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, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 35	$ 75	$ 47
Other Professional Liability and Management Liability	(15)	(69)	(127)
Surety	(69)	(92)	(70)
Warranty	(7)	(15)	(10)
Other	(5)	9	10
Total pretax (favorable) unfavorable development	$ (61)	$ (92)	$ (150)