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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 25	$ 29	$ 35	$ 59
Other Professional Liability and Management Liability	0	(18)	(6)	(37)
Surety	(40)	(43)	(70)	(83)
Warranty	0	0	(3)	(7)
Other	(1)	12	(3)	10
Total pretax (favorable) unfavorable development	$ (16)	$ (20)	$ (47)	$ (58)