned	0	0	0
Net earned premiums	$ 520	$ 530	$ 539
Percentage of assumed premiums earned to net premiums earned
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	4.60%	4.90%	5.30%
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	4.20%	4.50%	4.90%
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	9.60%	9.40%	9.30%

Amount of earned premiums assumed from other entities.

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