Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net investment income	$ 780	$ 742	$ 717
Other income	124	146	117
Revenues before realized gains (losses)	8,324	7,867	7,156
Realized gains (losses) on securities	0	(36)	(116)
Realized loss on subsidiary	0	(4)	0
Total revenues	8,324	7,827	7,040
Property and casualty insurance | Operating segments
Segment Reporting Information [Line Items]
Net earned premiums	7,036	6,531	6,085
Net investment income	784	729	683
Other income	8	16	12
Revenues before realized gains (losses)	7,828	7,276	6,780
Other | Operating segments
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	496	591	376
Property and transportation | Property and casualty insurance | Operating segments
Segment Reporting Information [Line Items]
Net earned premiums	2,793	2,519	2,487
Specialty casualty | Property and casualty insurance | Operating segments
Segment Reporting Information [Line Items]
Net earned premiums	2,967	2,886	2,659
Specialty financial | Property and casualty insurance | Operating segments
Segment Reporting Information [Line Items]
Net earned premiums	1,032	867	698
Other specialty | Property and casualty insurance | Operating segments
Segment Reporting Information [Line Items]
Net earned premiums	$ 244	$ 259	$ 241