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, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 75	$ 47	$ 30
Other Professional Liability and Management Liability	(69)	(127)	(126)
Surety	(92)	(70)	(84)
Warranty	(15)	(10)	4
Other	9	10	2
Total pretax (favorable) unfavorable development	$ (92)	$ (150)	$ (174)