Claim and Claim Adjustment Expense Reserves (Specialty - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 47	$ 30	$ 9
Other Professional Liability and Management Liability	(127)	(126)	(140)
Surety	(70)	(84)	(63)
Warranty	(10)	4	4
Other	10	2	(57)
Total pretax (favorable) unfavorable development	$ (150)	$ (174)	$ (247)