Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Earnings before realized gains (losses) and income taxes	$ 231	$ 242	$ 794	$ 809
Realized gains (losses) on securities	(2)	(19)	10	(67)
Realized loss on subsidiary	0	(4)	0	(4)
Earnings before income taxes	229	219	804	738
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	115	142	418	420
Investment and other income, net	176	156	532	527
Earnings before realized gains (losses) and income taxes	291	298	950	947
Operating Segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	34	42	129	117
Operating Segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	76	78	258	261
Operating Segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	22	29	80	65
Operating Segments | Property and Casualty Insurance | Other specialty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(15)	(6)	(45)	(22)
Operating Segments | Property and Casualty Insurance | Other lines
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(2)	(1)	(4)	(1)
Operating Segments | Other
Segment Reporting Information [Line Items]
Earnings before realized gains (losses) and income taxes	$ (60)	$ (56)	$ (156)	$ (138)