Segments of Operations - Components of underwriting profit, including significant segment expenses 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]
Premiums earned	$ 7,036	$ 6,531	$ 6,085
Losses and loss adjustment expenses	4,460	4,033	3,629
Commissions and other underwriting expenses	2,007	1,935	1,718
Operating segments | Property and casualty insurance
Segment Reporting Information [Line Items]
Premiums earned	7,036	6,531	6,085
Losses and loss adjustment expenses	4,455	4,017	3,629
Commissions and other underwriting expenses	1,961	1,883	1,680
Property and casualty insurance underwriting	620	631	776
Operating segments | Property and casualty insurance | Property and transportation
Segment Reporting Information [Line Items]
Premiums earned	2,793	2,519	2,487
Losses and loss adjustment expenses	1,952	1,741	1,735
Commissions and other underwriting expenses	630	594	544
Property and casualty insurance underwriting	211	184	208
Operating segments | Property and casualty insurance | Specialty casualty
Segment Reporting Information [Line Items]
Premiums earned	2,967	2,886	2,659
Losses and loss adjustment expenses	1,853	1,739	1,453
Commissions and other underwriting expenses	774	772	706
Property and casualty insurance underwriting	340	375	500
Operating segments | Property and casualty insurance | Specialty financial
Segment Reporting Information [Line Items]
Premiums earned	1,032	867	698
Losses and loss adjustment expenses	429	328	238
Commissions and other underwriting expenses	469	429	346
Property and casualty insurance underwriting	134	110	114
Operating segments | Property and casualty insurance | Other specialty
Segment Reporting Information [Line Items]
Premiums earned	244	259	241
Losses and loss adjustment expenses	215	207	199
Commissions and other underwriting expenses	88	88	84
Property and casualty insurance underwriting	(59)	(36)	(42)
Operating segments | Property and casualty insurance | Other lines
Segment Reporting Information [Line Items]
Losses and loss adjustment expenses	6	2	4
Property and casualty insurance underwriting	$ (6)	$ (2)	$ (4)