Segments of Operations - Components of underwriting profit, including significant segment expenses 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
Losses and loss adjustment expenses	965	912
Commissions and other underwriting expenses	530	503
Operating segments | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	1,580	1,546
Losses and loss adjustment expenses	965	907
Commissions and other underwriting expenses	521	486
Property and casualty insurance underwriting	94	153
Operating segments | Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	500	520
Losses and loss adjustment expenses	311	306
Commissions and other underwriting expenses	152	154
Property and casualty insurance underwriting	37	60
Operating segments | Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	794	783
Losses and loss adjustment expenses	536	502
Commissions and other underwriting expenses	238	220
Property and casualty insurance underwriting	20	61
Operating segments | Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	286	243
Losses and loss adjustment expenses	118	98
Commissions and other underwriting expenses	131	112
Property and casualty insurance underwriting	37	33
Operating segments | Other lines | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Losses and loss adjustment expenses	0	1
Property and casualty insurance underwriting	$ 0	$ (1)