Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] Claims in Millions, $ in Millions	Dec. 31, 2017 USD ($) Claims	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development [Line Items]
Incurred claims	$ 39,055
Cumulative Paid Claims	36,642
Outstanding liabilities for the periods presented, net of reinsurance	2,413
Outstanding liabilities prior to 2016	41
Liability for unpaid claims and claims expenses, net of reinsurance	2,454	$ 2,257	$ 2,112	$ 1,928
Reinsurance recoverable on unpaid claims	265	275	243	252
Total liability for unpaid claims and claims expenses	$ 2,719	2,532	$ 2,355	$ 2,180
Minimum [Member]
Claims Development [Line Items]
Percent of health claims paid within one year	95.00%
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred claims	$ 18,822	19,087
Cumulative Paid Claims	18,663	$ 17,052
Liability for unpaid claims and claims expenses, net of reinsurance	$ 159
Claim frequency | Claims	2.7
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred claims	$ 20,233
Cumulative Paid Claims	17,979
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,254
Claim frequency | Claims	3.3