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
	Mar. 31, 2021	Mar. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 8	$ 10
Other Professional Liability and Management Liability	0	3
Surety	(15)	(30)
Warranty	(8)	0
Other	0	6
Total pretax (favorable) unfavorable development	$ (15)	$ (11)