Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Claims Development [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,762	$ 2,454	$ 2,589	$ 2,257
Reinsurance recoverable on unpaid claims	243	265	259	275
Total liability for unpaid claims and claims expenses	$ 3,005	$ 2,719	$ 2,848	$ 2,532