Reinsurance - Effects of Reinsurance on Earned Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums Earned, Net [Abstract]
Direct	$ 14,315	$ 13,524	$ 12,997
Assumed	267	277	288
Ceded	5,102	5,134	5,110
Net	$ 9,480	$ 8,667	$ 8,175
Assumed/Net %	2.80%	3.20%	3.50%
Direct and ceded earned premiums	$ 2,900	$ 3,300	$ 3,600
Percentage reinsured under significant third party captive program	100.00%
Reinsurance recoveries	$ 2,800	2,600	3,100
Significant Third Party Captive Program
Premiums Earned, Net [Abstract]
Reinsurance recoveries	1,500	1,800	2,000
Property and casualty
Premiums Earned, Net [Abstract]
Direct	13,908	13,097	12,554
Assumed	223	231	240
Ceded	5,102	5,134	5,110
Net	$ 9,029	$ 8,194	$ 7,684
Assumed/Net %	2.50%	2.80%	3.10%
Long-term care
Premiums Earned, Net [Abstract]
Direct	$ 407	$ 427	$ 443
Assumed	44	46	48
Ceded
Net	$ 451	$ 473	$ 491
Assumed/Net %	9.80%	9.70%	9.80%