Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	1,309	1,246
Increase (decrease) in provision for insured events of prior years	8	(34)
Amortization of discount	50	47
Total net incurred	1,367	1,259
Net payments attributable to:
Current year events	(100)	(91)
Prior year events	(1,309)	(1,219)
Total net payments	(1,409)	(1,310)
Foreign currency translation adjustment and other	13	(9)
Net reserves, end of period	17,936	18,010
Ceded reserves, end of period	3,900	4,057
Gross reserves, end of period	$ 21,836	$ 22,067