Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
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	$ 168	$ 238
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.80%	1.20%
Net impact (unfavorable) of prior development on shareholders' net income	$ 66	$ 97
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	$ 93	$ 108
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.50%	0.60%
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	$ 66	$ 121
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.30%	0.60%
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	$ 9
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.00%	0.00%