Claim and Claim Adjustment Expense Reserves (Reconciliation of Claim and Claim Adjustment Expense Reserves) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Reserves, beginning of year: [Abstract]
Gross	$ 21,720	$ 21,984
Ceded	3,835	4,019
Net reserves, beginning of year	17,885	17,965
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	4,425	3,968
Increase (decrease) in provision for insured events of prior years	(68)	(65)
Amortization of discount	143	143
Total net incurred	4,500	4,046
Net payments attributable to:
Current year events	(556)	(599)
Prior year events	(3,285)	(3,547)
Total net payments	(3,841)	(4,146)
Foreign currency translation adjustment and other	39	29
Net reserves, end of period	18,583	17,894
Ceded reserves, end of period	3,951	3,702
Gross reserves, end of period	$ 22,534	$ 21,596