Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Earnings before realized gains (losses) and income taxes	$ 273	$ 257	$ 563	$ 567
Realized gains (losses) on securities	(2)	(2)	12	(48)
Earnings before income taxes	271	255	575	519
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	150	124	303	278
Investment and other income, net	169	175	356	371
Earnings before realized gains (losses) and income taxes	319	299	659	649
Operating Segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	39	32	95	75
Operating Segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	108	95	182	183
Operating Segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	25	10	58	36
Operating Segments | Property and Casualty Insurance | Other specialty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(21)	(14)	(30)	(16)
Operating Segments | Property and Casualty Insurance | Other lines
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(1)	1	(2)	0
Operating Segments | Other
Segment Reporting Information [Line Items]
Earnings before realized gains (losses) and income taxes	$ (46)	$ (42)	$ (96)	$ (82)