Claim and Claim Adjustment Expense Reserves (Specialty - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 5	$ 18	$ 23
Other Professional Liability and Management Liability	37	50	24
Surety	(43)	(83)	(73)
Warranty	(11)	(21)	(14)
Other	(2)	(4)	(5)
Total pretax (favorable) unfavorable development	$ (14)	$ (40)	$ (45)