Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,585	$ 1,507	$ 3,131	$ 2,944
Net investment income	188	198	386	415
Other income	27	25	66	57
Revenues before realized gains (losses)	1,902	1,842	3,794	3,628
Realized gains (losses) on securities	(2)	(2)	12	(48)
Total revenues	1,900	1,840	3,806	3,580
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,585	1,507	3,131	2,944
Net investment income	189	191	394	398
Other income	2	3	4	8
Revenues before realized gains (losses)	1,776	1,701	3,529	3,350
Operating Segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Total premiums earned	543	534	1,056	1,009
Operating Segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Total premiums earned	739	711	1,469	1,415
Operating Segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Total premiums earned	241	195	484	391
Operating Segments | Property and Casualty Insurance | Other specialty
Segment Reporting Information [Line Items]
Total premiums earned	62	67	122	129
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	$ 126	$ 141	$ 265	$ 278