Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 13	$ (17)	$ 44	$ (2)
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(5)	(15)	(9)	(35)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(2)	(2)	(17)	(26)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	0	0	15	(5)
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 20	$ 0	$ 55	$ 64