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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 8	$ 25	$ 16	$ 35
Other Professional Liability and Management Liability	0	0	10	(6)
Surety	(15)	(40)	(53)	(70)
Warranty	(6)	0	(14)	(3)
Other	(2)	(1)	1	(3)
Total pretax (favorable) unfavorable development	$ (15)	$ (16)	$ (40)	$ (47)