Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,683	$ 2,454	$ 2,514	$ 2,257
Reinsurance recoverable on unpaid claims	242	265	256	275
Total liability for unpaid claims and claims expenses	$ 2,925	$ 2,719	$ 2,770	$ 2,532