Goodwill (Details) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 02, 2018
Goodwill
Goodwill, impairment loss		$ 3,921,000,000	$ 0	$ 135,000,000	$ 3,921,000,000	$ 135,000,000
Goodwill
Goodwill, beginning balance					38,451,000,000
Acquisitions					102,000,000
Foreign currency translation adjustments					(14,000,000)
Divestiture of RxCrossroads subsidiary					(398,000,000)
Impairment		(3,921,000,000)	$ 0	(135,000,000)	(3,921,000,000)	(135,000,000)
Goodwill, ending balance	$ 34,220,000,000	34,220,000,000			34,220,000,000
Pharmacy Services Segment
Goodwill
Goodwill, beginning balance					21,819,000,000
Acquisitions					67,000,000
Goodwill, ending balance	21,886,000,000	21,886,000,000			21,886,000,000
Retail/LTC Segment
Goodwill
Goodwill, impairment loss					3,921,000,000
Goodwill
Goodwill, beginning balance					16,632,000,000
Acquisitions					35,000,000
Foreign currency translation adjustments					(14,000,000)
Divestiture of RxCrossroads subsidiary					(398,000,000)
Impairment					(3,921,000,000)
Goodwill, ending balance	12,334,000,000	12,334,000,000			12,334,000,000
Rx Crossroads
Goodwill
Goodwill, impairment loss				135,000,000		135,000,000
Goodwill
Impairment				$ (135,000,000)		$ (135,000,000)
Disposal group
Consideration							$ 725,000,000
LTC reporting unit
Goodwill
Goodwill, impairment loss		3,900,000,000			3,900,000,000
Fair value exceeding carrying value (as a percent)			1.00%
Intangible asset impairment	0
Goodwill
Impairment		(3,900,000,000)			(3,900,000,000)
Goodwill, ending balance	$ 2,700,000,000	$ 2,700,000,000			$ 2,700,000,000