Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	$ 130	$ 176
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.60%	0.90%
Net impact (unfavorable) of prior development on shareholders' net income	$ 43	$ 61
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	$ 71	$ 78
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.40%	0.40%
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	$ 50	$ 98
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.20%	0.50%
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	$ 0
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.00%	0.00%