Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Medical Claims Payable Activity [Abstract]
Beginning balance, unpaid claims, gross	$ 2,848	$ 2,577	$ 2,532	$ 2,355
Less: Reinsurance and other amounts recoverable	259	208	275	243
Beginning balance, unpaid claims, net	2,589	2,369	2,257	2,112
Incurred claims related to:
Current year	4,910	4,697	15,057	14,318
Prior years	(30)	(5)	(268)	(88)
Total incurred	4,880	4,692	14,789	14,230
Paid claims related to:
Current year	4,901	4,705	12,794	12,285
Prior years	46	82	1,730	1,783
Total paid	4,947	4,787	14,524	14,068
Ending balance, unpaid claims, net	2,522	2,274	2,522	2,274
Add: Reinsurance and other amounts recoverable	261	276	261	276
Ending balance, unpaid claims, gross	2,783	2,550	2,783	2,550
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	$ 2,600	$ 2,400	$ 2,600	$ 2,400