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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 0	$ 15	$ 10	$ 30
Other Professional Liability and Management Liability	(9)	(7)	(6)	(19)
Surety	0	(15)	(30)	(40)
Warranty	(3)	(7)	(3)	(7)
Other	(8)	(4)	(2)	(2)
Total pretax (favorable) unfavorable development	$ (20)	$ (18)	$ (31)	$ (38)