Business Segments (Segment Expenses Included Within Incurred Claims and Benefits) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year			$ 345	$ 316
(Favorable) unfavorable development	$ 91	$ 108	191	191
Property and Casualty Segments | Specialty
Segment Reporting [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	551	519	1,087	1,018
Catastrophe losses	0	0	0	0
(Favorable) unfavorable development	(1)	0	44	10
Property and Casualty Segments | Commercial
Segment Reporting [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	953	886	1,889	1,754
Catastrophe losses	53	57	137	143
(Favorable) unfavorable development	(5)	(4)	50	47
Property and Casualty Segments | International
Segment Reporting [Line Items]
Non-catastrophe net incurred claim and claim adjustment expenses related to current year	202	190	402	371
Catastrophe losses	7	5	11	16
(Favorable) unfavorable development	$ 0	$ 0	$ 0	$ 0