Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year			$ 659	$ 655
(Favorable) unfavorable development	$ (1)	$ 17	190	33
Property and Casualty Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	534	509	1,552	1,486
Catastrophe losses	0	0	0	0
(Favorable) unfavorable development	0	0	10	(8)
Property and Casualty Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	921	829	2,674	2,350
Catastrophe losses	39	127	182	285
(Favorable) unfavorable development	(1)	(3)	46	(11)
Property and Casualty Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	201	181	572	545
Catastrophe losses	2	16	18	28
(Favorable) unfavorable development	$ 0	$ (2)	$ 0	$ (5)