Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 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	10,906	10,077
Prior years	(168)	(238)
Total incurred	10,738	9,839
Paid claims related to:
Current year	8,557	7,823
Prior years	1,873	1,684
Total paid	10,430	9,507
Ending balance, unpaid claims, net	2,762	2,589
Add: Reinsurance and other amounts recoverable	243	259
Ending balance, unpaid claims, gross	3,005	2,848
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	$ 2,900	$ 2,700