Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,437	$ 1,302
Net investment income	217	230
Other income	32	30
Revenues before realized gains (losses)	1,786	1,603
Realized gains (losses) on securities	(46)	(15)
Total revenues	1,740	1,588
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,437	1,302
Net investment income	207	223
Other income	5	4
Revenues before realized gains (losses)	1,649	1,529
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	137	74
Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	475	443
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	704	639
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	196	163
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	$ 62	$ 57