Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019
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	$ 51,849
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	156		$ 149	$ 92
Claims and claim adjustment expense reserves at end of period	53,109
Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Reinsurance recoverables, allowance for estimated uncollectible reinsurance				53
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	51,836	$ 50,653
Reinsurance recoverables on unpaid losses	8,035	8,182
Net reserves at beginning of year	43,854	42,471
Estimated claims and claim adjustment expenses for claims arising in the current year	9,826	9,329
Estimated increase (decrease) in claims and claim adjustment expenses for claims arising in prior years	16	(118)
Total increases	9,842	9,211
Claims and claim adjustment expense payments for claims arising in current year	2,664	2,810
Claims and claim adjustment expense payments for claims arising in prior years	5,674	5,897
Total payments	8,338	8,707
Unrealized foreign exchange (gain) loss	(137)	62
Net reserves at end of period	45,221	43,037
Reinsurance recoverables on unpaid losses	7,876	8,022
Claims and claim adjustment expense reserves at end of period	53,097	$ 51,059
Amount of increase in gross claims and claim adjustment expense reserves	1,260
Amount of decrease in reinsurance recoverables on unpaid losses	159
Property-casualty [Member] | Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Reinsurance recoverables, allowance for estimated uncollectible reinsurance				$ 53
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	13
Claims and claim adjustment expense reserves at end of period	$ 12