Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions		3 Months Ended
	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 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]	us-gaap:AccountingStandardsUpdate201613Member
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	$ 92	142		$ 146
Claims and claim adjustment expense reserves at end of period		55,340
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		5,242	4,763
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years		(294)	(3)
Total increases		4,948	4,760
Claims and claim adjustment expense payments for claims arising in current year		1,059	947
Claims and claim adjustment expense payments for claims arising in prior years		3,012	3,439
Total payments		4,071	4,386
Unrealized foreign exchange (gain) loss		28	(210)
Net reserves at end of period	43,854	47,262	44,018
Reinsurance recoverables on unpaid losses	8,035	8,068	7,926
Claims and claim adjustment expense reserves at end of period	51,836	55,330	$ 51,944
Amount of increase in gross claims and claim adjustment expense reserves		820
Amount of decrease in reinsurance recoverables on unpaid losses		85
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