Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,767	$ 1,529	$ 4,462	$ 3,952
Net investment income	151	169	549	521
Other income	31	27	93	70
Revenues before realized gains (losses)	2,019	1,771	5,254	4,687
Realized gains (losses) on securities	(35)	(17)	(143)	103
Total revenues	1,984	1,754	5,111	4,794
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,767	1,529	4,462	3,952
Net investment income	145	165	524	467
Other income	2	4	12	9
Revenues before realized gains (losses)	1,914	1,698	4,998	4,428
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	105	73	256	208
Other | Real estate-related entities | Continuing operations
Segment Reporting Information [Line Items]
Net investment income	0	0	0	51
Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	857	700	1,805	1,547
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	677	613	1,973	1,772
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	171	163	505	477
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	$ 62	$ 53	$ 179	$ 156