Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Claims Development [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,712	$ 2,454	$ 2,522	$ 2,257
Reinsurance recoverable on unpaid claims	243	265	261	275
Total liability for unpaid claims and claims expenses	$ 2,955	$ 2,719	$ 2,783	$ 2,532