Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Reserves, beginning of year: [Abstract]
Gross	$ 21,984	$ 22,004
Ceded	4,019	3,934
Net reserves, beginning of year	17,965	18,070
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	2,615	2,552
Increase (decrease) in provision for insured events of prior years	(36)	(112)
Amortization of discount	98	92
Total net incurred	2,677	2,532
Net payments attributable to:
Current year events	(315)	(312)
Prior year events	(2,519)	(2,387)
Total net payments	(2,834)	(2,699)
Foreign currency translation adjustment and other	55	(70)
Net reserves, end of period	17,863	17,833
Ceded reserves, end of period	3,866	4,157
Gross reserves, end of period	$ 21,729	$ 21,990