Insurance Claim Reserves (Details) - Reserve Detail for Claims and Claim Adjustment Expenses - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	49,810
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	4,391	4,126
Estimated decrease in claims and claim adjustment expenses for claims arising in prior years	(116)	(50)
Total increases	4,275	4,076
Claims and claim adjustment expense payments for claims arising in current year	1,009	887
Claims and claim adjustment expense payments for claims arising in prior years	3,040	2,812
Total payments	4,049	3,699
Unrealized foreign exchange (gain) loss	(10)	34
Net reserves at end of period	41,726	40,359
Reinsurance recoverables on unpaid losses	8,068	7,942
Claims and claim adjustment expense reserves at end of period	49,794	$ 48,301
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