Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums Written, Net [Abstract]
Direct	$ 14,902	$ 14,264	$ 13,587
Assumed	262	281	303
Ceded	5,272	5,417	5,485
Net	9,892	9,128	8,405
Property and casualty
Premiums Written, Net [Abstract]
Direct	14,498	13,843	13,150
Assumed	219	235	255
Ceded	5,272	5,417	5,485
Net	9,445	8,661	7,920
Long-term care
Premiums Written, Net [Abstract]
Direct	404	421	437
Assumed	43	46	48
Ceded	0	0	0
Net	$ 447	$ 467	$ 485
Percentage of assumed premiums written to net premiums written
Premiums Written, Net [Abstract]
Assumed/ Net %	2.60%	3.10%	3.60%
Percentage of assumed premiums written to net premiums written | Property and casualty
Premiums Written, Net [Abstract]
Assumed/ Net %	2.30%	2.70%	3.20%
Percentage of assumed premiums written to net premiums written | Long-term care
Premiums Written, Net [Abstract]
Assumed/ Net %	9.60%	9.90%	9.90%