Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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	$ 50,668
Claims and claim adjustment expense reserves at end of period	50,718
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	50,653	$ 49,633
Reinsurance recoverables on unpaid losses	8,182	8,123
Net reserves at beginning of year	42,471	41,510
Estimated claims and claim adjustment expenses for claims arising in the current year	4,435	4,391
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(16)	(116)
Total increases	4,419	4,275
Claims and claim adjustment expense payments for claims arising in current year	970	1,009
Claims and claim adjustment expense payments for claims arising in prior years	3,320	3,040
Total payments	4,290	4,049
Unrealized foreign exchange (gain) loss	41	(10)
Net reserves at end of period	42,641	41,726
Reinsurance recoverables on unpaid losses	8,063	8,068
Claims and claim adjustment expense reserves at end of period	50,704	$ 49,794
Amount of increase in gross claims and claim adjustment expense reserves	51
Amount of decrease in reinsurance recoverables on unpaid losses	(119)
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	15
Claims and claim adjustment expense reserves at end of period	$ 14