Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 62	$ 19	$ 238	$ 83
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.30%	0.10%	1.20%	0.40%
Net impact (unfavorable) of prior development on shareholders' net income	$ 36		$ 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	$ 30	$ 12	$ 108	$ 63
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.20%	0.10%	0.60%	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	$ 23	$ 0	$ 121	$ 28
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.10%	0.00%	0.60%	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	$ 7	$ 9	$ (8)
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.00%	0.00%	0.00%	0.00%