Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 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	$ 189	$ 268
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.90%	1.40%
Net impact (unfavorable) of prior development on shareholders' net income	$ 85	$ 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	$ 114	$ 128
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.60%	0.70%
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	$ 131
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.30%	0.70%
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%