Reinsurance, Effects of Reinsurance on Earned Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Earned, Net [Abstract]
Earned premiums, Direct	$ 12,001	$ 11,491	$ 11,337
Earned premiums, Assumed	288	338	355
Earned premiums, Ceded	4,640	4,401	4,380
Earned premiums, Net	$ 7,649	$ 7,428	$ 7,312
Assumed/Net %	3.80%	4.60%	4.90%
Direct and ceded earned premiums	$ 3,500	$ 3,600	$ 3,700
Percentage reinsured under significant third party captive program	100.00%
Reinsurance recoveries	$ 3,200	2,700	2,800
Property and Casualty Operations [Member]
Premiums Earned, Net [Abstract]
Earned premiums, Direct	11,547	11,021	10,857
Earned premiums, Assumed	238	288	305
Earned premiums, Ceded	4,640	4,401	4,380
Earned premiums, Net	$ 7,145	$ 6,908	$ 6,782
Assumed/Net %	3.30%	4.20%	4.50%
Long Term Care [Member]
Premiums Earned, Net [Abstract]
Earned premiums, Direct	$ 454	$ 470	$ 480
Earned premiums, Assumed	50	50	50
Earned premiums, Net	$ 504	$ 520	$ 530
Assumed/Net %	9.90%	9.60%	9.40%
Significant Third Party Captive Program [Member]
Premiums Earned, Net [Abstract]
Reinsurance recoveries	$ 2,400	$ 2,100	$ 1,900