Claim and Claim Adjustment Expense Reserves (Specialty - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (308)	$ (288)	$ (210)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	5	(37)	(43)
Other Professional Liability and Management Liability	(131)	(130)	0
Surety	(84)	(63)	(69)
Warranty	4	4	(2)
Other	4	(61)	(27)
Total pretax (favorable) unfavorable development	$ (202)	$ (287)	$ (141)