Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Total premiums earned	$ 2,055	$ 1,855	$ 5,186	$ 4,799
Net investment income	200	168	586	583
Other income	26	43	92	100
Revenues before realized gains (losses)	2,371	2,187	6,165	5,815
Realized gains (losses) on securities	(2)	(19)	10	(67)
Realized loss on subsidiary	0	(4)	0	(4)
Total revenues	2,369	2,164	6,175	5,744
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	2,055	1,855	5,186	4,799
Net investment income	195	170	589	568
Other income	2	5	6	13
Revenues before realized gains (losses)	2,252	2,030	5,781	5,380
Operating Segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Total premiums earned	981	828	2,037	1,837
Operating Segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Total premiums earned	744	734	2,213	2,149
Operating Segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Total premiums earned	269	232	753	623
Operating Segments | Property and Casualty Insurance | Other specialty
Segment Reporting Information [Line Items]
Total premiums earned	61	61	183	190
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	$ 119	$ 157	$ 384	$ 435