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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 15	$ 3	$ 30	$ 23
Other Professional Liability and Management Liability	(7)	(34)	(19)	(68)
Surety	(15)	(15)	(40)	(30)
Warranty	(7)	(6)	(7)	(6)
Other	(4)	8	(2)	7
Total pretax (favorable) unfavorable development	$ (18)	$ (44)	$ (38)	$ (74)