Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions		6 Months Ended
	Dec. 31, 2019	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 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		$ 54,521.0
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	$ 92.0	135.0	$ 156.0	$ 142.0	$ 146.0	$ 149.0
Claims and claim adjustment expense reserves at end of period		55,906.0
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	53.0
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		54,510.0	51,836.0
Reinsurance recoverables on unpaid losses		8,153.0	8,035.0
Net reserves at beginning of year		46,357.0	43,854.0
Estimated claims and claim adjustment expenses for claims arising in the current year		10,430.0	9,826.0
Estimated increase (decrease) in claims and claim adjustment expenses for claims arising in prior years		(459.0)	16.0
Total increases		9,971.0	9,842.0
Claims and claim adjustment expense payments for claims arising in current year		3,113.0	2,664.0
Claims and claim adjustment expense payments for claims arising in prior years		5,274.0	5,674.0
Total payments		8,387.0	8,338.0
Unrealized foreign exchange (gain) loss		53.0	(137.0)
Net reserves at end of period	43,854.0	47,994.0	45,221.0
Reinsurance recoverables on unpaid losses	8,035.0	7,901.0	7,876.0
Claims and claim adjustment expense reserves at end of period	51,836.0	55,895.0	$ 53,097.0
Amount of increase in gross claims and claim adjustment expense reserves		1,390.0
Amount of decrease in reinsurance recoverables on unpaid losses		252.0
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	$ 53.0
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		11.0
Claims and claim adjustment expense reserves at end of period		$ 11.0