Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reserves, beginning of year:
Gross	$ 21,720	$ 21,984	$ 22,004
Ceded	3,835	4,019	3,934
Net reserves, beginning of year	17,885	17,965	18,070
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	5,793	5,356	5,358
Increase (decrease) in provision for insured events of prior years	(119)	(127)	(179)
Amortization of discount	183	184	176
Total net incurred	5,857	5,413	5,355
Net payments attributable to:
Current year events	(948)	(992)	(1,046)
Prior year events	(4,216)	(4,584)	(4,285)
Total net payments	(5,164)	(5,576)	(5,331)
Foreign currency translation adjustment and other	123	83	(129)
Net reserves, end of year	18,701	17,885	17,965
Ceded reserves, end of year	4,005	3,835	4,019
Gross reserves, end of year	$ 22,706	$ 21,720	$ 21,984