Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Claims Development [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,522	$ 2,589	$ 2,257	$ 2,274	$ 2,369	$ 2,112
Reinsurance recoverable on unpaid claims	261	259	275	276	208	243
Total liability for unpaid claims and claims expenses	$ 2,783	$ 2,848	$ 2,532	$ 2,550	$ 2,577	$ 2,355