Insurance Claim Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 49,650
Net reserves at end of year	42,471
Reinsurance recoverables on unpaid losses	8,197
Claims and claim adjustment expense reserves at end of year	50,668	$ 49,650
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	49,633	47,929	$ 48,272
Reinsurance recoverables on unpaid losses	8,123	7,981	8,449
Net reserves at beginning of year	41,510	39,948	39,823
Estimated claims and claim adjustment expenses for claims arising in the current year	18,614	17,846	15,675
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(406)	(458)	(680)
Total increases	18,208	17,388	14,995
Claims and claim adjustment expense payments for claims arising in current year	7,697	7,335	6,220
Claims and claim adjustment expense payments for claims arising in prior years	9,363	8,708	8,576
Total payments	17,060	16,043	14,796
Unrealized foreign exchange loss (gain)	(187)	217	(74)
Net reserves at end of year	42,471	41,510	39,948
Reinsurance recoverables on unpaid losses	8,182	8,123	7,981
Claims and claim adjustment expense reserves at end of year	50,653	49,633	47,929
Amount of increase in gross claims and claim adjustment expense reserves	1,020	1,700
Amount of increase in reinsurance recoverables on unpaid losses	$ 59	$ 142
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%
Reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, net of discount	$ 2,450	$ 2,320
Discount included in reserves for long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies	1,160	1,100
Accretion expense	49	50	$ 50
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	17
Reinsurance recoverables on unpaid losses	15
Claims and claim adjustment expense reserves at end of year	$ 15	$ 17