Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 23	$ 18	$ 16	$ 31
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(3)	(4)	(8)	(4)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(6)	(13)	(8)	(15)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(3)	0	(3)	15
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 35	$ 35	$ 35	$ 35