Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Earnings before realized gains and income taxes	$ 300	$ 227	$ 557	$ 421
Realized gains (losses) on securities	16	2	(2)	5
Earnings before income taxes	316	229	555	426
Operating segments \| Property and casualty insurance
Segment Reporting [Line Items]
Property and casualty insurance underwriting	142	113	298	207
Investment and other income, net	208	160	361	312
Earnings before realized gains and income taxes	350	273	659	519
Operating segments \| Property and casualty insurance \| Property and transportation
Segment Reporting [Line Items]
Property and casualty insurance underwriting	57	27	122	64
Operating segments \| Property and casualty insurance \| Specialty casualty
Segment Reporting [Line Items]
Property and casualty insurance underwriting	45	49	79	69
Operating segments \| Property and casualty insurance \| Specialty financial
Segment Reporting [Line Items]
Property and casualty insurance underwriting	42	38	99	75
Operating segments \| Property and casualty insurance \| Other lines
Segment Reporting [Line Items]
Property and casualty insurance underwriting	(2)	(1)	(2)	(1)
Operating segments \| Other
Segment Reporting [Line Items]
Earnings before realized gains and income taxes	$ (50)	$ (46)	$ (102)	$ (98)

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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