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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 0	$ 0	$ 8	$ 10
Other Professional Liability and Management Liability	10	(9)	10	(6)
Surety	(23)	0	(38)	(30)
Warranty	0	(3)	(8)	(3)
Other	3	(8)	3	(2)
Total pretax (favorable) unfavorable development	$ (10)	$ (20)	$ (25)	$ (31)