Claim and Claim Adjustment Expense Reserves (Specialty - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Specialty - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 0	$ 0	$ (2)	$ 9
Other Professional Liability and Management Liability	11	17	28	16
Surety	(20)	(21)	(46)	(28)
Warranty	7	(2)	20	(11)
Other	2	1	(8)	5
Total pretax (favorable) unfavorable development	$ 0	$ (5)	$ (8)	$ (9)