Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Medical Claims Payable Activity [Abstract]
Beginning balance, unpaid claims, gross	$ 2,719	$ 2,532
Less: Reinsurance and other amounts recoverable	265	275
Beginning balance, unpaid claims, net	2,454	2,257
Incurred claims related to:
Current year	16,287	14,952
Prior years	(189)	(268)
Total incurred	16,098	14,684
Paid claims related to:
Current year	13,915	12,689
Prior years	1,925	1,730
Total paid	15,840	14,419
Ending balance, unpaid claims, net	2,712	2,522
Add: Reinsurance and other amounts recoverable	243	261
Ending balance, unpaid claims, gross	2,955	2,783
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	$ 2,800	$ 2,600