Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,855	$ 1,767	$ 4,799	$ 4,462
Net investment income	168	151	583	549
Other income	43	31	100	93
Revenues before realized gains (losses)	2,187	2,019	5,815	5,254
Realized gains (losses) on securities	(19)	(35)	(67)	(143)
Realized loss on subsidiary	(4)	0	(4)	0
Total revenues	2,164	1,984	5,744	5,111
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,855	1,767	4,799	4,462
Net investment income	170	145	568	524
Other income	5	2	13	12
Revenues before realized gains (losses)	2,030	1,914	5,380	4,998
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	157	105	435	256
Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	828	857	1,837	1,805
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	734	677	2,149	1,973
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	232	171	623	505
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	$ 61	$ 62	$ 190	$ 179