Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017		Mar. 31, 2016
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		$ 176		$ 64
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage		0.90%	[1]	0.30%	[2]
Net impact (unfavorable) of prior development on shareholders' net income		$ 61		$ 14
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		$ 78		$ 51
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage		0.40%	[1]	0.30%	[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		$ 98		$ 28
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage		0.50%	[1]	0.10%	[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		$ 0		$ (15)
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	[3]	0.00%	[1]	(0.10%)	[2]

[1] Percentage of current year incurred costs as reported for the year ended December 31, 2016.

[2] Percentage of current year incurred costs as reported for the year ended December 31, 2015.

[3] Other amounts in 2016 related to increased medical costs in the Government segment resulting from additional provider risk sharing.