Reinsurance (Components of 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	[1]	8,175	[1]
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
Long-term care
Premiums Earned, Net [Abstract]
Direct	407	427		443
Assumed	44	46		48
Ceded	0	0		0
Net	$ 451	$ 473		$ 491
Percentage of assumed premiums earned to net premiums earned
Premiums Earned, Net [Abstract]
Assumed/ Net %	2.80%	3.20%		3.50%
Percentage of assumed premiums earned to net premiums earned | Property and casualty
Premiums Earned, Net [Abstract]
Assumed/ Net %	2.50%	2.80%		3.10%
Percentage of assumed premiums earned to net premiums earned | Long-term care
Premiums Earned, Net [Abstract]
Assumed/ Net %	9.80%	9.70%		9.80%

[1] As of January 1, 2023, the Company adopted ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts (ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance. See Note A to the Consolidated Financial Statements for additional information.