Segment Information - Special Item Charges (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
After-tax Impact [Abstract]
Debt extinguishment costs				$ 65	$ 65
Merger-related transaction costs		$ 28	$ 29		57
Charge related to reinsurance transaction							$ 507
Charge for disability claims regulatory matter							51
Charge for organizational efficiency plan							40
Costs associated with a pharmacy benefit management ("PBM") services agreement							24
Total Before-tax Impact [Abstract]
Debt extinguishment costs					100	$ 0	0
Merger-related transaction costs					66
Charge related to reinsurance transaction							781
Charge for disability claims regulatory matter							77
Total special items						$ 0
Other Benefits Expense [Member]
Total Before-tax Impact [Abstract]
Charge related to reinsurance transaction							727
Charge for disability claims regulatory matter							75
Other Operating Expenses [Member]
After-tax Impact [Abstract]
Debt extinguishment costs					65
Merger-related transaction costs					57
Charge for organizational efficiency plan							40
Costs associated with a pharmacy benefit management ("PBM") services agreement	[1]						24
Total Before-tax Impact [Abstract]
Debt extinguishment costs					100
Merger-related transaction costs					$ 66
Charge related to reinsurance transaction							54
Charge for disability claims regulatory matter							2
Charge for an organizational efficiency plan							60
Costs associated with a pharmacy benefit management ("PBM") services agreement	[1]						$ 37

[1] Under this agreement, the Company utilizes a vendor's technology and service platforms, retail network contracting and claims processing services.