Insurance Claim Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 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	$ 51,849
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201601Member
Reinsurance recoverables, allowance for estimated uncollectible reinsurance	$ 146	$ 92
Net reserves at end of year	46,357
Reinsurance recoverables on unpaid losses	8,164
Claims and claim adjustment expense reserves at end of year	54,521	51,849
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		$ 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	$ 49,633
Reinsurance recoverables on unpaid losses	8,035	8,182	8,123
Net reserves at beginning of year	43,801	42,471	41,510
Estimated claims and claim adjustment expenses for claims arising in the current year	19,285	18,854	18,614
Estimated increase (decrease) in claims and claim adjustment expenses for claims arising in prior years	(267)	164	(406)
Total increases	19,018	19,018	18,208
Claims and claim adjustment expense payments for claims arising in current year	7,497	7,734	7,697
Claims and claim adjustment expense payments for claims arising in prior years	9,092	10,060	9,363
Total payments	16,589	17,794	17,060
Unrealized foreign exchange loss (gain)	74	106	(187)
Net reserves at end of year	46,357	43,801	42,471
Reinsurance recoverables on unpaid losses	8,153	8,035	8,182
Claims and claim adjustment expense reserves at end of year	54,510	51,836	50,653
Amount of increase in gross claims and claim adjustment expense reserves	2,670	1,180
Amount of increase (decrease) in reinsurance recoverables on unpaid losses	118	(147)
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, net of discount	2,690	2,480
Discount included in reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies	1,140	1,160
Accretion expense	49	49	$ 49
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
Property-casualty [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Net reserves at beginning of year	$ 43,854
Net reserves at end of year		$ 43,854
Property-casualty [Member] | Minimum [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, discount rate (percent)	3.50%	3.50%
Property-casualty [Member] | Maximum [Member]
Reconciliation of beginning and ending reserve balances for claims and claim adjustment expenses [Roll Forward]
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, discount rate (percent)	5.00%	5.00%
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
Net reserves at end of year	0
Reinsurance recoverables on unpaid losses	11
Claims and claim adjustment expense reserves at end of year	$ 11	$ 13