Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Revenue Reconciling Item [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	$ 1,089	$ 1,093	$ 922
(Favorable) unfavorable development	185	48	48
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	2,097	2,010	1,937
Catastrophe losses	0	0	0
(Favorable) unfavorable development	37	(9)	(14)
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	3,605	3,217	2,801
Catastrophe losses	217	318	207
(Favorable) unfavorable development	39	(16)	(22)
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	767	730	680
Catastrophe losses	23	40	29
(Favorable) unfavorable development	$ (25)	$ (6)	$ 13