Claim and Claim Adjustment Expense Reserves (Reconciliation of Claim and Claim Adjustment Expense Reserves) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	1,355	1,309
Increase (decrease) in provision for insured events of prior years	(8)	8
Amortization of discount	51	50
Total net incurred	1,398	1,367
Net payments attributable to:
Current year events	(72)	(100)
Prior year events	(1,218)	(1,309)
Total net payments	(1,290)	(1,409)
Foreign currency translation adjustment and other	(88)	13
Net reserves, end of period	17,905	17,936
Ceded reserves, end of period	3,967	3,900
Gross reserves, end of period	$ 21,872	$ 21,836