Claim and Claim Adjustment Expense Reserves (Specialty - Detail of Development Recorded) (Details) - Specialty - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 20	$ 20
Other Professional Liability and Management Liability	(34)	(32)
Surety	(15)	0
Warranty	0	0
Other	(1)	0
Total pretax (favorable) unfavorable development	$ (30)	$ (12)