Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reserves, beginning of year: [Abstract]
Gross	$ 21,984	$ 22,004	$ 22,343
Ceded	4,019	3,934	4,094
Net reserves, beginning of year	17,965	18,070	18,249
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	5,356	5,358	5,201
Increase (decrease) in provision for insured events of prior years	(127)	(179)	(381)
Amortization of discount	184	176	179
Total net incurred	5,413	5,355	4,999
Net payments attributable to:
Current year events	(992)	(1,046)	(975)
Prior year events	(4,584)	(4,285)	(4,366)
Total net payments	(5,576)	(5,331)	(5,341)
Foreign currency translation adjustment and other	83	(129)	163
Net reserves, end of year	17,885	17,965	18,070
Ceded reserves, end of year	3,835	4,019	3,934
Gross reserves, end of year	$ 21,720	$ 21,984	$ 22,004