Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	5,447	5,125
Prior years	(130)	(176)
Total incurred	5,317	4,949
Paid claims related to:
Current year	3,423	3,183
Prior years	1,665	1,509
Total paid	5,088	4,692
Ending balance, unpaid claims, net	2,683	2,514
Add: Reinsurance and other amounts recoverable	242	256
Ending balance, unpaid claims, gross	2,925	2,770
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	$ 2,800	$ 2,600