Claim and Claim Adjustment Expense Reserves (International - Detail of Development Recorded) (Details) - International - 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	$ (2)	$ 2	$ (2)	$ 0
Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(6)	1	(6)	1
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	12	(10)	12	(11)
Energy and Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(5)	(3)	(5)	(3)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	9	16	9	15
Healthcare and Technology
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (12)	$ (2)	$ (12)	$ (2)