Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 266	$ 78	$ 210
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	1.40%	0.40%
Net impact (unfavorable) of prior development on shareholders' net income	$ 112
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	$ 124	$ 59
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	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	$ 133	$ 27
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.70%	0.10%
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%