Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions		9 Months Ended
	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
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		$ 54,521
Accounting Standards Update [Extensible List]	Accounting Standards Update 2016-13 [Member]
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	$ 92	142	$ 140	$ 135	$ 146	$ 156
Claims and claim adjustment expense reserves at end of period		56,805
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		54,510	51,836
Reinsurance recoverables on unpaid losses		8,153	8,035
Net reserves at beginning of year		46,357	43,854
Estimated claims and claim adjustment expenses for claims arising in the current year		15,813	14,809
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years		(398)	(108)
Total increases		15,415	14,701
Claims and claim adjustment expense payments for claims arising in current year		5,653	4,960
Claims and claim adjustment expense payments for claims arising in prior years		7,339	7,246
Total payments		12,992	12,206
Unrealized foreign exchange gain		(18)	(62)
Net reserves at end of period	43,854	48,762	46,287
Reinsurance recoverables on unpaid losses	8,035	8,033	8,119
Claims and claim adjustment expense reserves at end of period	51,836	56,795	$ 54,406
Amount of increase in gross claims and claim adjustment expense reserves		2,290
Amount of decrease in reinsurance recoverables on unpaid losses		120
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		11
Claims and claim adjustment expense reserves at end of period		$ 10