Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Net earned premiums	$ 1,580	$ 1,546
Net investment income	173	198
Other income	27	39
Revenues before realized gains (losses)	1,853	1,892
Realized gains on securities	3	14
Total revenues	1,856	1,906
Operating segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	1,580	1,546
Net investment income	170	205
Other income	3	2
Revenues before realized gains (losses)	1,753	1,753
Operating segments | Property and Casualty Insurance | Property and transportation
Segment Reporting Information [Line Items]
Net earned premiums	500	520
Operating segments | Property and Casualty Insurance | Specialty casualty
Segment Reporting Information [Line Items]
Net earned premiums	794	783
Operating segments | Property and Casualty Insurance | Specialty financial
Segment Reporting Information [Line Items]
Net earned premiums	286	243
Operating segments | Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	$ 100	$ 139