Insurance Claim Reserves (details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 47,949	$ 48,295
Net reserves at end of year	41,510
Reinsurance recoverables on unpaid losses	8,140
Claims and claim adjustment expense reserves at end of year	49,650	47,949	$ 48,295
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	47,929	48,272	49,824
Reinsurance recoverables on unpaid losses	7,981	8,449	8,788
Net reserves at beginning of year	39,948	39,823	41,036
Estimated claims and claim adjustment expenses for claims arising in the current year	17,846	15,675	14,471
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(458)	(680)	(817)
Total increases	17,388	14,995	13,654
Claims and claim adjustment expense payments for claims arising in current year	7,335	6,220	5,725
Claims and claim adjustment expense payments for claims arising in prior years	8,708	8,576	8,749
Total payments	16,043	14,796	14,474
Acquisition			2
Unrealized foreign exchange loss (gain)	217	(74)	(395)
Net reserves at end of year	41,510	39,948	39,823
Reinsurance recoverables on unpaid losses	8,123	7,981	8,449
Claims and claim adjustment expense reserves at end of year	49,633	47,929	$ 48,272
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	20
Reinsurance recoverables on unpaid losses	17
Claims and claim adjustment expense reserves at end of year	$ 17	$ 20