Claim and Claim Adjustment Expense Reserves (International - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - International - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Casualty	$ 0	$ 0
Property, Energy and Marine	0	14
Specialty	0	0
Total pretax (favorable) unfavorable development	$ 0	$ 14