Income Taxes (Details) - Effective Tax Rate - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
U.S. income before income taxes		$ 3,122	$ 3,101	$ 4,107
Foreign income before income taxes		249	253	351
Income before income taxes		$ 3,371	$ 3,354	$ 4,458
Statutory tax rate (percent)		21.00%	21.00%	21.00%
Expected federal income tax expense		$ 708	$ 704	$ 936
Tax effect of nontaxable investment income		(132)	(149)	(147)
Tax effect of audit reserve		(205)	(40)	1
Tax effect of other, net		9	(3)	6
Total income tax expense included in the consolidated statement of income		$ 380	$ 512	$ 796
Effective tax rate (percent)		11.00%	15.00%	18.00%
Tax benefit due to the expiration of the statute of limitations with respect to a tax item impacted by the repeal of Internal Revenue Code Section 847, which related to the discounting of property-casualty loss reserves	$ 211
Income taxes paid		$ 201	$ 817	$ 707
Current income tax payable		$ 285
Current income tax receivable			$ 32