Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 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
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	$ 140		$ 156	$ 92
Claims and claim adjustment expense reserves at end of period	54,418
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	14,809	14,193
Estimated increase (decrease) in claims and claim adjustment expenses for claims arising in prior years	(108)	214
Total increases	14,701	14,407
Claims and claim adjustment expense payments for claims arising in current year	4,960	5,174
Claims and claim adjustment expense payments for claims arising in prior years	7,246	8,034
Total payments	12,206	13,208
Unrealized foreign exchange (gain) loss	(62)	12
Net reserves at end of period	46,287	43,682
Reinsurance recoverables on unpaid losses	8,119	7,916
Claims and claim adjustment expense reserves at end of period	54,406	$ 51,598
Amount of increase in gross claims and claim adjustment expense reserves	2,570
Amount of increase in reinsurance recoverables on unpaid losses	84
Property-casualty [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