Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ (7)	$ 13
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(5)	0
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(2)	(2)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	0	15
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 0	$ 0