Claim and Claim Adjustment Expense Reserves (Reconciliation of Claim and Claim Adjustment Expense Reserves) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 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	2,899	2,615
Increase (decrease) in provision for insured events of prior years	19	(36)
Amortization of discount	98	98
Total net incurred	3,016	2,677
Net payments attributable to:
Current year events	(256)	(315)
Prior year events	(2,342)	(2,519)
Total net payments	(2,598)	(2,834)
Foreign currency translation adjustment and other	(35)	55
Net reserves, end of period	18,268	17,863
Ceded reserves, end of period	4,002	3,866
Gross reserves, end of period	$ 22,270	$ 21,729