Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Earned, Net [Abstract]
Direct	$ 12,001	$ 11,491	$ 11,337
Assumed	288	338	355
Ceded	4,640	4,401	4,380
Net	7,649	7,428	7,312
Property and casualty
Premiums Earned, Net [Abstract]
Direct	11,547	11,021	10,857
Assumed	238	288	305
Ceded	4,640	4,401	4,380
Net	7,145	6,908	6,782
Long term care
Premiums Earned, Net [Abstract]
Direct	454	470	480
Assumed	50	50	50
Ceded	0	0	0
Net	$ 504	$ 520	$ 530
Percentage of assumed premiums earned to net premiums earned
Premiums Earned, Net [Abstract]
Assumed/ Net %	3.80%	4.60%	4.90%
Percentage of assumed premiums earned to net premiums earned | Property and casualty
Premiums Earned, Net [Abstract]
Assumed/ Net %	3.30%	4.20%	4.50%
Percentage of assumed premiums earned to net premiums earned | Long term care
Premiums Earned, Net [Abstract]
Assumed/ Net %	9.90%	9.60%	9.40%