Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 18	$ 27	$ 31	$ 15
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(4)	(10)	(4)	(20)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(13)	(22)	(15)	(24)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	0	(5)	15	(5)
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 35	$ 64	$ 35	$ 64