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
	Mar. 31, 2019	Mar. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 15	$ 20
Other Professional Liability and Management Liability	(12)	(34)
Surety	(25)	(15)
Warranty	0	0
Other	2	(1)
Total pretax (favorable) unfavorable development	$ (20)	$ (30)