Claim and Claim Adjustment Expense Reserves (Specialty - Detail of Development Recorded) (Details) - Specialty - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (44)	$ (23)	$ (74)	$ (35)
Medical Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	3	2	23	22
Other Professional Liability and Management Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(34)	(37)	(68)	(69)
Surety
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(15)	0	(30)	0
Warranty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(6)	6	(6)	6
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ 8	$ 6	$ 7	$ 6