Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ 257	$ 194
Realized gains (losses) on securities	(18)	3
Earnings before income taxes	239	197
Operating segments \| Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	156	94
Investment and other income, net	153	152
Earnings before realized gains and income taxes	309	246
Operating segments \| Property and Casualty Insurance \| Property and transportation
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	65	37
Operating segments \| Property and Casualty Insurance \| Specialty casualty
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	34	20
Operating segments \| Property and Casualty Insurance \| Specialty financial
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	57	37
Operating segments \| Other
Segment Reporting Information [Line Items]
Earnings before realized gains and income taxes	$ (52)	$ (52)

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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