Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 78		$ 210		$ 159
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.40%	[1]	1.30%	[2]
Net impact (unfavorable) of prior development on shareholders' net income			$ 60
Completion Factors [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 59		$ 62
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.30%	[1]	0.40%	[2]
Medical Cost Trend [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 27		$ 115
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.10%	[1]	0.70%	[2]
Other [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ (8)		$ 33	[3]
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.00%	[1]	0.20%	[2]

[1] Percentage of current year incurred costs as reported for 2015.

[2] Percentage of current year incurred costs as reported for 2014.

[3] In 2015, the other balance primarily relates to an increase in the 2014 reinsurance reimbursement rate from CMS under the Health Care Reform Act.