Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
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
Claims and claim adjustment expense reserves at end of period	49,961
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
Reinsurance recoverables on unpaid losses	8,123	7,981
Net reserves at beginning of year	41,510	39,948
Estimated claims and claim adjustment expenses for claims arising in the current year	9,084	8,493
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(268)	(214)
Total increases	8,816	8,279
Claims and claim adjustment expense payments for claims arising in current year	2,851	2,699
Claims and claim adjustment expense payments for claims arising in prior years	5,454	4,966
Total payments	8,305	7,665
Unrealized foreign exchange (gain) loss	(102)	117
Net reserves at end of period	41,919	40,679
Reinsurance recoverables on unpaid losses	8,026	7,877
Claims and claim adjustment expense reserves at end of period	49,945	$ 48,556
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
Claims and claim adjustment expense reserves at end of period	$ 16