Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Total premiums earned	$ 1,546	$ 1,437
Net investment income	198	217
Other income	39	32
Revenues before realized gains (losses)	1,892	1,786
Realized gains (losses) on securities	14	(46)
Total revenues	1,906	1,740
Operating Segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned	1,546	1,437
Net investment income	205	207
Other income	2	5
Revenues before realized gains (losses)	1,753	1,649
Operating Segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Total premiums earned	513	475
Operating Segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Total premiums earned	730	704
Operating Segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Total premiums earned	243	196
Operating Segments | Property and Casualty Insurance | Other specialty
Segment Reporting Information [Line Items]
Total premiums earned	60	62
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	$ 139	$ 137