Segments of Operations - Earnings before income taxes by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting [Line Items]
Earnings before realized gains and income taxes	$ 394	$ 255	$ 951	$ 676
Realized gains (losses) on securities	(35)	12	(37)	17
Realized gain on subsidiaries	0	1	0	1
Earnings before income taxes	359	268	914	694
Operating segments \| Property and casualty insurance
Segment Reporting [Line Items]
Property and casualty insurance underwriting	130	138	428	345
Investment and other income, net	276	190	637	502
Earnings before realized gains and income taxes	406	328	1,065	847
Operating segments \| Property and casualty insurance \| Property and transportation
Segment Reporting [Line Items]
Property and casualty insurance underwriting	68	55	190	119
Operating segments \| Property and casualty insurance \| Specialty casualty
Segment Reporting [Line Items]
Property and casualty insurance underwriting	42	33	121	102
Operating segments \| Property and casualty insurance \| Specialty financial
Segment Reporting [Line Items]
Property and casualty insurance underwriting	23	51	122	126
Operating segments \| Property and casualty insurance \| Other lines
Segment Reporting [Line Items]
Property and casualty insurance underwriting	(3)	(1)	(5)	(2)
Operating segments \| Other
Segment Reporting [Line Items]
Earnings before realized gains and income taxes	$ (12)	$ (73)	$ (114)	$ (171)

Net investment income plus other income less other expenses.

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