Global Health Care Medical Costs Payable - Activity (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Medical Claims Payable Activity [Abstract]
Beginning balance, unpaid claims, gross	$ 2,770	$ 2,646	$ 2,532	$ 2,355
Less: Reinsurance and other amounts recoverable	256	207	275	243
Beginning balance, unpaid claims, net	2,514	2,439	2,257	2,112
Incurred claims related to:
Current year	4,986	4,796	10,147	9,621
Prior years	(62)	(19)	(238)	(83)
Total incurred	4,924	4,777	9,909	9,538
Paid claims related to:
Current year	4,674	4,595	7,893	7,580
Prior years	175	252	1,684	1,701
Total paid	4,849	4,847	9,577	9,281
Ending balance, unpaid claims, net	2,589	2,369	2,589	2,369
Add: Reinsurance and other amounts recoverable	259	208	259	208
Ending balance, unpaid claims, gross	2,848	2,577	2,848	2,577
Total of incurred but not reported liabilities plus expected claim development on reported claims, including reported claims in process	$ 2,700	$ 2,400	$ 2,700	$ 2,400