Claim and Claim Adjustment Expense Reserves (International - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - International - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Casualty	$ (20)	$ (17)	$ 9
Property	23	19	(12)
Energy and Marine	2	(19)	(12)
Specialty	16	13	6
Total pretax (favorable) unfavorable development	$ 21	$ (4)	$ (9)