Claim and Claim Adjustment Expense Reserves (International - Detail of Development Recorded) (Details) - International - 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	$ (2)	$ 1	$ (4)	$ 1
Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(5)	6	(11)	7
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	1	(7)	13	(18)
Energy and Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(5)	(6)	(10)	(9)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	8	5	17	20
Healthcare and Technology
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (1)	$ 3	$ (13)	$ 1