Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ 255	$ 231	$ 676	$ 794
Realized gains (losses) on securities	12	(2)	17	10
Realized gain on subsidiaries	1	0	1	0
Earnings before income taxes	268	229	694	804
Operating segments \| Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	138	115	345	418
Investment and other income, net	190	176	502	532
Earnings before realized gains and income taxes	328	291	847	950
Operating segments \| Property and Casualty Insurance \| Property and transportation
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	55	33	119	133
Operating segments \| Property and Casualty Insurance \| Specialty casualty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	33	63	102	210
Operating segments \| Property and Casualty Insurance \| Specialty financial
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	51	21	126	79
Operating segments \| Property and Casualty Insurance \| Other lines
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(1)	(2)	(2)	(4)
Operating segments \| Other
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ (73)	$ (60)	$ (171)	$ (156)

Net investment income plus other income less other expenses.

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The net result for the period of deducting operating expenses from operating revenues.

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