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, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Casualty	$ (13)	$ (20)	$ (17)
Property, Energy and Marine	13	25	0
Specialty	(2)	16	13
Total pretax (favorable) unfavorable development	$ (2)	$ 21	$ (4)