Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Medical Claims Payable Activity [Abstract]
Beginning balance, unpaid claims, gross	$ 2,532	$ 2,355	$ 2,180
Less: Reinsurance and other amounts recoverable	275	243	252
Beginning balance, unpaid claims, net	2,257	2,112	1,928
Incurred claims related to:
Current year	20,233	19,087	18,564
Prior years	(266)	(78)	(210)
Total incurred	19,967	19,009	18,354
Paid claims related to:
Current year	17,979	17,052	16,588
Prior years	1,791	1,812	1,582
Total paid	19,770	18,864	18,170
Ending balance, unpaid claims, net	2,454	2,257	2,112
Add: Reinsurance and other amounts recoverable	265	275	243
Ending balance, unpaid claims, gross	2,719	2,532	$ 2,355
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	2,600	2,400
Amounts due for physician incentives and other medical care expenses and services payable	$ 100	$ 100