Claim and Claim Adjustment Expense Reserves (Specialty - Detail of Development Recorded) (Details) - Specialty - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (53)	$ (99)	$ (127)	$ (134)
Medical Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	15	8	38	30
Other Professional Liability and Management Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(45)	(19)	(113)	(88)
Surety
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(20)	(82)	(50)	(82)
Warranty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(1)	(1)	(7)	5
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (2)	$ (5)	$ 5	$ 1