Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 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	$ 30	$ 5	$ 268	$ 88
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.20%	0.00%	1.40%	0.50%
Net impact (unfavorable) of prior development on shareholders' net income	$ 19		$ 116
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	$ 20	$ 2	$ 128	$ 65
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.10%	0.00%	0.70%	0.30%
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	$ 10	$ 3	$ 131	$ 31
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.10%	0.00%	0.70%	0.20%
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			$ 9	$ (8)
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage			0.00%	0.00%