Global Health Care Medical Costs Payable - Unpaid Claims Development (Details) - Global Health Benefits Segment [Member] Claims in Millions, $ in Millions	Dec. 31, 2016 USD ($) Claims	Dec. 31, 2015 USD ($) Claims	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Claims Development [Line Items]
Incurred claims	$ 37,574
Cumulative Paid Claims	35,357
Outstanding liabilities prior to 2015	40
Liability for unpaid claims and claims expenses, net of reinsurance	2,257	$ 2,112	$ 1,928	$ 1,856
Reinsurance recoverable on unpaid claims	275	243	252	194
Total liability for unpaid claims and claims expenses	$ 2,532	2,355	$ 2,180	$ 2,050
Minimum [Member]
Claims Development [Line Items]
Percent of health claims paid within one year	95.00%
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred claims	$ 18,487	18,564
Cumulative Paid Claims	18,305	$ 16,588
Liability for unpaid claims and claims expenses, net of reinsurance	182
Claim frequency | Claims		2.5
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred claims	19,087
Cumulative Paid Claims	17,052
Liability for unpaid claims and claims expenses, net of reinsurance	$ 2,035
Claim frequency | Claims	2.9