Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net earned premiums	$ 6,531	$ 6,085	$ 5,404
Net investment income	742	717	730
Other income	146	117	113
Revenues before realized gains (losses)	$ 7,867	$ 7,156	$ 6,438
Investment, Type [Extensible Enumeration]	Real Estate Funds [Member]	Real Estate Funds [Member]	Real Estate Funds [Member]
Realized gains (losses) on securities	$ (36)	$ (116)	$ 110
Realized gains (losses) on subsidiaries	(4)	0	4
Total revenues	7,827	7,040	6,552
Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums	6,531	6,085	5,404
Net investment income	729	683	663
Other income	16	12	27
Revenues before realized gains (losses)	7,276	6,780	6,094
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	591	376	293
Other | Continuing operations
Segment Reporting Information [Line Items]
Net investment income	0	0	51
Property and transportation | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums	2,519	2,487	2,144
Specialty casualty | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums	2,886	2,659	2,408
Specialty financial | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums	867	698	642
Other specialty | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums	$ 259	$ 241	$ 210