Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,507	$ 1,393	$ 2,944	$ 2,695
Net investment income	198	168	415	398
Other income	25	32	57	62
Revenues before realized gains (losses)	1,842	1,632	3,628	3,235
Realized gains (losses) on securities	(2)	(93)	(48)	(108)
Total revenues	1,840	1,539	3,580	3,127
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,507	1,393	2,944	2,695
Net investment income	191	156	398	379
Other income	3	6	8	10
Revenues before realized gains (losses)	1,701	1,555	3,350	3,084
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	141	77	278	151
Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	534	505	1,009	948
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	711	657	1,415	1,296
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	195	171	391	334
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	$ 67	$ 60	$ 129	$ 117