Claim and Claim Adjustment Expense and Future Policy Benefit 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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 29	$ 15	$ 59	$ 38
Other Professional Liability and Management Liability	(18)	(45)	(37)	(113)
Surety	(43)	(20)	(83)	(50)
Warranty	0	(1)	(7)	(7)
Other	12	(2)	10	5
Total pretax (favorable) unfavorable development	$ (20)	$ (53)	$ (58)	$ (127)