Global Health Care Medical Costs Payable - Prior Year Development (Details) - Global Health Benefits Segment [Member] - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
(Favorable) Unfavorable incurred costs related to prior years' claims payable	$ (64)		$ (210)
Incurred costs related to prior years' claims payable percentage	0.30%		1.30%
Favorable (unfavorable) impact of prior development on shareholders' net income	$ 14	$ 25
Completion Factors [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
(Favorable) Unfavorable incurred costs related to prior years' claims payable	$ (51)		$ (62)
Incurred costs related to prior years' claims payable percentage	0.30%		0.40%
Medical Cost Trend [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
(Favorable) Unfavorable incurred costs related to prior years' claims payable	$ (28)		$ (115)
Incurred costs related to prior years' claims payable percentage	0.10%		0.70%
Increased Medical Costs Government Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
(Favorable) Unfavorable incurred costs related to prior years' claims payable	$ 15
Incurred costs related to prior years' claims payable percentage	0.10%
Reinsurance Reimbursements Under Health Care Reform [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
(Favorable) Unfavorable incurred costs related to prior years' claims payable			$ (33)
Incurred costs related to prior years' claims payable percentage			0.20%