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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 0	$ 8	$ 9	$ 17
Other Professional Liability and Management Liability	17	9	16	22
Surety	(21)	(20)	(28)	(48)
Warranty	(2)	(13)	(11)	(22)
Other	1	1	5	(4)
Total pretax (favorable) unfavorable development	$ (5)	$ (15)	$ (9)	$ (35)