Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ 227	$ 273	$ 421	$ 563
Realized gains (losses) on securities	2	(2)	5	12
Earnings before income taxes	229	271	426	575
Operating segments \| Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	113	150	207	303
Investment and other income, net	160	169	312	356
Earnings before realized gains and income taxes	273	319	519	659
Operating segments \| Property and Casualty Insurance \| Property and transportation
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	27	40	64	100
Operating segments \| Property and Casualty Insurance \| Specialty casualty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	49	86	69	147
Operating segments \| Property and Casualty Insurance \| Specialty financial
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	38	25	75	58
Operating segments \| Property and Casualty Insurance \| Other lines
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	(1)	(1)	(1)	(2)
Operating segments \| Other
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ (46)	$ (46)	$ (98)	$ (96)

Net investment income plus other income less other expenses.

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Amount of realized gain (loss) on investment in debt and equity securities.

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

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