Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Claims and claim adjustment expense reserves at beginning of year	$ 49,650
Claims and claim adjustment expense reserves at end of period	50,430
Property-casualty [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Claims and claim adjustment expense reserves at beginning of year	49,633	$ 47,929
Reinsurance recoverables on unpaid losses	8,123	7,981
Net reserves at beginning of year	41,510	39,948
Estimated claims and claim adjustment expenses for claims arising in the current year	13,707	13,261
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(255)	(197)
Total increases	13,452	13,064
Claims and claim adjustment expense payments for claims arising in current year	5,112	4,799
Claims and claim adjustment expense payments for claims arising in prior years	7,419	6,831
Total payments	12,531	11,630
Unrealized foreign exchange (gain) loss	(82)	215
Net reserves at end of period	42,349	41,597
Reinsurance recoverables on unpaid losses	8,066	8,136
Claims and claim adjustment expense reserves at end of period	50,415	$ 49,733
Accident and health [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Claims and claim adjustment expense reserves at beginning of year	17
Claims and claim adjustment expense reserves at end of period	$ 15